Retrofit insulation cathedral ceiling: Foam/Roxul sandwich?
So recently we bought a 1950s story ranch with two upstairs bedrooms (mostly under adormer) in northern NJ. In one upstairs bedroom which we dismantled, I’m looking to upgrade the insulation to the exterior wall and cathedral ceiling (sad looking R13? between 2×6 roof rafters with a a tongue and groove wood behind it – which looks in perfect condition).
Technically someone added soffits along the eaves but not to this secton of the roof. There is probably a large air path from this section to the soffit areas at the peak of the roof but no vent at the top of the roof. The other upstais bedroom has one knee wall exposed to the soffits/uninsulated garage (which I need to tackle later).
I’m trying to get the best bang for my buck and have a DIY job. I was thinking of putting up 1″ wood spaces and 1″ polyiso to create a vented space. Then have 3.5″ of roxul. Then lining over the rafters with 1″ (?) polyiso foam and then sheetrock. I’m thinking it would give me ~R34. Not exactly the highest, but it wouldn’t eat up too much headroom of the sloped walls and the poly would create a thermal break and air seal.
Do you think there might be a moisture issue with the sandwich of foam?
Or do you have a better suggestion?
Should I skip the polyiso channel entirely? If I did, would I need to somehow airseal the batts (Tyvek wrapping?)
Any help would be much appreciated. There seem to be some many opinions floating around and I”m not sure which way is “up”.
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Replies
Nicole,
1 inch of polyiso (R-6.5) + 3.5 inches of Ruxul (R-14) + 1 inch of polyiso (R-6.5) = R-27, not R-34.
Q. "Do you think there might be a moisture issue with the sandwich of foam?"
A. No, as long as all of the materials are dry when they are installed.
Q. "Do you have a better suggestion?"
A. If you can afford it, closed-cell spray polyurethane foam will give you the best thermal performance in a small cavity.
Q. "Should I skip the polyiso channel entirely?"
A. You can skip the ventilation if you are using spray foam. However, if you are using Roxul, you have to have it.
For more information, see How to Build an Insulated Cathedral Ceiling.
Yes, my math was wrong. It was too late at night!
The R34 was for if I put up 2" of foam.
Definitely can't afford closed cell, so it's out.
Do I need ventilation if I stuff the entire cavity with rigid foam (~R38) and seal the edges with can spray foam? That's still pricy.
The big orange box store by me sells two types of 2"polyiso. One is foil faced and says its for interior/exterior. The cheaper one is not foil faced but only says exterior. Is it really just for the exterior? is there any difference besides the foil?
Nicole, rigid cut to fit bays is not good. Do your idea with vent chutes replacing rigid for venting and use high R glass batts instead of rock wool. And use taped foil backed rigid foam under your drywall. Caulk or foam all air leaks except the chutes can be leaky.
AJ,
Thanks for the help.
Why do you say to use high R glass batts instead of rock wool? I was leaning toward roxul because it is mouse/mold resistant and seems to be less finicky to install properly.
Do I "need" vent chutes? Or can a simple gap also be sufficient? Or would that kill the R value due to air infiltration? Can I just wrap the roxul with tyvek for air infiltration purposes?
The more I think about my roof lines, I don't see how any soffit air would travel up these rafters. And from the ridge, I doubt much air moves to this section of the house (no ridge/gable vents anyway).
Thanks again
Nicole,
There is no R-value advantage to high-density fiberglass batts over Roxul. You can use Roxul if you want.
You still need the ventilation channel, however, unless you use closed-cell spray polyurethane foam -- or (perhaps, if approved by your local code official) a stack of rigid foam rectangles, each one air sealed, that fill the joist cavity.
Nicole, vent chutes need vents. The rest of my advice stands. Your complicating a simple project IMO.
Gotcha. I think.
