Window options for passive solar home

Hello, Really appreciate this site. First time poster. We are replacing all windows and doors on our older passive solar cabin. We’re at 7000’ in CO with warm summer days and hot west afternoon sun that lasts about 3 months, about 2-4 weeks of that gets the house very hot. Winters are cold, our wood stove does fine, but windows are clearly cold when near them. After having been in the house for many years, we have seen the south windows with well designed overhangs work good for full sun in winter onto our concrete floor and no sun in summer to keep it cool. Our views are out of the west windows and so we have a lot of windows on that side, which heats the house too much in summer. We’d like to pick glass that will do a lot of the work rather than rely on window coverings, if possible, as we really like light/brightness/view.
Here’s what we’re considering:
1) No argon. One company says we can do it but it would deflect the glass, which to us sounds like more strain on the seals. It seems Capillary tubes are more recommended and LoE i89 gets us a good U value.
2) LoE 180 with LoE i89 on N, E, S sides for the higher SHGC and better heat trapping in winter. I’ve seen some recommendations for LoE 270 or 366 on E and N but others say 180 and I’m thinking 180 because the U value is nearly the same, sun isn’t an issue on these aspects, and the VT will be better.
3) LoE 270 with LoE i89 (or is 272 better?) on W side for the lower SHGC, albeit not as good as 366 but we are concerned of the even larger reduction in VT with 366 and if there will be a larger noticible difference between 180-270 vs 180-366 since we can see all aspects inside. In our climate, does anyone think 270 or 272 would be a good compromise of halving the SHGC we have now (clear double pane) for the less than 3 months we have hot sun coming in the west side vs almost 2/3 reduction with 366? It seems that the little bit of additional heat in the shoulder months that 270/272 would let in would be nice too (in winter the sun sets behind trees too early to give much heat).
Thanks for any help on this.
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1) Depending on where the IGU was assembled, using argon at 7000' might or might not be a viable option. If the window company that you are talking with uses Cardinal IGU's then Cardinal states that their seals are safe up to 5000' above the altitude where the IGU was built. Personally I like the idea of cap-tubes at 7000'.
2) While i89 does offer U factor improvement, it does so by reflecting room side radiant energy back into the room that would have otherwise escaped through the glass. The upside is that the room side air closer to the window feels warmer than it would feel without the coating. The downside is that some of that reflected heat would have been absorbed by the glass resulting in a higher glass temperature. With any surface 4 coating, the glass is colder than it would be without the coating and that colder glass results it a potential increase in glass surface condensation over glass that isn't surface 4 coated.
Leaving the potential condensation issue aside, I would never recommend using a surface 4 coating on west facing windows. If you think that your west facing rooms are uncomfortably hot with clear glass windows, using a coating that enhances solar heat gain by keeping that heat in the room and not letting it escape to the outdoors will introduce you to a different level of warm room. That's what i89 does and it's really good at it, and while LoE-180 does the same thing when installed on surface 3, i89 is even better at it on surface 4.
3) Since you are uncomfortably warm for just a few weeks a year, I would install L0E²-272 on windows facing west to help help limit solar heat gain during the summer months.
While LoE²-272 might not be as good as 180 for letting sun heat in during winter, and not as good as 366 in keeping sun heat out in summer, I would define it as a workable compromise for what you are wanting.. Using 272 will help in limiting the uncomfortable few weeks in the summer, but not overly affecting the VT as much as would using a low solar gain coating, such as 366.
As you noted, U factor difference isn't huge and while 366 will block more direct solar heat in summer, it also blocks it in winter, so while using 272 won't be quite as comfortable in summer as will 366, the slight increase in solar gain for 272 over 366 on your west facing windows can be a nice addition during the winter.
Super helpful response oberon476, thank you so much. The only thing I don’t quite understand is about not using the i89 on the west. Wouldn’t that reduce the U value of those windows from 0.23 to 0.29 (without argon)? The main floor is open concept. So if those west windows were the less efficient U value of 272 without i89 in the roughly 6 months of cold, would that affect the whole floor level by letting more heat escape? Maybe it’s a compromise of while not having the better U factor, 0.29 is still better than what we have now on the west for the roughly 6 months of cold and not cooking ourselves even worse for the 3 months of hot in summer?
You’d still go with 180 i89 on the N, E and S, correct?
