Most of the world, but especially the Greenbuilding Community assumes, “European” implies “more energy efficient”. When it comes to windows, this automatic presumption of superior energy efficiency is both so common and so misplaced that it deserves a name; Presumptive Euro Superior Energy Efficiency Syndrome.

Presumptive Euro Efficiency Syndrome applies not just to a window’s glass but its frame as well. Afterall the European PH (Passiv Haus) window frames have all that insulation in them, they must be better !

I may be biased, but I’m not so sure. Let’s take a closer look.

The PH system for rating windows emphasizes insulating ability over solar gains. Choosing windows based on a lower U, as long as SHGCglass > 0.5 does not always result in the lowest energy bills.

Recognizing the flaw, PHI (Passivhaus Institute) has introduced a letter rating scheme that rewards slimmer frames; (for an explanation see pg 3 of: http://www.passiv.de/downloads/03_certification_criteria_transparent_components_en.pdf ). Slimmer frames allow greater solar gains than bulkier frames, which reduces heating loads, making it easier to achieve Passive house heating load targets. While the letter rating scheme does help, it still doesn’t really fix the problem. But that’s another story for another day…

Typically European Passive House window frames, even the ‘A’ rated ones, are bulkier than North America’s most energy efficient windows.

The problem with too much frame is that no matter how well it insulates it will never be as energy efficient as the glass it surrounds. That’s because unlike the glass which contributes heating season solar gains, the frame can not gain energy, it can only lose energy. So, while bulkily framed windows typically have a lower Uframe than slimmer frames, they also always contribute fewer solar gains than slimmer frames.

Some common Euro Passive House frames are almost 125mm (5” ) high, but most are more like 100mm (4”) tall. Even Euro ‘A’ rated frames are still typically 85mm (3 1/4”) high. And they are often that high for both operable and fixed windows.

North American outswing windows have frames that are about 70mm (2 3/4”) high. This is an advantage over Euro Passive House windows. On smaller windows it can be a significant advantage. For a 24”x35” window, a 100mm tall frame leaves you with a window that is only 50% glass. A slimmer 70mm high frame results in a window that is 63% glass.

More significantly North American fixed frames are about 50mm (2”) high – providing noticeably more glass area than their more bulkily framed Euro counterparts. For a 48”x48” fixed window an ‘A’ rated window with a 85mm high frame is 73% glass. The same window with a 50mm high frame is 84% glass.

To be very clear, just as there is more to energy efficient cars than tire pressure, there is more to energy efficient windows than frame height.

It certainly possible to have a long and tortured discussion about the myriad of often confounding and conflicting factors that affect window energy efficiency. But once again, the best way to assess ‘better’ is not to argue over more beer, but to run PHPP.

So once again, I ran PHPP. I compared frames using the same Lancaster NH PHPP model I used for comparing glass. For more information about the house see; http://www.garlandmill.com/articles/passivehausstandards.htm

Lancaster House
image from www.garlandmill.com/articles/passivehausstandards.htm

I took the PHPP spreadsheet for the house and kept everything constant, except for window frame characteristics.

I compared 3 frames; 2 Euro Passive House frames and our insulated fiberglass frame. The first Euro frame is based on a common PVC frame. The second Euro frame is based on a more advanced PHI listed ‘A-rated’ (slimmer)frame.

The results are in the Table below:

Keep in mind that your mileage may vary. Your mileage will especially vary, sometimes wildly, depending on building design, climate and of course the window’s characterisitics.

The best bet to compare windows is to run PHPP for your building.

PHPP predicts this house as built will use 14.2 kWh/m^2/yr (4.5kBTU/ft^2/yr). Note that the common Euro PVC frame’s bulk, would’ve increased the heating requirements of the house by 9%. This increase is enough to put the house over the 15.0 kWh/m^2/yr (4.75 kBTU/ft^2/yr) qualification threshold for the Passive House program.

For this house the PH version of our frames produce a 14% lower Specific Heat Demand than a common Euro PVC window.

The results also show the advantage of slimmer frames. Although the ‘A’ Rated PHI listed window frame insulates much better than the PH version of our insulated fiberglass frames, its extra bulk results in a marginally higher heating bill.

Again, your mileage may vary; but in this case a North American offering produced the lower space heating demand. Results like this are project specific. Nobody should make any sweeping statements based on one house.

