Air Sealing & Insulating Attic
I’m air sealing & insulating my attic. I believe I have a reasonable plan but have a few nagging questions that I hope can be put to rest.
The details are lengthy – I apologize in advance. We own a 1950’s 2 story house with 1000 sq ft attic located in Saskatoon, Saskatchewan, Canada (Continental climate – very cold winters & hot summers). We do not use air conditioning in the summer and do not humidify in winter. Under the attic 2×6 floor joists are 3/4 ” ship lap, Kraft paper, 3/8″ rock lath and finally 3 skim coats of plaster. Current ventilation consists of 4 soffit vents 1 sq ft each, 2 end gable vents 5 sq ft each and 4 roof vents 0.4 sq ft each (areas quoted do not include area losses due to screens & louvers). Since I will be crawling on my belly for some of this work I removed the old dusty insulation & vacuumed out the attic.
The plan is to air seal all attic penetrations, top plates, etc with caulk &/or foam, seal the chimney chase and install an air tight insulated attic hatch. I also plan to install air chutes in each rafter bay along with home made wind stops made out of rigid foam sealed in place with foam. I plan to insulate with approximately 18″ of loose fill cellulose. Finally I will install continuous soffit vents.
In our region it is a common practice to include a 6 mil poly vapor barrier on the warm side of the insulation. My plan does not include one because it would be a pain to install & from my reading at GBA it doesn’t seem necessary (especially if I increase my ventilation). Is this a correct assumption or should I (because of our very cold climate) attempt to add a poly vapor barrier or even seal the joints between the ship lap. Secondly, I am assuming that the more ventilation from soffit vents the better – can I add to much? Although there is nothing I can do – should I be concerned that there will be considerably less insulation at the rafter bay ends due to the roof pitch? Are there any other details I have missed or should consider?
Thank you in advance.
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Replies
Colin,
Your plan sounds good. I'm glad to see that you understand and value air sealing.
1. There is no need to install polyethylene. Most attic moisture problems are due to air leakage, not vapor diffusion. If it helps you sleep better at night, you can always paint your ceiling with vapor-retarder paint -- although the multiple layers of paint in older homes are usually pretty close in permeance to vapor-retarder paint anyway.
2. Don't obsess about ventilation or the total areas of your vents. Once you have addressed air leakage, there really is little need for ventilation. Attics with a wide range of ventilation rates perform very well, as long as the ceiling plane has been air sealed.
Colin,
I agree with Martin about not needing a vapor barrier if the attic floor is properly air-sealed.
But I disagree about the value of attic ventilation. Right now, you have far too much exhaust ventilation for an unsealed attic floor, since it will create a negative attic pressure which will increase leakage from the conditioned space.
However, once the attic floor is sealed, this imbalance will be less of a problem. But I would recommend eliminating the gable and roof vents (they both create uneven vent flow and can introduce rain and snow) and installing balanced soffit and ridge vents - and the ridge vents must have external wind baffles (such as AirVent Shingle Vent II or Lomanco OR-4).
Good roof venting requires a 1:150 vent to attic area ratio. For your 1000 sf attic, you would need about 6.7 sf of net free venting, split evenly between soffit and ridge. A soffit strip vent offers 9 square inches per linear foot of net free venting, and a good ridge vent offers 18 square inches per foot. So you would need about 27' of soffit vent per side and about 27' of ridge vent.
Roof venting:
- removes extraneous moisture
- maintains a cold roof in winter, thereby eliminating ice dams
- maintains a somewhat cooler roofing, sheathing and attic in summer, thereby reducing cooling loads and extending shingle life (the darker the roofing, the more valuable the summertime venting)
- expedites drying of roof sheathing and framing if it should get wet, thereby making the assembly more durable and resilient