Windows Make a Big Difference

This is from the Marvin Windows Website. It shows many of the aspects I would like to see in my design including the copper clad vault, etc.

Windows will likely be the single largest expense category for any home, and even more so for a passive-solar earth-sheltered home.  I have not actually ordered my windows yet, as I am still in the design stage, but here is what I have learned so far.  I am sure that I will add to and revise this page due to experience.

I already know that I am violating my own advice by adding too many “interesting” windows that are not cheap, but “the heart (or eye) wants what it wants” and I am not walking in blindly.

Under development

Save Money?

If you want to save money on your windows, you need to go with standard sizes and features.   Mass produced builders’ windows in the most common sizes are the cheapest.  The interesting thing is that the standard sizes change from window company to window company.  Some go with even sizes such as 28″, 30″ and 32″, while a competitor may use odd sizes such as 29″, 31″ and 33″.   I am not sure if this is just a historical/legacy thing or a cunning plan to make it more difficult to switch window manufacturers.  Some times window companies buy other window companies and add them as a new line.  These window companies continue with the same equipment and standard sizes which may be different from others under the same brand.   This makes it hard to lay out your window dimensions before you choose your brand/line, so stay flexible as long as you can.

Main windows often have more standard sizes than the transoms that are aligned with them.  For instance, Marvin has standard sizes every 2 inches for windows, so 38″, 36″ and 34″ are all standard.  But their transoms are available in fewer standard sizes and 38″ and 34″ would be considered custom sizes.

Not all window sizes are available.  Most companies limit their windows with a fixed maximum area.  This is to limit the weight and area of the window to reduce the chance of failures due to sagging or high winds.  This limits the height of wide windows or the width of tall windows.  Lower cost lines are typically more limiting.  The window frame material and glass strength also affect the largest window sizes.  For instance, stronger aluminum extrusions allow for wider casement windows than wood or vinyl windows.

The more interesting windows cost a lot more.  First, they are usually only available in the more expensive window lines, but even within those lines, expect a large price increase.  Here, I am shopping in the USA market, these options may be more standard and affordable elsewhere.

  • Want a round top? They are 4 times the price of an equivalent rectangle.  Adding a fixed half round transom is much cheaper.
  • Want a French casement?  It is at least 3 to 5 times more than a regular casement of the same overall area.
  • Want a push out casement instead of that annoying little crank?  Expect to pay at least double even though the mechanism is simpler.
  • Want to add a few inches past (or even between) the standard sizes?  You could easily see the price double.  For instance, the widest standard Pella Pro casement window is 35 inches (odd sizes), adding 5 inches to increase the size to 40″ wide is an option, but more than doubles the window cost.

Of course, specialty shapes such as chord windows are a terrible idea, if you are concerned with cost.

My design actually features Chord windows to fill the gap under the ends of the vaults.  If I use a single large (8ft wide and 2 ft tall) chord window, even fixed and without a sash (just the glass placed on site and held in with caulk and trim ) it costs about 3 times what the more standard casement style rectangle window costs.  However, my wall is also curved, so I can’t just use a single window to span the 30 degree segments, I needed to break it up into 3 equal segments and mull them all together.  The split and mull increased my cost by more than a factor of 3 for the same size overall window opening.  I have been told that the additional cost has to do with the trickiness of creating three windows that will fit together perfectly, and then mull them together, etc.  So at this point, quotes I have received from a number of companies show the segmented Chord window is about 10 times the price of the large regular casement windows on the front of my house.  They had not even factored in that it was bowed,  but I can only assume that would raise the price further.  If I had a single window like this, it would be bad, but I have 8, which makes it un-affordable.  We considered using flat walls so we could go with a single large chord window across each vault end, but ended up deciding against it.  Instead, I am considering cutting the shapes I need from 1/4 inch thick clear poly-carbonate which is about 1/70th the price and has an R value of about 1.5.  It is also many times stronger/tougher than glass and is UV stable.  It is less scratch resistant, but located high in the wall where nothing has an opportunity of scratching it.  It would then be held in with trim exactly as the glass version would have been.  Comments?


This Marvin Casement window with the “simulated transom” is much cheaper than it would be if purchased as a casement and a transom mulled together.

It seems like cost is not as directly related to window size as you may expect.  Combining a window and the transom above it into a single larger window will usually save a lot of money (provided it is a standard size).



Design professionals suggest you keep to just 3 different window sizes plus one specialty window.  More variety than that can make your home look cluttered and a bit odd.  (more to come on this topic)


Low-E high-SGHC controversy

There has been a lot of research and development into improving window glass.  They have invented coatings that keep out solar heat gain and coatings that let it in.  The climate you live in, as well as the orientation of the window should determine which type of windows you purchase, but somehow, politics got involved.  I was not able to unravel all the reasons behind how the tax breaks are applied, perhaps it is due to the greater use of glass in the southern states, or maybe it is California’s fault, or some lobby group for the natural gas companies, but there is a regulatory bias in favor of glass with a low solar heat gain.

As a consequence of the federal tax breaks associated with this regulatory bias, window manufacturers responded to the misinformed public demand and almost abandoned Low-e glass with high solar heat gain coefficients.  As a result, those of us in the north half of the country no longer had easy access to the kind of glass that we needed to take full advantage of passive solar heating.