Again, the best bet to compare windows is to run PHPP for your building.

However, I do think it’s fair to say, that when it comes to energy efficient window frames, that there should be no automatic presumption of Euro thermal superiority.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

Different Standards

Higher Euro window R-values are for the most part, a product of a different Standards rather than a refined mastery of physics.

Euro window standards (CEN) are based on a 0C (32F) outside temperature, while North American Standards (NFRC) are based on a -18C (0F) outside temperature. At the warmer 0C, there is a thermal benefit to a wider to a 18mm (11/16”) pane spacing. This benefit does not exist at the colder North American design temperature of -18C.

In other words, at 0C, glass lites optimally spaced for 0C, insulate better than the same glass lites optimally spaced for -18C. That’s why for the same 3 lites of glass, Euro optimized glass can have a Euro Uglass lower than a North American Uglass.

For example, consider a High Solar Gain triple with two of low-e coatings as shown in the drawing below:

Depending on the interpane spacing and the calculation procedure, this glass unit can have very different U-values. The Table below compares the U-values of optimally spaced glass for each of the NFRC and the CEN Standards.

This Triple Glazed unit, when optimally spaced for and evaluated according to each Standard, has very different values for Uglass. In this case the CEN Uglass is 19% lower than the North American Uglass. ( 0.61 vs 0.76 W/m^2K or R-9.3 vs R-7.5).

So Euro Windows have a significant marketting advantage. Their Uglass is calculated differently, so they claim a significantly better insulating value for their glass. It’s sort of like runners comparing times for the 100, without specifying yards or meters, only worse.

More bluntly, stating a Euro Window Uglass-value to North Americans, without identifying it as a Euro Uglass-value is misleading. For example tabulated above, it is very misleading.

Different Low-e 1

So Euro glass looks better because of a more favourable Standard. Some people, usually Passive House enthusiasts, will say that Euro Windows have a lower Uglass not only because of differences in standards, but also because ‘they have better glass than we do’.

I may be biased, but I’m not so sure. Take a closer look.

Because the Passiv Haus program is based on European (CEN) Standards, my analysis is based on calculating Uglass to CEN conditions.

The best insulating glass on the Passivhaus Institute’s Component Database for Glass is Guardian Europe’s ‘ClimaGuard Premium2′. It is listed as having a metric U of 0.49 when filled with Krypton and having a G/SHGC of 0.53. ( http://www.passiv.de/komponentendatenbank/verglasung ).

In imperial units that’s; U 0.086 or R 11.6. That’s a mind boggling good insulating value. WINDOW7.1 produces the same results for ‘Climaguard Premium’ under CEN conditions with a glass unit constructed as shown below. So, not surprisingly, the Passivhaus Institute’s listiing is a legitimate result.

It is true, no single North American low-e can produce exactly this combination of SHGC & U. European low-e’s are different. European low-e’s have SHGCs and U-values part way between those of North American High Solar gain low-e’s and Low Solar Gain low-e’s.

Logically then, the best way to create a Euro-like unit with North American glass is to create a Hybrid unit as shown in the diagram below. The North American Hybrid unit has 1 High Solar gain low-e and 1 Low Solar Gain low-e. This hybrid unit comes awfully close to Guardian Europe’s ClimaGuard2. Such a hybrid unit according to WINDOW7.1, has a G/SHGC of 0.52 and a metric U of 0.53. In imperial units that’s; U 0.090, or R 11.1.

This Guardian version of a North American Hybrid Unit insulates slightly less well (about R 0.5 lower), and allows just slightly less Solar Gain (SHGC about 0.01 lower) than Guardian Europe’s ‘Climaguard Premium’. We could argue all day about how close is close, but the only way to evaluate the difference is not to argue over more beer, but to run PHPP.

So i ran PHPP. I compared not only a North American Hybrid unit based on Guardian North America’s glass, but also 3 other North American manufacturable (Cardinal, PPG & AGC) Hybrid units. I compared them all to Guardian Europe’s ‘Climaguard Premium2′ glass in a Lancaster, NH PHPP model.

Lancaster House
image from www.garlandmill.com/articles/passivehausstandards.htm

Built in 2010 the Lancaster, NH house is a highly glazed house in a heating climate. Its south facing glass area is equivalent to about 17% of the floor area. This is high, but most of our Custom House customers, whether building a Passive House or not end up with houses with large south facing glass areas. So, by my logic, it’s not an unreasonable example. For more information about the house see; http://www.garlandmill.com/articles/passivehausstandards.htm .

