Vaulted / cathedral ceiling insulation
Ok, I’ve been reading on here and FHB for days about insulation. Here’s my situation:
Home (kitchen) remodel:
A small porch was put on the back of my Cape Cod style Maryland home some time ago. Roughly 25’x6’. Shed roof 3/12 pitch and 2×4 rafters. Eventually it was later closed in to become a galley style kitchen.
When the porch was closed in, a poor flashing/ water management job led to severely rotted walls.
I bought the house 6 months ago with intentions of installing a rear deck. After lifting the siding and discovering the damage (a not so pleasant conversation was had with my 900$ home inspector.)
After weighing my options, let the kitchen remodel ensue.
Trying to save money and time here’s what I decided:
-Rear exterior wall and 6′ of each side wall completely replaced.
-New walls built with conventional 2×4 framing and zip system sheathing.
-Re-attached roof to new exterior walls.
-Wife decided that the slanted ceiling looks “Nice” as opposed to a flat ceiling.
– Spending several hours researching insulation practices brings me here.
Roof has a vented soffit that circulates into attic space behind a knee wall. Air vents out through installed vents high on the side walls (I’m not too sure about this practice, but that’s for a different question.)
I know I need at 1″ air space. I like the Idea of using 1″ rigid foam (Site Built) baffles to let air circulate. I’ve read the article by Mike Gurertin where he uses foil faced rigid foam, but most articles on here recommend using Owings corning XPS. Why the difference in practices?
After I figure out what type of foam to use my main question is what the heck to put below that?
Most cost effective for me would be fiberglass.
Faced/ Un-faced? Do I need a vapor barrier or retarder?
From what I can tell the best practice would be to use un-faced batts with air sealed drywall and vapor retardant paint?
I’d also like to put painted T&G Pine on the ceiling.
Any guidance with this mess of information would be greatly appreciated.
Thank you
Andy
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Replies
Andy,
You are mistaken when you write that "most articles on here recommend using Owings Corning XPS." In fact, green builders try to avoid the use of XPS (since XPS is manufactured with a blowing agent that has a high global warming potential).
Your biggest challenge is the fact that the rafters (2x4s) aren't deep enough to allow you to install enough insulation to meet minimum code requirements.
If you read these two articles, all of your questions will be answered:
How to Build an Insulated Cathedral Ceiling
Site-Built Ventilation Baffles for Roofs
I might be guilty of writing comments that could be interpreted as recommending Owens Corning XPS. For the record, I do not recommend Owens Corning XPS. It's not quite as terrible as Dow XPS, but it's plenty terrible and about a hundred times worse than foil-faced polyiso.
Andy,
My article on site-built ventilation baffles goes on to explain why, even though some people insist on installing a vapor-permeable ventilation baffle, I don't think that the vapor permeance of the baffle matters.
You wrote, "My concern with the foil faced was that it would trap air in the cavity by being another vapor barrier." This sentence confuses two concepts. To untangle them: trapping air is good. (You always want to strive for airtightness.) Trapping vapor may be a problem in some assemblies -- but in the case of ventilation baffles, there aren't any reports of moisture problems, even when vapor-impermeable materials are used to make the ventilation baffle.
In Maryland (Climate Zone 4), the 2012 IECC mandates that roofs should have a minimum R-value of R-49, not R-24. So if at all possible, R-49 (or better) should be your target.
Thank you both for the info!
Martin,
Thank you for the quick response! I apologize, I neglected to mention that I sistered those 2x4 rafters with 2x8's so I could at least get R19 batts in there plus the foam which would give me value of R-24.
The polyiso foil faced board at home depot is a higher R-value at 6 and is what I initially wanted to use.
My concern with the foil faced was that it would trap air in the cavity by being another vapor barrier.
I guess my confusion with the XPS comes from one of the articles you provided where it says..
"(If you use rigid foam for your baffles, it probably makes more sense to choose thin EPS or XPS rather than foil-faced polyisocyanurate, to allow a bit of outward drying, however slow, by diffusion. A thin layer of EPS or XPS is somewhat vapor-permeable, while foil facing is a vapor barrier.)"
I'd like to be as energy conscious as possible without spending an arm and leg.
Martin,
Thanks for breaking that down for me man, as you can tell i'm pretty novice at this stuff. I'll stay away from the XPS and stick with my original plan to use the foil faced foam.
As far as achieving R-49.. it's going to be tough, but i'll continue to research ways i can get closer.
This website is such a great resource and i appreciate all knowledge and advise you guys share!