Cathedral Roof & Insulation Question(s)
Hello
I was wondering if I could get some input on my insulation “plan” that may have gone sideways.
Here is the info:
I have a small older farmhouse 1 3/4 story approximately 1000 sq ft that has a roof sheathed with boards.
Climate zone 7.
House is stripped to the studs including the ceiling which was ventilated loose packed with cellulose (and mouse poop).
House has now been completely double studded including the ceiling.
Walls are 10″ cavities and ceiling 8″ cavities (no room to go deeper)
Walls are going to be dense packed with Cellulose.
ERV will be installed.
All ceiling lights are flush mount LED with vapour shields.
This is where it may have gone sideways with the roof.
I sprayed roughly 1/2″ (some spots a little thinner some thicker) of closed cell spray foam on the inside of the ceiling.
The plan was;
Dense pack cellulose in the roof cavity and have conditioned space behind the knee walls.
The roof is currently 2 layers of shingles which are going to be stripped off and I was going to install a steel roof.
I was hoping to insulate from the outside as well to get to my R60 goal.
Issues to be resolved:
I have been reading that Dense packing Cellulose in a roof is bad without some sort of ventilation channel and that sandwiching ISO board or other foam board on the outside decking will increase the moisture issues in conjunction with the spray foam inside.
I have reviewed the insulation of cathedral ceilings somewhat after the fact and now I am concerned that I’ve right buggered this up with the closed cell foam.
How do I get this to work without breaking the bank? Advice is greatly appreciated.
Might have to eat some humble pie now.
Marc
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Replies
Have you read this article?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Getting away from the plans in this article is a risky game with high stakes.
Walta
1/2" of closed cell foam actually isn't a problem, as it's somewhat vapor permeable at that thickness. But the cellulose definitely needs ventilation--as required by codes and conventional building science knowledge. If the roof surface will be redone, you could vent above the sheathing, or you can add enough foam above the roof that it's at least 60% of the total R-value. In your case, if you have 8" of cellulose at about R-29, you would need at least R-43 in the foam layer. Because this is for long-term resistance to moisture damage, I don't use the values for new or slightly aged polyiso; I use R-5, so you would need at least 8" of polyiso over the roof. Or spray foam the interior with closed cell foam.