I do like to complicate simple things. :)
I have a similar situation but not all houses are same. So I wanted to get the experts advice on my insulation plan. I have a 1950's 1.5 story bungalow in Chicago. The attic is hot in summer and cold in winter probably because of lot of air leaks and poor insulation. The attic is finished and has 2 bed rooms and one bathroom (within a 5 ft dormer). I also plan to run ductwork for a supply and a return in the top attic. My plan is to insulate underside of the rafters so I can make the side attic space conditioned and to maximize floor space. The roof has no soffit vents, no ridge vents and no gable vents but only two standard roof vents. Again, closed cell spray foam is not an option for me because I can't afford it. I plan to use Roxul R-23 comfortbatts and 2 inch polyiso or XPS foam board so I can get an R-value of R33 with XPS or R36 with polyiso. My question is will this combination work in my context and if I need to use any baffles or vent chutes considering I don't have soffit or ridge vents. Please help!
John,
If you want to mix rigid foam and air-permeable sheathing in an insulated unvented roof assembly, you need thicker foam. In your climate zone (zone 5), the building code requires the foam layer to have a minimum R-value of R-20. If your foam is thinner, you risk moisture problems.
More information here: How to Build an Insulated Cathedral Ceiling.
Thanks for your response Martin. Just to confirm, from the attic living space looking upwards, this the order of layering materials: Is my understanding correct?
1. Drywall (1/2" thick)
2. 4" XPS foam board R-20 laid perpendicular to the rafters
3. Roxul R-23 comfortbatt that fills the 6" rafter cavities
4. Baffles (is this required??)
OK, I got this wrong. I just saw a figure "Rigid board foam unvented roof assembly" on buildingscience.com that illustrates clearly the layers. Since I can't get the rigid foam board on top of roof sheathing from an inside installation, I think spray foam is the only option...argh!!! a 5K job...yikes..
Builder science verses building science....
Foam board on the inside with a decent air stop barrier and venting IMO works and the price is right.... Repeated in many threads, batt insulation works best if the bays it's installed in are caulked and taped into airtight compartments.
To me most homes lose most heat via leakage. Slow the leakage along with even a little continuous insulation and you are now better than almost all existing homes.
John: You don't need the whole R20 on spray foam on the underside to protect the roof deck- even 1-2" will do if you're only looking at 2x6 cavities. It would technically be a code violation, but it wouldn't be a problem, read this document carefully:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
The more signficant issue would be the moisture trap created by 4" of XPS (about 0.25 perms) on the interior, and the even lower permeance roofing material. If you're going to use rigid foam below the rafters, first make an air barrier layer by stapling kraft paper, a smart vapor-retarder (MemBrain, Intello + etc) or permeable housewrap on the underside of the rafters after installing the batts, and use unfaced Type-II EPS (about 1perm @ 5") or paper-faced roofing iso (0.5-1 perm).
The greenhouse gas footprint of XPS is also quite high- more than 100x that of EPS or polyiso due to the ubiquitous use of HFC134a as the primary component of the blowing agents for EPS, which has a global warming potential more than 1000x C02, and leaks out over a handful of decades. by contrast the pentane used for blowing EPS & polyiso has a GWP of only ~7x CO2, and unless there's a foil facer, it's mostly gone by the time it hit's the distributor's lot, and barely detectable a year after manufacture.
Thanks for the great document Dana.
I believe I have three options here to create an unvented roof conditioned attic space:
1. Full depth 6" open cell SPF under rafters and then sheetrock with latex paint. seems like Zone 5 less than 4500 heating degree days doesn't require a vapor retarder. But this will give me only 22.8 R-value. I don't have to break the bank to do this. But this a far cry from R-38 code for cathedral ceilings in Zone 5
2. Hybrid plan: 2" close cell SPF -R 13 + R-15 batts + 1.5" thick polyiso ~ R-10 gives me R38, then sheetrock with latex paint. It's a bit laborious, but doable
3. Full depth close cell SPF and then sheetrock with latex paint. Expensive and my last option.
Please help me decide. I don't know what kind of insulation I have between the roof shingles and above the roof sheathing. If there is any, it's a bonus. But I am trying to achieve R-38 under the roof rafters. Appreciate your help here.
Thanks,
John