Cooling an Attic with Soffit Vents
How much of a difference will additional soffit vents make for cooling our attic? We live in a 1950’s 1960 sq. ft. 2 story house in Saskatoon, Canada which has a cold semi-arid climate with warm summers (getting hotter) and cold winters. We have no AC but have successfully (until now) used closed blinds during the day and open windows in the evening to cool our house.
Recently we have had unusually hot weather spells and have not been able to rely on cool evenings to cool our house. I poked my head in the attic and it was hot and there did not seem to be much air movement. Ten years ago, I air sealed and insulated the attic with 22 inches of loose fill pink fiberglass. I also added draft stops and air chutes in each rafter bay. Unfortunately, I also discovered that my vented aluminum soffits were covering the original solid wood soffits. The only attic ventilation was coming from two small gable vents and four roof vents. I added 4 soffit vents to increase attic ventilation and prevent ice dams until all the soffits could be renovated in the future. We have not had any ice dams since these four soffit vents were installed. I had not been concerned about completing this job until now. Removing the solid wood soffits on a 2 story house and replacing them with vented soffits would come at a price. I would be willing to pay it if it would significantly help to keep our house cooler, but I’m concerned that this might be money wasted. Any advice as to if this ventilated soffit reno would help with our overall house cooling would be appreciated.
As added information our Blower door results after the attic air sealing and other improvements was 1000CFM @ 50 Pa. To the best of my knowledge our wall construction is stucco, tar paper, 1 ½ inches EPS, ½” ship lathe, 3 ½ inches mineral wool, rock board and three skims of plaster.
Thank you in advance for your help,
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> additional soffit vents make for cooling our attic?
The complicated answer is it depends on how much more airflow you are going to create. The short answer is "not a big effect on whole house heat gain".
Hi Jon, Thank you for your comment. I think you may be right.
Do you have a ridge vent or just the two gable vents? For best airflow, you want soffit vents and a ridge vent. Venting needs an "in" (soffit vents) and an "out" (usually a ridge vent). If you only have one or the other, your attic ventilation will be severely limited.
If you have a "regular" attic, entirely ABOVE your living space, attic ventilation will help, but probably won't make a huge difference -- especially if you've done a good job with your air sealing and insulating project on the attic floor. If your attic is AROUND the living space (cape cod style), then the walls can be a big source of heat gain on the upper level of the home (I have that problem in my own home, which I've been gradually working to correct). In a home with walls between living and attic spaces, it can help to put "exterior" (attic side) rigid foam up. Another thing to look for is any open stud bays with batts in them -- if there is no attic-side air barrier, the batts performance will be reduced. To correct open stud bays like that, use rigid foam and get both additional continuous insulation AND an air barrier. That's what I've been doing in my own home -- I swap out the old mouse-tunneled fiberglass batts with new mineral wool batts (which upgrades me from R11 to R15), then I put a few inches of polyiso on the attic side right over the studs. The end result is very noticeble. Note that I also wrap the polyiso over the top plate, since the top plate is a weak spot in the insulation and is noticeably warmer than the rest of the wall on the interior side.
Hi Bill, Thank you for your reply. For venting out I have four roof vents near the point or ridge of the roof as well as the two gable vents and for airflow in I have four soffit vents. Our attic is entirely above the living space. I did a pretty good job of air sealing all the fixture penetrations in the attic floor (using the advice of GBA!) but didn't spray foam the whole floor.
It's not usually necassary to spray foam the attic floor, you can do a good job using "traditional" methods, which is probably what you've already done.
I would consider adding a ridge vent, which may help. Another thing to check is to make sure any existing roof vents aren't clogged with debris -- I've seen tree fuzzies and other stuff clog up insect screens on vents many times.
With 22 inches of insulation on the attic floor, you're not getting much heat flow down into the house. I doubt that opening the soffit vents would have much impact on your interior comfort. With the house as you describe it,, most of your heat load is probably through the walls, possibly through the windows. Your wall system seems to be close to R-21 or so - not bad. Do you know your window specs? Adding shading for south and west facing windows might help significantly. Possibly even some sort of lattice shading for the west wall of the house, if it is currently very exposed to low-angle afternoon sun.
How strong is the wind at night in your area? Natural passive ventilation can be much less effective without some wind to move the air. Using window fans at night can help to move much more air and remove the day's heat from the space.
Thank you Peter for your analysis. You have me convinced that the attic ventilation (while important to have) is likely not the main factor contributing to overheating of our house during hot spells.
Our house was renovated prior to our purchasing it and most of our windows are mid grade Dual-pane Ply-Gem windows. In our recently renovated kitchen we have High end Triple-pane Loewen windows. We have never experienced frost on the Loewen windows in winter but have on the coldest days with the Ply-Gem.
Our West side of the house is shaded by the neighboring house but we have a mostly exposed South side. So far we have been using interior blinds against the South Sun but I agree that exterior shading would be more beneficial and I will consider it.
We do not have particularly strong evening winds and do use two fans when needed. I guess the problem during those newly experienced hot evenings is that there is no cool air to exchange with.
Once again - thank you for your comments. Keeping the hot air out with more shading and possibly better South side windows may be a better investment.