Increasing Insulation R-Value in Cathedral Ceiling
Good afternoon,
Recently purchased a mid-70’s Viceroy kit cottage north of Parry Sound, ON (cz 5). It’s a basic cottage with a cathedral ceiling. The previous owner added a new metal roof added couple years back and had the cathedral ceiling spray foamed. The original roof structure was 2×6 and it looks like 2” venting channels were put in each joist bay and the ceiling was built down with a 2×3 to get roughly 5 +/- inches of spray foam. It was then strapped and ½ plywood screwed up.
I would like to increase the insulation R-value to get it closer cz 5 minimum R-values. With the new steel roof adding insulation from the inside is my only real practical choice. Headroom near the peak of the cottage is not an issue; however, at the exterior walls there is only 3” of play to add extra insulation.
I have read the various cathedral blogs and articles on the site (great information) along with the various cathedral questions and conversations under Q&A. My likely similar but slightly different question to the group is –
Where I have headroom could I “frame down” to add additional roxul (3.5 or 5.5”) and then add 2-3” rigid insulation that is vapor permeable as my air barrier before strapping and putting up T&G pine? Where headroom is limited I would just add the rigid insulation. The rigid insulation would be taped and foam sealed around the edges for an air tight fit. I am assuming because of the closed cell spray foam I would want any interior moisture to dry inwards through the rigid foam or do I need to worry about permeability and can use something a little tighter around 0.5 perm?
I can upload a couple pics when I figure it out if that helps.
Thank you for your input.
Nick
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Replies
Hi Nick,
Just want to confirm that the existing foam is closed-cell (that is, rock hard).
Hi Steve,
Thanks for the reply. It is closed-cell foam. The previous owner gave me a copy of the quote and invoice; and after Looking at the foam it is closed-cell - it’s “rock hard”.
Thanks,
Nick
Hi Steve,
It sounds like you have 5.5" of closed cell foam which translates roughly to R-33. Also, you mention there is a plywood layer below this, correct? The ccsf will act as a pretty good air barrier to begin with and if you wanted to make the air barrier more robust, why not just tape the seams of the plywood?
I would use a vapor open fluffy insulation if you are going to frame down below this. The ccsf will be sufficient to prevent condensation (and meets current code in your CZ) even if you add another say 5.5" of fluffy for an additional R-23 or so. I would advise against putting a closed foam insulation board below this assembly as it will create a 'vapor sandwich'. You currently have a good 'dry to the inside' strategy and I would keep it as such. This would also likely be the most cost effective option.
Let me know if I'm not envisioning the assembly correctly...
Josh
Josh,
Thanks for your reply. You are envisioning the assembly correct. I was worried about the vapour sandwich as well, that was why I had initially thought using a permeable rigid foam and removing the need to put the plywood back up could work; but your comments and ease of completing makes more sense!
Only issue I have to wrap my head around, I cannot build down near the exterior walls with roxul batts (cover top of door/window frames) however, could I use 2” comfortboard 80 there covered with the plywood and seems taped?
Thanks,
Nick
Hi Nick,
(Sorry I got your name wrong before!)
Glad I could be helpful. In regards to your subsequent Q: Welcome to the world of retrofits... I always tell folks to not let the perfect be the enemy of the good. In your case I can see how one would obviously not want to block windows (or move them). One just has to do the best one can and in this case that may be what you are describing. I would pay attention to keeping the plywood air barrier continuous. If it is below the ccsf, but above the newly installed fluffy, I'm not sure how you make the transition to comfortboard with the ply outside of it. I would continue the plywood to the wall and tape it. Then you could attach the comfortboard below it. You may have to adjust your framing to a taper at the walls in order to accommodate this and allow for attachment of your finishes. The comfortboard would then get installed between the new framing.
Make sense?
Josh
Hi Josh,
No worries. Attached is a pic of what the interior ceiling structure looks like. I was thinking near the exterior walls strapping 2x3 and then adding comfortboard and putting back up the 1/2 plywood - keeping me at that 3" ceiling height. However, to your point Josh, maybe going that extra little depth and I can just add 3.5" roxul batts and then plywood and make it that much easier.
Taping the seams and chalking the seems enough or should I paint the plywood as well before putting up the T&G?
Thanks for the great input!
Nick