With open concept and some of the windows on different aspects close to each other, it looks like we’ll be able to tell a difference between the 180 and 272. But do you think it’s worth it to use the 272 on the west for the better SHCG (rather than do 180 all around, which would still be better than what we have now)?
Actually I forgot, it was 2 companies that said they could do argon and they both use Cardinal. Andersen dealer said their windows (series e) are good up to 10k elevation because we are using tempered glass and Weathershield dealer (premium series) said as long as we use the high altitude glass (which we understand is actually just less argon) then we could use argon, regardless of tempering. Both said there’d be at least some pillow effect. We don’t want to risk worse seal failure, which with our harsh sun and wind is hard on windows already.
Luckily we’re in semi-arid CO but condensation may be an issue in the bathroom north window with the addition of i89 as it does fog up now. We need to do a better job of turning the exhaust fan on. The extra heat is just so nice.
Thanks again!
The downside of i89 is that it lowers the glass temperature of your windows and that means an increased chance for window condensation.
If you are in zone 6 or 7, I would recommend triple pane windows rather than dual pane with a surface 4 coating, however if your current windows consist of two clear lites without a LowE coating and you haven't had condensation issues in the past then I would be less concerned about using i89 because while the coating does lower glass temperature, it's still warmer than two sheets of clear glass would be.
My apologies for confusion on using i89 on west facing windows. For some reason I was thinking i89 with 180 facing west when I wrote about concerns for room overheating with that configuration. Using the 180 and i89 combination for west facing windows would be a very very bad idea but i89 behind a dual or triple layer coating should be okay since the outer coating is blocking a percentage of direct solar heat gain through the window.
I do think that 272 on the west facing windows offers the best combination of visible light transmittance with reasonable U and SHGC values for your situation. I also think that with proper overhangs, 180 would be my choice facing south.
mmeerryy,
I see a cat stepping on your keyboard. ;)
I live at 8,00o in a semi arid part of Colorado and wanted to be near PassivHaus level while retaining my views. Don't overthink this. It is unlikely that you will have a net gain of any useful heat during the winter without excess gain in the summer. Yes, I can certainly feel the winter sun on my east windows, but overall energy losses the rest of the day and night do not balance out. Even with the large overhang you have it is hard to make a case for high solar gain in that location. The winter sun will feel good, but that's about it.
Do go to ThinkAlpen and talk to them. I have had their windows for 11 years now and have no regrets. I missed out on the newest option of thin glass triple pane, but I am quite happy with the film type. The newer type have a different spacer technology that may help with the edge condensation I see in the bathroom, but bathrooms are an extreme situation.
Do not give up argon fill, the energy hit is too much. The little bladders Alpen uses to equalize the trip up in altitude are not that big a bother to deal with and all our windows are un stressed. Ignore the internet horrors about a house on the front range that had fun house glass after installing un bled triple panes.
As others have noted the interior i89 coating is not a good idea. I have no problem sitting next to my windows even at minus 10. General air sealing might be a more useful path to help cure and drafty feelings.
Stop worrying about the VT. I fussed a lot and the only way you can tell if a window has low VT is if you have an operable window that allows you to see full day vs. window day. For all our view windows, it is not a perceptible issue. I chose to go minimal SHGC and highest U value for all and not kid myself that winter sun was going to be useful. I believe I made the right choice.
Thanks both for your answers. I appreciate the different perspectives. Cost is a consideration so I’ll need to research what triple panes would add. Especially because our house is old school construction with an older passive solar design (overhangs, trombe wall, some passive air currents through the house, some concrete floor but that’s about it). So presumably it would take a bunch of retrofits to bring the house up to any kind of modern standard, therefore putting in the best/most expensive windows may not be feasible (or make sense?) for us. For example, it’s off-grid on a tiny PV system so we can’t run an active ventilation system without an upgrade to the PV system.
But for the sake of my understanding this more, does anyone have a good source of a retrofit guide for improving efficiency of an older home?
Barring a full undertaking and to keep costs in check, I guess we went into this project thinking the most we can do is a compromise of upgrading the windows to provide some additional summertime coolness, and wintertime heat retention. We’re in zone 4b. We are already compromising efficiency because we need aluminum clad for wildfire considerations and want to continue to have wood interiors due to the rustic nature of the house. The air infiltration is indeed the window units themselves and not the window installation so all of the new air infiltration numbers would be far superior to what we have. But sealing is important in general and we continue to make progress on that front.
