Avoid higher SHGC windows?
Hello,
First, allow me to say how much I’ve enjoyed reading and learning from GBA- great resource. We’re in the process of designing a pretty good house in Climate Zone 6A (SW WI) that will take advantage of some southern views of our property. We’ll face 17 degrees east of south and will have appropriately sized overhangs. Currently, I estimate the southern glass area will be 10.5% of the floor area. I’d been leaning toward some windows in the .52 to .61 SHGC range until I realized how high the percentage of southern glass would be. The west side will have only one small window while the north and east sides will have average to lower amounts of glass and some covered porch space. No extra thermal mass is planned. Despite reading about every window article and comment on GBA I could still use some advice since it seems this topic has been a moving target over the years and I was unsuccessful in getting RESFEN to function. Not wanting to reduce our window area, should we be looking for lower SHGC windows on the south side to avoid overheating? If so, what value would be recommended?
Thank you for your help.
Matt
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Replies
Matt,
I'm not a big fan of using rules of thumb developed in the 1970s for passive solar design, but -- for what it's worth -- most passive solar design guides said that south-facing window glazing area should be no more than 6% of the floor area unless the designer includes thermal mass. Some design guides went as high as 7% or even 8%, but never 10%.
If you are boosting the south glazing ratio out of a belief that these south-facing windows will act as heating appliances, the idea is misguided. What you want is a glazing ratio that is aesthetically pleasing -- enough to provide a view without (a) imposing too much glare, (b) overheating the house on sunny days, or (c) imposing an energy penalty at night and during cloudy weather.
For more on why it's a good idea to limit south-facing glazing to levels chosen for aesthetic reasons (rather than the mistaken idea that south-facing windows are heating appliances), see this article: Reassessing Passive Solar Design Principles.
Matt: For what it's worth, here's my experience. We're in zone 6 Maine. Our South facing glass is about 10% of floor area, with decent overhangs. SHGC is .49. Triple pane windows, around
U .11. We've never experienced overheating. On a really sunny day this time of year, it gets comfortably warm, but never approaches too warm. I'd make my decision based on the views and other esthetic concerns.
Thank you, Martin. I've picked up some great information from that article and others. While we like the idea of gaining some heat in the winter, the main idea with the southern windows is to afford great views from the southern half of the house. Since we're above what would've been recommended for glazing area in a passive home, my main question is if lower SHGC values would be a good alternative if we wanted to preserve the glazing area? If so, what value range? That question comes from reading this article: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/cost-effective-passive-solar-design. I will also re-think how we could reduce the area and still enjoy the view.
Thanks again,
Matt
Matt, although rules of thumb can be helpful, when you have a somewhat unusual situation such as yours, it's a good idea to invest in an energy model. The results may not be perfectly accurate, but within reason--enough to allow you to see the results of different specifications and details, on everything from an hourly to annual basis. It sounds like low SHGC, low-U glazing would be best for you, but it depends on many factors that an energy model would take into account.
When choosing glazing you also want to look at VT to make sure the glazing does not have an objectionable tint, something energy models do not consider.
Thank you, Michael and Stephen. Much appreciated insight!
I wouldn't worry about overheating. My house in central CO has S glass at 9% of floor area. We get a lot more sun than you do and overheating hasn't been a problem. I used high SHGC low E double pane on the south side, low E triple pane on the other sides. Fairly inexpensive Milgard. From what I recall, from reading studies a couple of years ago, high SHGC makes sense in most of the US on the south side. I modeled my house with energy plus and the S windows are a big help to my energy budget.
I chuckled when I saw Martin's comments this morning, knowing he isn't a big fan of passive solar. I live in a passive solar (sun tempered) home. It also has geothermal (ground source heat pump), radiant heated floors, and was built using SIPs. All things Martin discourages folks from using. It's a wonder the house hasn't imploded yet.
Seriously though, there are reasons to think again about passive solar, some that might not be so obvious. For instance:
1. I looked at some "days cloudy" maps and SW Wisconsin seems to be about the same as Central PA where I live, not exactly stellar.
2. I paid a pretty steep upcharge for the special LoĒ-180 glass on the south side of my home. Maybe five years later that isn't so much the case?
3. I wasn't able to use certain finishes and materials in my home due to fears of fading and discoloration.
4. Insects love warm south facing walls, particularly in the fall.
5. Large expanses of glass are great for bird watching, but the birds also fly into them.
6. The amount of daylight you get in a passive solar home shows every little bit of dirt.
7. There are no tax credits or rebates for special glass.
That said, it is an unbelievably cool thing to get all of your heat from the sun on a day when the temp is in the single digits.
If you are still game, yes find someone to do the energy calculations that Michael Maines suggest. I cracked open my copy of Manual J when I saw your question, particularly after I noted that your home will be shifted "17 degrees east of south". I was curious to see what table addressed such a shift in orientation, seems to be table 3D-2, "PSF Values for Selected Latitudes", under "Default Cooling HTM for NFRC Rated Fenestration Windows, Glass Doors and Skylights". Sounds intense doesn't it? Unfortunately I think it's the level of detail that needs to be gone through for your scenario.
So there is enough information in Manual J to make some pretty good calculations of what the solar gain would actually be in the winter, and more importantly on days in the fall and spring when your overhangs aren't as effective at shading and you could get overheating. It may need to be done by hand, but I am confident it can be accomplished. (I reran all the calculations for my home by hand, using an ASHRAE spreadsheet as the template.)
BTW, a bit of warm morning sun is better than the same in the afternoon. Did I mention that I also have a large expanse of western glass for my views? Yet another "horrible mistake", ha! (I have motorized and regular blinds on those windows, it is still heat load though.)
In summary, I think what you are asking can be done, but it will take very careful execution. You have definitely come to the right place for help with that. Although I tease, what Martin has built here with Green Building Advisor is an unsurpassed compendium of information and discussion.
Good to hear this may not be impossible, based on Brad and Stephen's experiences. Andrew- thank you for all the food for thought. It sounds like I need to dedicate some (more) time and dig into the models. I second your nod to the great value of GBA as well as the willingness of members to share their knowledge.