More Insulation vs. More Airflow
Better insulation or better airflow in cathedral ceiling?
Hello,
Looking for some advice about what to do with cathedral ceiling insulation.
I’ve read the article and it got me thinking, which is better: more insulation or more airflow?
We live in Canada, climate region 5. We have a dormer window above which there is a cathedral ceiling. It’s about a 5’ x 6’ zone that is constructed out of 2x10s. It’s also vented with a 2 in soffit vent and 1 in baffles will be installed throughout the whole section.
We had moisture issues before (hence the addition of the soffit vent and baffles) and don’t want to use polyurethane spray foam.
So our options are to use only fiberglass batting and/or polyiso sheets.
Question is: our baffles are 1 in tall and provide 20.3 sq in of airflow. We could use those and have a combination of batting and polyiso to get to R42, but would it be better to have more airflow instead?
Thanks
Pascale
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Replies
Based on Building Science Corp reports, I prioritize the depth of the vent channel. I typically specify 1 1/2" Accuvent. 1" meets code but 1 1/2" allows significantly more air flow.
Pascale,
That photo sure doesn't help! What is going on there with the truss flying through the middle of the window - and is there a header over that opening? I'm probably just not understanding what I'm looking at.
Canadian building codes require a 63mm (2.5") clearance between the underside of the sheathing and the insulation below, so the typical detail often shown on GBA with 1" baffles doesn't meet them. Like Michael, I'd go for prioritizing air-flow over R-value.
(Edited based on DC's comment)
Since the rafter is sitting on the top of the window frame furring doesn't look possible.
I'm wondering how that rafter is supported.
Hello again,
Thank you for your replies.
We have updated this question with more details and photos here: https://www.greenbuildingadvisor.com/question/dormer-window-insulation-vs-ventilation
Your help would be much appreciated.