Maybe we’re limiting ourselves in what we can achieve with double pane LoE coatings type windows or does what I’m saying sound reasonable?
Thank you again.
Joe Lstiburek once said something like "in the 1970's it was passive solar vs. super-insulation. Super-insulation won."
You want to make your house as tight and well-insulated as you can. To the extent the passive solar features contribute, they will work better in a well-insulated house.
mmeerryy,
Guiding a retro upgrade for energy is fraught with peril when done remotely. Nonetheless, I will ignore the warning angels and rush in.
To start, I am guessing you live in or near Las Animas county based on your elevation and the climate zone of 4B. I have only been to that part of the state during summer, so I can't judge your winter experience. The googles says the average is 54F annually, which doesn't provide much useful input. One source indicates an annual range of roughly 4F to 95F, but fails to note any elevation information. The average elevation for the county is approx 4000' despite the many mountains in the area. You say that your cabin is at 7,000', so despite the general 4B zone for the county, I would think using 5B as your zone category would be more appropriate. Any heat loss analysis will likely model better. I live 2000' higher than the nearest town and I find my elevation to be 10-15 degree cooler than the data provided for the town.
You describe the cabin as set up for older passive solar design standards. A bit late for DC's observations. I imagine that your glass to wall area ratio is higher than most cabins. Standard wall construction is a bit vague, but lets assume 2x6 with R-19 batts. Functional R value for a whole wall rating is probably R-15, possibly less depending on framing losses if you have a high wall/window ratio. A quick look at your wall to window glass heat losses may illuminate possible choices. For the moment I will ignore attic and ground losses.
An R-15 wall is the same as U=0.0666. One of your window choices seems to be U=0.29 without the i89 or ~R-3.45. This means the windows would lose heat more than 4 times faster than your walls. Cutting your window heat loss is probably more easily achieved than improving wall losses and you will feel much more comfortable sitting next to them.
You will have to do some measuring to figure out your current wall/glass ratio, but for arguments sake, lets say the house footprint is 28x40 with 8' interior wall height. Perimeter is 28+40+28+40 = 136' x 8' = 1088 s.f. A 70/30 split for wall to window means ~762 sf wall and ~326 sf window. For a winter temp of 5F and interior of 70F the delta T is 65. Heat loss in btu/hr is (area x U x deltaT)
Your proposed U=0.29 window option and (theoretical) existing wall of U=0.0666 would lose ~9500 btu/hr at 5F. (Hourly window loss -(326 x 0.29 x 65) ~6145) (Hourly wall loss - (762 x 0.0666 x 65) ~3299 btu.) With the U=0.23 windows (~4874 btu), you can shave that to ~8200 btu/hr. Either way, you can see the windows are a major component of heat loss relative to wall area during winter months. Cheaping out on the windows and searching for other places to save energy can be difficult. Depending on the structure, adding insulation in the attic might gain you savings and is usually a lower cost avenue. If you are not entirely on a slab there might be ways to stem losses at the foundation/sill level. Savings can be had by adding exterior insulation when re-siding, but there are many details around windows and doors to work out and execute. The details tend to be labor intensive and also complicate window installation.
To continue this example, a triple pane fixed unit with argon should be able to achieve U=0.15 (R-6.6), an operable unit U=0.18 (R-5.5) If half the windows are fixed triple pane with argon at U=0.15 and half operable triple with argon at U=0.18, it would yield a combined loss of 3496 btu/hr at delta T 65. About 28% better than your lower U=.23 option. You could seek to insulate the walls over more costly windows, but the windows are being replaced anyway, so why not take do the energy saving there. And be more comfortable.
The conundrum of all this calculating is that these values are for the coldest temperatures which for you may exist only two months a year. It takes a great deal more calculating to assess true yearly losses based on much more information than I have. Do remember that improved U value windows work the same when the air is hotter on either side. The coatings to reduce solar heat gain are on the outside because that's where the sun is. You might think of the i89 coating as an interior version of the outer coatings. I have a work around for the bathroom windows. I have a hot air fan aimed at an angle to the window to warm the room and reduce condensation around the frame. I only need it when very cold out.
The SHGC you choose should be based on the level of direct sunlight exposure. You might desire the extra heat of high SHGC glass in winter, but suffer wild overload much of the year.If you want to make your west wall of windows more summer tolerable, consider a fabric shade sail. Greenhouse suppliers might be able to guide you. The large overhang you have to the south might allow for a high SHGC window while the west would not. The trombe wall is a new wrinkle you mentioned. They are a whole 'nother ball of worms.