I used the PHPP spreadsheet for this house to compare the effect of several different glass options. The Table below compares 4 North American Hybrid glass options to the most energy efficient Euro glass option. The Table compares their respective G/SHGC and Uglass characteristics, along with the PHPP generated Specific Heat Demand.

All of the above options, some of today’s most crazily energy efficient, most optimally spaced, with rather expensive Krypton gas fills, are more energy efficient than what was used in the 2010 built NH house. That’s why all of the above glass options produce much lower Specific Head Demands than the 14.2 kWh/m^2/yr (4.45 kBTU/ft^2/yr) in the as built PHPP model.

Running the PHPP model with the 4 North American made glass units produced Specific Space Heat Demands ranging from 10.4 to 11.0 kWh/m^2/yr.

So the North American Hybrid units made glass results in 10%-20% higher projected heating bills for this cold climate PH house than the best European glass. For this house the difference is about 160 – 320 kWh/yr. As a worst case scenario, with $0.15/kWh electric resistance heating this 10% difference amounts to about $25/yr – $50/yr.

I suspect to most people, the differences aren’t much. But the bragging rights do go to the European glass. Your mileage may vary, but my guess is that most heating climate PH’s would probably get similar results. So that’s it then, the European glass is better?

Not so quick Poindexter.
Different Low-e 2

There is at least one other worthwhile option to consider. It’s an option that takes advantage of North America’s sunnier climate. Both of its low-e coatings are a North American High Solar Gain low-e. Combined with a Low Iron outer lite, call it a Super Gain glass unit. An example of this unit is shown below.

For these low-e’s the G/SHGC is higher than the highest rated PHI listed unit, which is good. On the other hand, the Uglass is also higher, which is not so good. Again, the best way to assess ‘better’ is not to argue over more beer, but to run PHPP.

The Table below shows G/SHGC and Uglass for 4 North American High Solar Gain low-e glass options. More importantly, it also shows the respective Specific Heat Demands when input into the Lancaster NH PHPP model.

Again, and as always, your mileage may vary. However, for this heating climate example the North American high solar gain low-e results in about a 20% lower space heating demand than ‘the best’ European low-e.

While this 20% spread at Passive House efficiency levels, because this house so energy efficient does not result in a large dollar savings, it is important to note that there is likely no incremental cost to achieve this savings.

The key point is that despite not insulating as well as the Euro low-e, North American high solar gain low-e’s take advantage of the fact that, for the most part, North American heating climates are sunnier than European heating climates.

So not only can North American glass match the performance of European glass, but in heating climates it can also surpass it.

I do think it’s fair to say, then, that when it comes to energy efficient glass, that there should be no automatic presumption of Euro thermal superiority.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

When I was a pre-teen, my father bought me my first 10 speed bicycle.  By the fall of Grade 10, I had both ridden it into the ground and outgrown it.

I bought a police auction special and spent the winter re-building it.  It was great. I rode it everywhere.  But, by the time I got to university I was tired of its quirks.  I wanted a new bike.

So I shopped and shopped and shopped.  I was young, cynical and most importantly, I knew it all.  I didn’t listen to the bike shops and bought too cheap.  Two years later I admitted my mistake and slunk back to the bike shops for my fourth10-speed.

This time I was a little older and a little wiser.  I knew I had to spend double or triple what I spent on the previous bike.  I made a long list of models, their costs and their dozen or so main components (gears, brakes, hubs, rims, tires, seat..).  There was one very clear choice.  It was a great bike.  I can still remember that absolute exhilarating first evening – it felt like flying – even uphill.

I didn’t stop for a second to wonder how they could offer such a great deal.   It was not until a few years later that I found out how they did it.  The bearings in the bottom bracket and steering column completely, and I mean completely, self destructed.

In a way I suppose it was perfect marketing; everything I could see was name brand top quality.  They hit their price point by going cheap on what I couldn’t see.

Unlike bicycles, windows are not easy to replace.  Similar looking windows that have different prices, do so for a reason.  Eventually the differences appear.  Damaged frames in particular, whether they be rotted wood, cracked vinyl or cracked fiberglass are an owner’s nightmare.

