How to reduce summer heat highs with insulation and ventilation in a conditioned attic with 2×4 roof rafters?
Hi there,
I live in Vancouver (zone 4C, I guess) and am renovating the attic of my 70-year old 2-and-a-half story house to make it a more livable conditioned space. The roof is an open gable design with each roof section roughly 20 by 30 feet. The roof rafters are 2×4 (actually 2 inches by 4 inches) at 16 OC. There is some very old (not vermiculite) batt insulation between the rafters of the sloped ceiling with signs of mold/discoloration on the roof sheathing (no rot is currently visible). The rafters behind the knee wall are uninsulated and have no signs of mold. The roof sheathing is 1 1/4 wood planks with 3 layers of shingles (lazy previous owners, I guess).
The problem is that the attic is rather hot in the summer (cold in winter has not been a big issue).
I have read the various posts here about spray foam (not interested due to the high embodied GWP of currently available blowing agents), cathedral insulation, cut-and-cobble, etc., and am still uncertain about the best approach. From the reading I have done here (and elsewhere), my thoughts for reducing the summer heat peaks are as follows:
1. Air quality
a. Hire someone to fog the attic with mold control stuff.
2. Ventilation:
a. Install ventilation baffle (at least 1″, maybe more) from soffit vents to peak.
b. Install ridge vent (none present currently)
c. Install whole house fan into rafter tie space
3. Insulation:
a. Install mineral wool into remaining 2-3″ space between rafters (not enough head room in attic to scab additional framing)
b. Install 1″ polyiso sheets below rafter. Air seal properly.
c. Drywall on top of polyiso.
Will this approach lead to joyous comfort or rotten rafters? Would polyiso cut-and-cobble give better performance than the mineral wool?
Thanks,
John
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Replies
John,
1. It sounds like you are describing a Cape style home, with living space on the 2nd floor. I'm assuming that the 2nd floor includes kneewalls, a sloped ceiling section, and a level ceiling section in the center with an unconditioned attic above the level ceiling. Is this correct?
2. If I'm correct, the best approach to your problems, by far, is to install one or more layers of thick rigid foam above the roof sheathing, followed by a second layer of roof sheathing and new roofing. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.
3. For more information on different ways to handle a Cape-style home, see Two ways to insulate attic kneewalls.
4. Your plan to "fog the attic with mold control stuff" should be scrapped. I've never heard of this "stuff," but it sounds nasty. The best way to avoid mold is to provide dry conditions in you house. That means installing enough exterior rigid foam to keep your roof sheathing above the dew point.
5. If you can come up with a way to adequately insulate your roof, you may not need a whole-house fan.
6. Your current plan does not include enough R-value to meet minimum code requirements. You need to come up with a plan that results in an R-38 assembly, if possible.
Hi Martin,
Thanks for the reply. Yes, you are right that the house is a Cape style with the knee wall, sloped ceiling and level ceiling in the center. The unconditioned attic you note is tiny, maybe 2' high and 4' wide, but it is there.
I appreciate that insulating the roof sheathing would be the best approach but the roof shingles still have 5 years of life left so sadly that is not an option now. Is there a next best solution that would perform moderately well until we replace the roof and complete the insulation with rigid on top of the sheathing?
Thanks again,
John