For me, personal comfort is a very high value item, perhaps less so for you. By selecting low U and low SHGC windows, I can sit anywhere in my house next to the windows and not bake or sense that vague chill of the crypt feeling I got from my previous homes' windows. (approx. U 0.34 Weathershields) I also no longer need to mop the windows each morning after lifting the "energy saving" shades we bought to make up for the poor energy efficiency. I of course designed for very well insulated walls and windows on my new house, but this is a retrofit not a new build you are asking about. Trying to justify higher cost windows on fuel cost savings alone will almost never work out. Just the same, like the old credit card commercial, "being comfy" - priceless.
On other fronts. Air sealing will help you. An exterior air feed for your fireplace will help you. More attic insulation, if possible, will help you. Each option will show the same reduced cost benefits as the daily temperatures rise. None of these will help with solar heat gain. Upgrading the wall insulation can help but light colored siding will too. Minimizing the window SHGC will provide the most relief and as you are planning on replacing windows. This is where I would look to spend the dollars first.
I would be very careful about the materials choices for the window frames. The Alpen window use a fiberglass pultrusion which is touted as having very similar thermal expansion to the glass units set into them. Now 10 years on, I can say that it must be true as no corner separations have occurred and the stucco to frame intersects are stable as well. We see a 30-40 degree shift in air temps most days outside of winter and the sun load is crazy at these altitudes. While building, I lived in a rental that had vinyl windows and the sun death they were suffering was quite extensive. They also did not air seal anything like my current windows.
I can't accurately address (without a lot of research) who is currently offering argon fill for high altitude windows. I can say that twelve years ago, three major manufacturers told me that they would not ship with argon over 5,000'. One told me that air was just fine and argon was over rated. Their best windows at the time were only U 0.26. Didn't go with them. Yes setting the Alpen bladders and capillary tubes can be a bit daunting at first, but after the third one it went quite well.
Any way, it is your money and your call as to where it is best spent. Hopefully, this foray into the basics of windows and wall heat loss will give you a sense of where to spend your dollars more effectively.
This is extremely helpful. I will be rereading it several times as I continue to research all of this further.
A question on installation- all the window companies I’ve talked to have instruction that we can use either insulation batting (as long as not over compressed) or non-expanding foam around the window units (along with the special tape and caulking). Any consensus on which is actually better?
Batting provides zero air sealing. If you're not controlling infiltration you're wasting your money with new windows.
Foam, without question.
Thanks, it seemed that would be the case but then I started thinking about what a pain it will be to remove the windows when it’s time to replace them in the future.
For window type, on the lower level we currently have mostly double hungs due to the rustic nature of the house, and so they don’t swing over the deck when open. I’ve looked at the air infiltration specs of double hungs vs casements/awnings and while double hungs are somewhat less tight, they are dramatically better than what we have now. Of course casements/awnings would be nicer for their even better tightness (especially on the windy west side) and less rails to block the views, but changing to them would mean redoing custom timber trim and having them swing over the deck when we open one of those windows about 5 months of the year. Neither of those problems are a deal breaker but wondering what your thoughts are on making the decision based on the air infiltration differences of those window types.
A related question is if there is such a thing as making the house too tight? Like I mentioned we have no active air ventilation system and have no plans to add one. Far and away it’s the leaky windows we have now that provide most of the fresh air we get into the house when they are closed. Would that little bit of extra air infiltration from new double hungs actually be a benefit in our situation?
When it's time to remove the windows, hold a hacksaw blade in your hand and go around the edge of the window. Takes about a minute.
I have to ask what is wrong with the current windows?
Assuming the frames are not rotten I see no reason for replacement.
If you spend a ton of money on replacement windows, I think you will be disappointed with the almost imperceptible comfort improvement. Like with most energy products you need a 50% change in the R or U value to make a perceivable difference
If you are somehow convinced the latest tech is that much better then simply replace the glass units at likely 25-40% of the cost.
The way I see it the passive solar house was a failed idea the few that claimed to work are really well insulated homes that happen to work despite south facing glass not because of it.
