Upgrade to vented cathedral ceiling
Now that we’ve reached the conclusion that (in the current market) it will be more affordable to buy a resale and remodel/upgrade than to custom build and “do it right,” I’m reading the GBA advice from a different angle.
My current question deals with upgrading a vented cathedral roof assembly, a popular bit of poor construction judgment (in hindsight) that flourished here from about 1990 – 2003. My wife likes one of these houses: 4-sided brick, hip roof, 1 story, with a vented cathedral roof over the main living and dining areas (where the ceiling reaches about 20 ft.). The remaining roof above each end of the building is vented attic truss assembly.
From what I’ve seen, I anticipate that in the cathedral there will be about 6” of blown fiberglass and nothing else; I can see the soffit vents and ridge vent; perhaps (scissor) trusses were used but I won’t be sure till I pull a couple of the can lights. I’ll assume the soffits aren’t blocked …. The interior is finished with gypsum.
I want to air seal this inaccessible assembly and add insulation, but I’m not finding info on this reno situation. Since I have such height to work with:
Can I add rigid foam board to the cathedral on the interior ceiling? 3” of polyiso tape sealed with an added layer of gypsum (ADA) airsealed and screwed from the interior? Is a gypsum sandwich around RFB troublesome? Will the cart-before-the-horse airseal turn into a ‘hot mess’?
Or: Would it be better (for permeability) to add a 2×6 drop ceiling, densepak with cellulose (giving 12” total), and add the new layer of gypsum?
Or: Do I need to strip the old gypsum before adding insulation to get rid of the gypsum sandwich?
Or: Of course I wish the roof was unvented so I could add foam to the exterior, but I have trouble imagining a useful way of adding insulation to the exterior of this (vented) assembly.
If your answer is too long to explain, please point me at whatever I should read! And thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Any reason you can't seal the vents and foam the exterior?
See this page, top drawing set (roof insulation retrofit):
Leaving the added cost of a new roof to the side, wouldn't the 1-2" vent space between the fiberglass and the current sheathing be an issue?
Yes, I could be wrong.
Cathedral ceilings are nice. Low heating and cooling costs are too. Not easy to have both Joe. You might want to choose one or the other or another home. Lowest cost best insulation system would be to add a lower ceiling and insulate it with blown insulation IMO.
Nearly every house down here -- pre WWII and 1990 on -- has high ceilings because of the heat, AJ.
And yes, your rec for a lower ceiling (my option 2) is probably preferred. But I'd like to understand --
why would it be "lowest cost"? Labor?
is there any reason it is "best insulation system" other than the carbon footprint?
Joe, there are choices. Out of your narrowed down list, the least cost is to buy and do no insulation upgrade. Sounds like both of you like your high ceiling. Leave it come north when its too hot to visit and go south when the heat bill gets too big. This site has cellulose rated as most green. You can do a Larson truss, cellulose thicker cathedral set up or you could use two layers of foam, or frame and spray foam, there are too many ways. For me I would do the interior drop cathedral w cells, or possibly factory foam sheet seconds, and make sure drywall is air tight. And then keep humidity under control. With so many homes on the market, the best money way to go is pick up a home for cents on the dollar.
Q. "Can I add rigid foam board to the cathedral on the interior ceiling? Three inches of polyiso, tape sealed, with an added layer of gypsum (ADA) airsealed and screwed from the interior?"
Q. "Is a gypsum sandwich around RFB troublesome?"
A. No. (Assuming that RFB is some kind of acronym referring to polyisocyanurate.)
Q. "Will the cart-before-the-horse airseal turn into a ‘hot mess’?"
Q. "Would it be better (for permeability) to add a 2x6 drop ceiling, dense-packed with cellulose (giving 12” total), and add the new layer of gypsum?"
A. You never had a permeability problem with Option A, but your suggested Option B will also work.
Q. "Do I need to strip the old gypsum before adding insulation to get rid of the gypsum sandwich?"
A. No. The gypsum sandwich is not a problem.
Thank you, Martin. As always.
Actually I learned RFB here as a short for 'rigid foam board.'
I don't doubt that some writers on our site use "RFB" -- but it happens to be one of the acronyms I'm not familiar with. (This week, I was also tripped up by "RRP.") I figure that if three letters make me scratch my head, some other readers are probably scratching their heads, too.
So even though writing things out in full words takes a few more keystrokes, it aids communication.