Several years ago, when I started my research, only two companies in the USA had moderately high SHGC windows, now I see a few more are responding to demand and providing some options.  But before believing the high SHGC marketing claims, look at the numbers.   Those manufactures that did keep higher solar heat gain windows used methods that kept the same manufacturing processes as their low SHGC windows, so the performance is much lower than is possible.  In Canada, you can easily find low E windows with a SHGC of 0.7; it is tough to get even half of that with the American equivalents.  This is because the truly high SHGC windows require a totally different process to apply the coating.

Why is this regulatory bias and tax credit unreasonable?  According to the U.S. Energy Information Administration’s (EIA) 2001 Residential Energy Consumption Survey, Americans consume more than 7 times more energy for space heating than for air conditioning.  By some estimates, if average homes had high-SHGC glass on their southern exposures, it would save an average of 10% of their heating costs and this would add up to nearly a quadrillion (1 with 15 zeros) BTUs or hundreds of billions of dollars worth of energy.   It makes me wonder if the energy companies were involved with making sure that tax break only applied to low SHGC glass?  Or maybe congress just didn’t know what they were doing 😉

Sputtered low-E coatings provide low solar heat gain coefficients

Sputtered low-E coatings provide low solar heat gain coefficients

At least in the United States, “energy efficient” windows are designed for warm environments.  They feature low U values and low SHGC (solar heat gain coefficient), such as SHGC = 0.27.  The low E coating is usually on the inside of the outer pane.  Low-solar-gain low-E glass is typically made with sputtered low-E coatings consisting of either two or three layers of silver (also called double-silver or triple-silver low-E).

–Sputtered low-E. A multi-layered low-E coating deposited on glass or plastic film. Typically consists of three primary layers with at least one layer of metal. Sputtered coatings often use one, two or three silver layers and must be protected from humidity and contact. For this reason, they are often referred to as “soft coat.” Sputter coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to plastic and used in suspended films or retrofit window films.
Pyrolytic low-E coatings provide high solar heat gain coefficients

Pyrolytic low-E coatings provide high solar heat gain coefficients while still retaining much better R values than clear glass

It is also possible to get glass with the same low U values, but much higher SHGC  = 0.70.  These are much better for passive solar, but since the government incentives work against this glass, it is typically not available in the USA.  For these windows, the Low-E coating uses a very different “pyrolytic” technology that is applied on the outside of the inner pane.

–Pyrolytic low-E. A low- E coating which typically uses tin oxide with some additives deposited directly onto a glass surface while it is still hot. The result is a baked-on surface layer that is hard and durable and thus sometimes referred to as a “hard coat.” Pyrolytic coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to single-pane glass and separate storm windows.
Sputtered low-E coatings can be applied differently to produce moderate solar heat gain coefficients

Sputtered low-E coatings can be applied differently to produce moderate solar heat gain coefficients. This is what most American window manufacturers call high SHGC.

Since the truly high SHGC glass actually requires a totally different technology, many american glass companies that market high SHGC actually use the same Low-E coatings from their low SHGC glass, but applied slightly differently, such as on the inside of the inner pane, to slightly improve the SHGC.  If this is what you get, it is better than nothing, but be aware.

A 4th option is to go with clear glass windows.  With no low-E coatings to lower your SHGC, this is the highest SHGC you can get (~0.79).  However, I have seen various scholarly papers done by passive solar researchers (not glass companies) illustrating that while the solar heat gain is higher, it doesn’t make up for the higher nightly losses unless you have some good thermal shutters to apply whenever the sun isn’t shining.


When the government gets serious about Global Warming maybe they’ll fix the cooling bias in the regulations, but until then here’s where to go to get a Low-e windows with a high SHGC:  This list was current as of Jan 2012, but the market is flexible and may be out of date now.  For instance, I heard that several mainstream companies have started providing moderate SHGC glass in 2012.

–The Efficient Windows Collaborative provides a list of companies that sell high SHGC glass

Accurate Dorwin, Manitoba, Canada

Alpen Windows, Boulder, CO  Talk about a range of SHGC, but max out SGHC = 0.51, R=5.3

Fibertec Windows, Ontario, Canada   Listed on the Efficient Windows Collaborative site, but their own Website doesn’t really get into the topic.

Thermotech Windows, Ontario, Canada   Website really talks about high solar gain…  A wide range of glass including low-e up to SHGC=0.70, U-0.29

INLINE Fiberglass, Ltd., Ontario, Canada   Listed on the Efficient Windows Collaborative site, but didn’t include high SHGC in their default literature…

Vinyl Window Designs, Ontario, Canada

Marvin Windows & Doors, Warroad, MN (much nicer looking website than most, but you will need to call their technical support for architects and design professionals @ 1-800-346-3363 and ask to specify your own glass)

Some passive solar forums recommend Pilkington North America as a source for true high-SHGC glass.  The don’t sell windows themselves  but they provide high SHGC glass to other window manufacturers across North America.  There were two in my area, but both turned out to be small operations that couldn’t get it together enough to properly put a quote together for me.


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