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cathedral ceiling reinsulate from roof side

13 years ago

I have an unusual opportunity to remove the roof (including decking) on a small house in which the ceiling is mostly cathedral style. This comes about because the roof is shot and the decking is shot and warrants replacement. House built in 1980, no idea what is in there now.

The on-line information seems mostly to be about retrofitting from the inside, not the outside.

What do I do?

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Comments (4)

worthy
13 years ago
Insulating from the exterior is consistent with an unvented attic. Details of R Value, vapour permeability requirements and venting depend on climate and relevant codes. Unless you are familiar with those details, get input from a local design professional.

See page 8 on link.

Here is a link that might be useful: Vented and Unvented Roofs
brickeyee
13 years ago
Depending on how much eight you are willing to add you can even have a vented roof.

All you need is enough clearance between the new roof deck and the insulation below.

You could also look into insulating and then applying a 'cold roof.'
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you are correct....open cell is the cheaper, less R and higher perm (allows more water through) then closed cell. Closed cell will be a added cost, but would probably be worth it in the basement at least. Closed cell, in general, will seal up your entire house better then open cell, as well as add structural rigidity to your 2x4 framing making your house stronger. Open cell does not. Closed cell is not water proof, as it seems your questions are hinting toward. Cellulose is just fine for the attic, and going with more closed cell is a waste of money IMO. An animal can rip apart open cell just as easy as getting into cellulose. Besides, if something gets in there you have bigger issues anyway! Also, your roof is suppose to be water proof....not your insulation. If your roof is failing, you do NOT want a water proof insulation anyway. This traps water and allows wood to rot, thus a possible structural failure in your roof without you really even knowing. You may also be money and r value ahead to practice air sealing your walls with caulk and then using a blown fiberglass or dense packed cellulose in your walls. A side note...spray foams are THE most expensive possible way to insulate a home, and most cases you will never see a ROI compared to dense packed cellulose and blown fiberglass with air sealing especially in your climate. (note, I never mentioned batts).
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Hmmm, that's interesting. We have a fan unit in the attic that vents 2 bathrooms that are side by side. From the unit, the vent duct goes straight up to the roof. You could certainly insulate the duct work, I am pretty sure we did that mostly for noise (we used rigid ducts and you hear the turbulence, not the fan). We have a cathedral type ceiling in both bathrooms, and we mounted the vent on the vertical wall above the shower head, routed to the motor, then as above (does that make sense to you?). This does not eliminate the heat loss into the duct, but as it is on a vertical wall, rather than the ceiling, maybe it is less like a "chimney", although I am sure we loose heat there too. This routing avoids running what sounds like a "heat conduit" between the roof and ceiling. But now I think I will go outside in the morning and look at our roof after over a foot of snow...
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Hi, We had half of the second floor of our house removed to make a loft style upstairs. The light that poors in from the second floor makes it well worth the effort. However, we brought in three contractors and a structural engineer before making our decision. We also hired an excellent contractor to do the work. Our house is post and beam construction which made the work possible. However, it also required tension poles coming from the basement and through the first floor. The structural engineer provided guidance as to the exact location of the poles. Without the input of an expert the renovation could have been a real disaster. The money spent retaining experts is money well spent. It costs a lot more to try and repair structural damage to a home. Good luck with your project, but please get some expert advice. Your home will thank you.
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Stacey Collins
13 years ago
If it were me, I'd look into the feasiblilty of the foam-in-place stuff. That's the proper thing for an unvented "hot roof" like a cathedral ceiling, anyway. It also has more R value per inch than fiberglass. The only thing I'm not sure of is that if you blow it from above, any air gaps will on the cold side. I'm not sure if they can overfill and slice it off flush or not. Or if that matters much at all. I'd call one of the spray foam installers and ask them!

You could also carefully cut rigid foam and spray/caulk it tightly between the rafters, but that would be a lot of labor!
sierraeast
13 years ago
I would run rigid foil faced foam boards tight,(friction fit) between the rafters minus two inches down from the top of the rafter to allow an air space. I would install contious run soffit/ridgeventing making sure the air spaces remain unobstructed and have a clear path from the soffit venting to the ridgeventing. The foil facing will act as a radiant barrier and trap the heated air in the air space and it will be pushed out the ridgevent in the hotter months. The foil facing down tight against the top of the ceiling material will help hold the heat in in the colder months.This is all dependent on your location, weather extremes, and moisture/humidity levels.

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