When it comes to fiberglass, process differences make some fiberglass lineals more susceptible to cracking and chipping than others.   In our view, our supplier, Winnipeg MB based, Omniglass SCT makes better frames than other frame suppliers. They use materials and processes that while hard to see, create a stronger, more crack & chip resistant frame.  To understand how and why this is possible you need to understand how fiberglass window frames are made or in the language of the trade – pultruded.

Fiberglass window frames start, perhaps not surprisingly, with glass fibers. Individual fibers are loosely grouped together into bundles of long continuous filaments that resemble yarn. These bundles of fibers are called rovings.  A typical frame has many, usually dozens, of rovings.

The rovings pass through a resin bath and are formed into a shape with a heated die.  The resin, although not particularly strong on its own, serves an important role. It locks the fibers in place.  Without the resin, the glass fibers, like rope, have no strength in any direction, other than longitudinally, and then only when they are in tension.  Once locked in place by resin, the fibers can then resist lateral and compression loads.  Although the resin dramatically stiffens the fibers, the frames remain stronger along their length; in the direction of the fibers, than they are across their width; perpendicular to the direction of the fibers.  In this way, pultrusions are similar to wood. They are strongest along their grain.

To give the frames more strength across their ‘grain’, the frames are wrapped in a mat or cloth.  A mat can be made of either woven fibers or randomly oriented fibers. The regular pattern of a woven matt would be visible would be apparent on the surface of the frame. This is why most mats used in fiberglass window frames consist of randomly oriented fibers.

Structurally speaking, the most useful fibers in the mat are those that are somewhat perpendicular to the rovings.  These across the “grain” fibers give the frame lateral strength.  Without a mat the frames would be prone to longitudinal cracking.  However, it’s hard to wrap a mat around a complex shape like a window frame.  Consequently, many pultruders will use several pieces of mat to encircle a complex shape.  Omniglass SCT on the other hand nearly always uses a 1 piece mat.  The advantage of a 1 piece mat is strength.  Every joint in the mat is a weak link – a crack waiting to happen.

While frames and sashes are loaded across the fibers under high wind conditions, some parts of the frame and the sash are subject to larger local loads across the “grain”.  Small ridges near glass stops, couplers or accessory grooves can experience large cross “grain” or lateral forces.  Ridges that retain glass stops are loaded laterally under large wind loads.  These ridges are subject to even higher lateral loads when glass stops and accessory profiles are pried out or snapped in – like when glass is replaced.

A mat provides uniform strength; it is constant around the perimeter of the profile. For example, it can’t be stronger around a glass stop ridge than on the face of a frame.  Wrapping the entire frame with a heavy mat, the transverse strength of which is only needed in a few spots around the frame, is expensive and wasteful.  Another type of reinforcing is needed.

That’s where texturized rovings or texos enter into the picture. A texo is a very fuzzy roving. It gets its extra ‘body’ from blowing air across a roving.  It is a similar process to using a hairdryer to add body.  The added bulkiness or fuzziness means, the texo intertwines with its neighbor rovings more effectively than a regular roving. “In engineer speak” a texo provides extra local lateral strength.

Besides providing additional local reinforcement, texos can help fill sharp outside corners. They prevent the corner from becoming resin-rich. Unreinforced resin-rich sharp outside corners tend to be brittle. In other words they have low impact resistance.  Even routine handling can cause resin rich corners to chip or flake.

While texos are important in preventing damage their most important role is minimizing damage. Their bulkiness is because most fibers are more diagonally meandering and less straight. The meandering nature of texos’ glass fibers is what helps stop micro cracks from becoming major cracks.  On the other hand, frames without texos can easily suffer noticeable cracks.  Even with diagonal rather than truly lateral fibers, a texo can make a big difference in in-situ performance and ongoing serviceability.

Texos cost more than regular rovings. They are an obvious ‘value engineering’ target by less quality conscious pultruders.  Deleting them is particularly tempting because their absence only shows up long after the sale. As an example, their absence can show up as cracks when glass is changed. Their absence can also show up as cracks when frames are subject to wind loads.

We cannot vouch for other pultruders. However, we can tell you that Omniglass SCT does use texos, to strengthen both ridges near glass stops and sharp outside corners.   The presence of these texos is likely why when we hear other fabricators bitterly complain about brittleness we have such blank looks on our faces.

My fourth bicycle taught me that with the fullness of time, some deals are too good to be true.  Missing texos are just one of a myriad of hidden factors that will, over time, convert a lower price into a higher price.