Walta
Thanks for the straight talk. After the input here and looking into it, I see the house is likely worse energy wise than even a conventional home built these days. But at this point we plan to stay in this home and definitely want to replace the windows for both aesthetic and energy/comfort reasons beyond the overheating and air infiltration I mentioned previously- blistering aluminum paint, actual dirt blows in with the air, can’t see out of them because of the glass haziness from busted seals, etc. We should be able to improve the U factor by about 42% with the specs oberon476 and I have been discussing and air infiltration by a whole bunch more than that so we are excited for the project for those reasons alone. We’ve considered the option of rehabbing them but have decided that doesn’t make sense for us.
Thanks for everyone’s ideas. I welcome more on all of it, including the DH and casement air infiltration and too tight questions above.
mmeerryy,
(some points not needed now, I see)
Aside from pointless ragging on your house design, I am not sure where Walta is getting his cost estimates from. A friend in town just recently paid $300 to have the (two pane) IGU replaced in just the bottom half of a double hung window. This was the low bid, the other two options came in at $375 or $410. His double hung is an aging PVC builder window that leaks air like a sieve, but at least the glass is meant to be replaceable. Some 20 years ago, I redid the porch windows of my own home with Pella Pro double hung units from HD. Fairly nice for the price with one flaw.
A roofer goofed when dropping materials into their dumpster and broke the glass in the lower half of one window. We both got a big surprise when he went to HD to order a replacement glass unit. He was told that the glass is glued in and he would need to replace the sash unit for something like $400 because it would be special order. I had paid barely $200 for the whole unit one year earlier. We ended up just buying a whole window and swiping the bottom sash. The rest of the window lived in my garage until I moved in case an upper sash got busted. Do you know how your window glass is set?
It would be a very unlikely circumstance that you could affordably upgrade just the glass in your current windows. Besides, as you have noted, the air infiltration is as much a problem as the cold glass. Leaving them leaking would severely undercut the whole exercise.
I will also strongly disagree with Walta's notion that the comfort upgrade of new windows would be imperceptible. In the same fixer-upper house I referenced earlier, we didn't get to replacing all the windows in the main house for five years while I re-wired, re-plumbed, re-plastered, etc. The original 1940 Sears windows leaked worse than a sieve. Without felt seals of any kind, the only option was to use a clear sealant goo to completely the window sashes every fall. It would peel out come spring. Point being that just stopping the air influx was a HUGE comfort upgrade without any R increase. The insulating shades we added helped more though the condensation issues were monstrous. The aforementioned Weathershield windows we finally put in were better still except for the fatal flaw caused by their aluminum cladding details. Still monstrous condensation, but at the perimeter of the glass. Another story for another time. Anyway, you will likely get more than a 50% R increase over your current windows and vastly less air leakage.
If you are concerned about the custom trim around some windows, you could ask your potential suppliers about flangeless installation. I didn't make note of the option first round as you said you were looking to upgrade fire resistance with new siding. New windows, flanged or flangeless, will need proper flashing or you will risk water intrusion from rain. I only get about 11" of precipitation where I am, but it is still important to flash properly. If you are needing to keep the trim, be sure to really think on the rain details. Alternatively, you could consider down sizing the opening to allow for flange installation, but this means work inside and outside that may create more problems.
In regard to the window install sealing; foam over batt definitely. DC is correct that a hacksaw blade or the giant size snap knives available will cut them free. Bigger issue would be any trims applied over the flanges or sealants around flangeless. I would worry about that in the (hopefully) distant future. I would suggest you use the non-urethane foams like DAP. I used the low expansion extensively and it can get very messy. Also, urethane foam is water activated and I discovered that hidden condensation will make it go nuts. I had to do my window frame sealing in November, which set me up for the surprise. Cleaning up over foam goo with acetone is problematic on many finishes. DAP is water clean up.
In regard to being "too tight" a house, some myths never die. You need controlled ventilation, not free will air offerings. There are energy efficient thru wall units that could be a good fit for your needs, but that is a long post in itself. The woodstove will not draft well if you do achieve good sealing everywhere. Crack a window or put in an exterior air feed since you seem to rely on it for winter heat. Running a stove fan hood can cause backflow down the flue and cause an inactive fireplace to smell. The same with a dryer running. Not sure that you will find your house quite that tight with the window upgrade. Personal habits and hobbies might be a better place to review if you don't install a controlled ventilation source. Certain cooking styles can choke up a house with smells and both aquarium and plant fanatics will tend to keep interior humidity higher than normal. You can wait if budget won't allow windows and ventilation to be done at one go.