windows in maine
Hello, i am designing a house and its time to choose windows. I inherited 4 4’x8′ plate steel plate glass windows and I would like to use them on this build. The house is 1000 square feet, 2/12, single pitch shed roof with the gable end facing south. I have a 2 foot overhang and deciduous trees a good ways off that should block summer sun and allow sun to come through in winter. I should get r-40 in the walls and an r-50 roof or so and it will be tight. I am shooting for 10% of my livable space footage in the south wall glazing so, about 90 square feet. I am considering vertical installation of the plate glass, side by side. I’m embarrassed, but I don’t know if the windows are low E (deep in storage at this point). The question is…should I be using low e windows on the south side in this situation? Also, should I use low E on the other walls? I am in mid-coast Maine, zone 6. Thanks!
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Replies
Stephenr,
If you are still at the design stage I'd urge you to increase the pitch of your roof to 3/12. That removes many restrictions on how you can roof the house, and effectively vent it.
It would be helpful to know more about your location, to be able to get more data on cloud cover and solar gain at your location. There is great data available for your location on weatherspark.com (see attached examples). It appears that you will get mostly or fully sunny skies about 40% of the time mid-winter, not bad but not ideal.
For solar gain from south-facing windows, as you know, they need to be mostly unobstructed mid-winter when the track of the sun is lower over the southern horizon. Deciduous trees can potentially block a fair percentage of that solar gain. Estimate that amount of shading by looking at tree trunks and branches considering their distance from the windows. I’m assuming there are no buildings, hills or other obstructions that are tall enough to block the sun very much from 9 or 10AM to 2 or 3PM on December 21st, when the sun is lowest near the horizon midwinter.
For your location, the sun will be only 22 or 23 degrees up from the horizon (ground) at solar noon on Dec 21, and lower than that at 10AM or 2PM. That’s only about a quarter of the way up from the horizon to overhead at noon. Much lower than people realize. (See: https://gml.noaa.gov/grad/solcalc/azel.html
Solar panels on your roof will do better than south-facing windows near ground level. Look it up for your location to analyze how much shading would occur on sunny December days. Regardless, unobstructed south facing windows typically result in three times as much mid-winter solar gain as east and west facing windows, and about 15 times as much solar gain as north facing windows that get only indirect solar radiation. So more glazing on the south, and less in other directions, is helpful for exploiting free wintertime heating from the sun. (With the sun almost overhead at solar noon in June-July, south-facing windows gets far less solar gain than east and west windows, so plan on shading east and west sides mid-summer. And north-facing windows get almost no direct solar gain anytime of the year for your location.)
Solar-tempered buildings will get enough solar gains through south-facing windows to heat the home’s interior from late morning to evening, about a third of the day. Your well-insulated home would do well in harvesting free heating from solar gains. If you intend to have a passive solar home that provides the majority of winter heating by exploiting south-facing solar gains, the solar gains need to be tempered by thermal mass, which would stabilize interior temperatures. Otherwise, your home’s interior gets too warm in the afternoon, and too cold by dawn. For example, a (polished or tiled) 4” concrete slab floor would absorb heat during the afternoons when the interior air temp is warmer than the slab, and radiate heat back into the interior overnight when the room is cooler than the slab. Stone, brick, water containers or other thermal mass can work too.
To really tune your home to work ideally, you need to calculate solar gains vs. building envelope heat losses on fully sunny days, average days and overcast days for January, your coldest month. For more data on climate and solar heat potential, see:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
or tips for effective passive solar design:
https://www.greenbuildingadvisor.com/article/cost-effective-passive-solar-design
There is also a fair amount of misinformation on GBA by those who don’t understand how to design buildings to exploit solar gains effectively, or how climate differences impact solar gains.
It sounds to me like you want to design and build a passive solar home.
For the last 50 years people have been mostly unsuccessful pursuing this dream. At this point the numbers say build a tight well-insulated envelope with the fewest and smallest windows you are comfortable with, it will cost less to build and heat while being more comfortable.
The chances of a random used window happening to have the optimal U value and solar heat gain coefficient are very low.
Walta
Thanks for the kind replies. With regards to Malcolms' feedback...I like a 2/12 pitch for a shed roof because of the aesthetic ( closer to flat in the modern style) and the smaller volume which means less need for heat. The roof will be metal (non standing seam), as it is also a water catchment for my water (no well, no traditional septic). I hadn't yet considered the roof venting and am glad that you offered that observation. I hope to vault it and will, of course, need to vent it. I have solar aspirations (non PV) and was imagining the 2/12 as being suitable for this..
And Robert, here is a more detailed description of the building site...Its on 12 acres of forest in coastal Maine. There is a long, 400 foot long ledge that runs in a north-south direction through the property. The ledge is essentially a very long eight foot wall. 2oo feet from the road, there is a break in the wall and it opens up into a ledgy boulder field with no trees. I am placing my house in this opening. The bottom floor of the house (the lower step of the wall ledge) is on grade and I plan to do a slab on grade with radiant heat here ( possibly using solar collected from the roof). The second floor will be twice the size of the first (it will rest on the first floor and the top of the step ledge) and almost all of the glazing will be on the second floor, which will be the living space. Sadly, the slab will not receive any direct sunlight and the first floor will function more like a basement. The forest is thick but will be thinned a bit by me (to manage, not clear) in order to open up passive and active solar options and remove a couple of trees that could fall on the house. We get nor'easters here . It seems that there will be enough (more than enough, its a mature forest) shading from leafy trees on the east and west sides during the summer months. Luckily, the forest is less thick on the south side and with some clever thinning, I think I can have that sweet spot of appropriate shade in the summer and direct sun in the winter. I could also raise the height of the first floor to make it more solar friendly. Thanks for the detailed post and the links. I would like to get this right.
stephenr,
If you stick with a 2/12 roof, make sure you work out a viable venting strategy before you go too far. With a low slope that typically involves a deeper vent channel, which then means a deeper structure to get adequate insulation. https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
Surface area, not volume, is what dictates energy exchange with the environment. This point may also say something about designing a second story with a significant overhang above the first.
There's good articles here discussing window specs (like SHGC - solar heat gain coefficient).
Could you elaborate on the energy exchange of a house that is 2xs over 1? I think i can get r40 in the floor of the second floor section that overhangs the first. Thanks.
Yeah. Sorry my comment was really just because you mentioned volume in reference to roof pitch, but it seemed to apply to another aspect of your design.
There may be many reasons you would want to design the second story the way you have, I just felt it worth mentioning that from a purely energy efficiency perspective, shapes that depart drastically from a simple cube become less efficient due to having greater surface area (energy losses/gains occur through exterior surfaces.) A sphere is the most efficient shape in terms of surface area to volume ratio. A cube is pretty good. Rectangle slightly less good. And a house with lots of bump-outs and angles less good still. This doesn't mean to never use a bump out or ell, etc., but surface area is being added at a higher rate than volume.
I'll agree with everything Tyler said, and add that every time you change plane you have to worry about making sure all of the layers of your building envelope transitioning continuously. That adds to the cost of construction and makes for a less reliable building.