For the record, I still own my fifth bicycle.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

Talk to some people about increasing insulation levels beyond Building Code minimums and you get snorts of derision, often followed by “it’ll take more energy to make the extra insulation and framing material than you’ll ever save”. These people would be almost always be wrong.

The Nay Sayers are referring knowingly or otherwise to initial embodied energy – the energy used to create a product. Although important, initial embodied energy nearly always dwarfed by the energy consumed by a building over its lifetime.

One study of Canadian office buildings concludes that over 50 years, the operating energy is more than 6 times greater than the total embodied energy. The total embodied being the sum of:

–          initial embodied energy (used to make material used to build the building)

–          recurring embodied energy (used to make material used to maintain & renovate the building)

After 50 years, there is a roughly 50/50 split between initial and recurring embodied energy. Put another way, over the first 50 years, the initial embodied energy is less than 1/12^th of the operating energy.

Embodied energy does start to be significant when a Factor 10 building – one that uses 1/10^th the energy of a ‘Code Building’.  However, few buildings make that standard. Until a designer tackles a Factor 10 building, they need to focus on annual energy consumption rather than initial embodied energy.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

When it comes to green building; recycled carpets, natural materials and natural finishes are usually front and centre.  Recycling old carpet into new carpet is an example of “cradle to cradle” that is the essence of sustainability. Natural materials have the possibility of being renewable in ways man made materials can’t.  Natural finishes beat their solvent rich cousins every time.

But at the end of the day, green priorities need to be more than skin deep. We are responsible for a myriad of environmental impacts; all worthy of attention. However, of these, excessive energy use is the largest of all of man’s environmental impacts. That’s because carbon based fuels are the single largest contributor to climate change – by far our most pressing environmental issue.

The United States Green Building Council recognizes this along with the design community’s aversion to the technicality of low energy design.  Recently it revised LEED NC to make 2 of the 10 energy points mandatory.   Previously it’d been possible to build a LEED building without garnering any energy points at all.  Of the first 420 LEED NC version 2 certified buildings, 12% didn’t claim any of the 10 energy points, including, incredibly, 1 Gold building.

Here in Canada, Canadian Green Building Council’s LEED NC has always had an energy prerequisite of 25% better than code – roughly the same as LEED’s first energy point.

When it comes to reporting on green issues in the built environment there are few more authoritative than Environmental Building News. It weighed in on the green priorities question in their landmark article “Establishing Priorities with Green Building”.  In it they noted; “Ongoing energy use is probably the largest environmental impact of a building, so designing and constructing buildings for low energy use should be our number one priority”

Recognize too that energy use is related to the myriad of other environmental issues besides climate change. Although embodied energy is nearly always a secondary concern (see link to EMBODIED ENERGY column), it does reflect resource and hence habitat depletion. Buildings with lower embodied energy, consume fewer resources, deplete fewer habitats, and contribute less pollution.

So when it comes to green priorities, highly visible “aesthetic green” priorities are not as important as less visible “technical green” priorities. Skin deep green is not deep enough. Green Building’s most important gains are those that reduce energy use.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

Over time, HVAC systems have evolved to more effectively and efficiently counteract the heat loss from poorly insulated, drafty buildings.  However, they are getting very expensive to buy. And even the most evolved still cost money to operate.  Additionally, they also need to be replaced in whole or in part, every few decades.

If the truth were said, they are temporary patches for inadequate building envelopes.

Put another way, HVAC systems treat the symptoms, rather than the disease.  A good Envelope, on the other hand, does the reverse. It minimizes the need to ameliorate symptoms.

Besides their fuel costs, HVAC systems also suffer from the requirement for regular maintenance, something an Envelope usually does not require.  The bigger and more complicated the HVAC system, the greater the potential for problems.

As mentioned earlier, HVAC systems are also relatively short-lived. They or their key components will need to be replaced several times over the life of a building.  The Envelope is more permanent. It usually lasts many generations without the need for any modification.

HVAC systems like ground source heat pumps and radiant floor heating systems can easily add tens of thousands of dollars to the cost of a house.

While these systems may use less fuel over their lifetime than other approaches, they still use fuel. An equivalent investment in the Envelope will typically produce even larger savings for many more years than these complex HVAC systems.

It is currently not realistic to completely eliminate HVAC systems. Controlled ventilation is not yet easily achievable by strictly passive means. So while they can’t be eliminated, they can be simplified.

A good Envelope means perimeter heating systems are nearly always unnecessary. This can be a big savings in commercial buildings. In houses duct runs can be shortened, ending at inside, not outside walls. In fact, a building with a good Envelope should not require a dedicated furnace.

This is old news to some people.  It was about 20 years ago when we had our first customer build a house without a furnace. Their water heater satisfies his family’s space heating requirements. Their HRV (Heat Recovery Ventilator) preheats the incoming fresh air with the outgoing stale air. If required, a coil connected to their water heater further heats the incoming air. (A coil is fluid to air heat exchanger and looks like the radiator in your car).

A more recently, another customer uses a 1000 W electric heater, downstream of their HRV to heat her house. (For the record 1000W is the equivalent of a hair dryer set on medium.)

It’s not complicated to build a building with a simple HVAC system. The common threads among these houses are high levels of air tightness and insulation with some passive solar design.

A high level of air tightness means tested air tightness of at least 2.5 air changes per hour (ach) @ 50Pa (the Canadian Energy Star standard), preferably 1.5 ach @50Pa (the Canadian R-2000 standard). Some houses have tested below 0.25 ach @ 50Pa. Consider giving your contractor a bonus based on the level of air tightness they achieve.

A high level of insulation means levels of at least: RSI 10+ (R-60+) attic, RSI 6+ (R-40+) walls and : RSI 4+ (R-22+) below grade and, of course, – triple glazed windows.

Passive solar design means biasing windows towards the south. Most sources suggest south facing window area equal to 6-8% of the floor area. Passive solar also means making some effort to store daytime gains in thermal mass like masonry elements or a double thickness of drywall.  A simple way to increase thermal mass is to leave drywall scraps within internal walls.

The exact costs involved in the trade-offs between HVAC and Envelope vary. They vary not only from region to region, but also from house to house, so it’s not possible to make blanket statements about how and where to make HVAC/Envelope compromises.

Never the less, since a good Envelope means HVAC systems can be downsized and simplified without compromising comfort, you’ll almost always save money by spending more on Envelope and less on HVAC.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

Ask nearly any window sales person (North American or European), about energy efficiency and you will get the same answer.  They’ll start talking about insulating value (either U-value or its inverse, the R-value).  They will talk as if a window is no different from a wall.  They promote the idea that the lowest U-value produces the lowest energy bill.

In doing so, they are getting the science 100% incorrect, (see Stephen’s Column – All Low E is not created Equal).

Besides from being wrong, equating a window’s insulating value with its energy performance is brutal marketing.  Even a 2×6 wall normally insulates about 600% better than a typical Energy Star (US northern zone) window.  It follows then, that an energy efficient home would minimize the number and size of windows.  This is not exactly clever marketing for window makers.

Marketing aside, the simple fact is that windows are a unique element of the building envelope.  Unlike walls, roofs and foundations, windows can be a source of heat gain.  For buildings that need to be heated, this can be a very good thing.

Unlike most other window makers, Thermotech Fiberglass recognizes this fact.  Although we offer a multitude of glazing options, we are biased towards high solar gain glazings.  When combined with our slim insulated fiberglass frames these glazings produce super energy efficient windows.  Windows that not only insulate well, but also capture solar gains.

Compared to a typical Energy Star (US Northern zone) window, our best triple glazed windows insulate about 50% better, and capture about 1/3^rd more solar gain.  This means that Thermotech’s windows are dramatically more energy efficient than a typical Energy Star window.

Consider a 600mm x1200mm (about 24” x 48”) casement window that faces south in Ottawa ON.  If that window was a typical Energy Star casement, it will gain 28 kWh over the heating season.  If it was Thermotech’s best casement, it will gain 118 kWh over the heating season.  That’s a 400% improvement.

Surprisingly to most, that same south facing window will not cause extensive overheating in the summer.  The sun is just too high in the sky.  Consider a south facing window at 40 degrees north, in mid-June.  It receives about half the solar gain it does in February.  In addition, this diminished solar gain is only 40% more than the solar gain of a north-facing window.

Usually overheating comes not from south facing, but from east or west facing windows.  On the longest day of the year, they can experience 115% more solar gain than a south facing window.

It is true that summertime solar heat gain can be controlled with low solar heat gain glass. However, doing so means the substantially more valuable wintertime solar gains for south facing windows are largely wasted.

It’s usually more effective to control excessive solar gains with exterior shading.  Exterior measures such as deciduous trees, vines, overhangs and awnings can all provide cooling season shading; without sacrificing cherished heating season gains.  Interior window treatments such as blinds, shades and curtains are never as effective as exterior shading.  Even white interior blinds only reduce solar gains by 20%.

Clearly then there is more to window energy efficiency than insulating value alone.  Thermotech Fiberglass has taken this less travelled road; a road that recognizes the unique opportunity presented by windows.  This is the opportunity to not only reduce heat loss, but also maximize usable solar gains.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

There are two ways to install a new window where one already exists. Frame in means that only the sashes of the old window are removed. – The new window is installed into the old frame. Surprisingly enough, frame out means that both the sashes and the frame of the old window are removed. – The new window is installed into the original rough opening and new trim is installed.

Most replacement windows are installed into the existing frame – that is “frame in”. This approach has several advantages…..

1. From the customer’s point of view – since the original frame is left untouched, the original trim remains in place so there is no need to repaint the trim.

2. From the contractors point of view – leaving the existing frame in place greatly simplifies measuring and installation. Even the least sophisticated installer can install a window into an existing frame.

As you may tell from my tone, these are about the only advantages to installing into the existing frame. It truly is a lousy way to replace windows – for several reasons….

1. There is a significant reduction in glass area. I’ve seen some old glass on glass sliders that when replaced with vinyl casements lost half their glass area. This not only reduces the view, but also cuts down on the free heat from the sun.

2. Bypass air leakage. When we remove old frames there is often glass wool insulation between the window frame and the rough opening. Its usually dirty – not because it was dirty when it was installed, but rather because it filters out the dust in the air that’s been leaking around the window. So if you don’t remove the old frame how do you stop that air leakage? How many farmers fence in only 95% of their fields? It just doesn’t make sense.

3. Exterior appearance. While it’s usually possible to make the inside of a “frame in” installation look OK, the exterior is another matter. On the outside of a “frame in” installation, the aluminum trim covers the old frame and its brickmould. No matter how carefully this is done this exterior trim is much bulkier looking that the original trim. Once you know this – you can spot a “frame in” installation from half a block away. – it just doesn’t look like an original window.

4. False Economy. The knuckle dragging proponents of “frame in” always try and tell people its much cheaper and faster to leave the existing frames. But this is not true. Our crews are used to doing “frame out” installations and they can remove existing sashes and frames and install the new window complete with trim for almost the same price as they can if they only removed the existing sashes.

There is only one possible exception to the rule-take the blasted old frame out -that is in homes more than 50 years old where the existing trim is very expensive or impossible to duplicate with new trim. In this case, and this case only, it’s wise to consider installing into the existing frame.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

If you picked the higher R-value window you could be wrong. Wrong to the tune of many hundreds of dollars over the life of the windows.

The answer is not always the higher R-value window. Why?

Windows contribute solar gains that help heat your house. The type of Low e glass that insulates the best also stops more than half of the free heat from the sun. If your heating bill is bigger than your cooling bill this is a bad thing, a very bad thing,. To understand how a window affects your heating bill you need to know not just how well it insulates but how well it collects free heat from the sun.

The Canadian Energy Rating (ER) is the only one number system that permits comparison of energy efficiency because it accounts for all the R-value losses as well as the solar gains. I’ll use it to show you how more is less and less is more when it comes to windows and your heating bill.

The two extremes in the R-value/solar gain trade-off are Cardinal’s E2 and Libby Owens Ford’s Energy Advantage II. The example below is for our double glazed fiberglass casement.

Fiberglass Casement (600mm x 1200mm / 24″ x 48″)

The Fine Print:

1. Glass Code1st digit – number of panes of glass
   2nd digit – number of SuperSpacers
   3rd digit – number of Low-e coatings and argon gas fills

   The number after the Low e manufacturers name shows the location of the Low e coatings – surface #1 is the exterior surface

2. Glass properties are as determined by VISION.
   
3. Window Properties were determined by FRAME and FRAME PLUS

Note that the location of the Low e surface is different for the two options, but reflects where most manufacturers put the coating. Feel free to e-mail me if you want a more detailed explanation of this.

Comparing glass properties you can see the two glass options both transmit about the same amount of visible light. In other words they look very similar. But they aren’t. The LOF glass transmits 60% more solar heat than the EE. Solar energy includes visible light energy as well as invisible UV energy and invisible infrared energy. All three ‘flavours’ of solar energy contribute to reducing your heating bill.

When the comparison moves onto the U-value (R = 1/U), the ranking has reversed. Cardinal’s EE insulates 14% better than the LOF.

Comparing window properties for this example you can see similar results. SHGC stands for Solar Heat Gain Coefficient. It tells us how much of the solar energy that strikes the outside of the window (including the frame), makes it through the window. Looking at the SHGC you can see not suprisingly, that the LOF Low e delivers more solar energy than Cardinal’s EE. Similarly the Cardinal EE window has maintained its lead in the U-value, although the lead has been reduced by the effect of the spacers and frame.

As far as your heating bill goes, the ER tells the tale. In this case the difference is 11.5 W/m2. Over a 212 day heating season (typical for Canada and the Northern US) this amounts to 1170 kWh or 4 million BTU’s for a typical custom house with 200 ft2 of windows. Over the next 20 years that difference accumulates to an expense that, depending on local energy rates, is about $1600 for electric resistance heating or about $600 for natural gas heating. (for more info on ER, see section 6.3 of Consumer’s Guide)

This may or may not be a lot of money to you, but when you consider that Cardinal’s EE is either the same price as the LOF Low E or more expensive than LOF’s Low e it’s a pretty easy decision to make.

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.

With such a humungous market why don’t we have a fibreglass double hung?

Thermotech’s focus is on energy efficiency and a double hung is inherently not as energy efficient as a hinged window, such as a casement. Firstly a double hung window is not as air tight as a casement window. The double hung window has to slide, so its weatherstripping can’t be as tight as a casement or it won’t slide. That’s obvious to most people.

What is not so obvious is that sliding windows aren’t as air tight as casements because the weatherstripping changes plane. Think of the bottom sash of a double hung. The bottom and side rails are weatherstripped along the edge of the rail. But what about the meeting rail? It’s weatherstripped along the outer face. It is difficult, very difficult, to maintain the integrity of the air seal through this change of plane.

Despite the fact that double hung windows are inherently draftier than casements, this does not have a major effect on your annual heating bill. The reality is that as long as new windows can maintain their tested air tightness (this topic will be the subject of a future column) very little of the window’s heat loss is due to air leakage.

Most of the annual heat loss from reasonably tight windows is from conductivity losses through the frame, spacer and glass. This is where double hung windows really fall down, (so to speak). All sliding windows have inherently cold frames compared to hinged windows, like casements. The sash in a casement window is ‘insulated’ on the inside by the hardware cavity. The sash in a double hung window has no such advantage.

Typical Double Hung Sill

Typical Casement Sill

Another inherent thermal weakness in any sliding window is the meeting rail. Thermally it is the weakest part of any sliding window. The upper sash in a double hung not only loses heat through its outer face, but also through its lower edge. It is cooled on two sides. That’s why the bottom edge of the upper sash experiences condensation before the lower sash.

On top of the inherent thermal drawbacks to the double hung there are some nuts and bolts engineering reasons for dragging our heels on the double hung. The window world is fast becoming a vinyl world. One of the inherent advantages to fibreglass compared to vinyl is its strength. This means that compared to vinyl windows many internal walls can be eliminated. Larger single cavity sections allow the economical insertion of polystyrene insulation. The strength advantage can also be used to reduce the size of some sections. This is especially true for casement windows as shown by Spilka’s conversion below:

Fibreglass as a material is more expensive than vinyl so this section simplification is important to being competitive. The same degree of simplification is not possible with a double hung frame. Nearly all of the internal walls in vinyl windows have a role other than adding stiffness. This means that compared to vinyl the percentage upcharge for fiberglass is likely more for a double hung than it is for a casement.

So why, then, do we have a sliding patio door, single hung and single slider? For the same reason that we will have to have a double hung very soon.

Customers want them. (Besides a fibreglass double hung still has the advantages of fiberglass’s low coefficient of thermal expansion.)

Stephen Thwaites is Thermotech’s Technical Director. He’s a professional engineer and a long time window and energy nerd. His irregular columns provide some insight into topics where there is at least as much fiction as fact.