Roof insulation for a West Baltimore row house
I’ve read some earlier GBA discussions about insulating brick buildings and those with flat roofs, and I’m planning to follow the guidelines I’ve gathered from those. My question is about the full insulation process for the roof when you use spray foam on the interior side of a flat roof. I recognize that the best advice is to put the insulation on the exterior side on top of the sheathing, but for a variety of reasons, that wasn’t possible when I had my roof replaced in the fall.
I plan to spray closed-cell foam (Froth-Pak) on the interior side of the roof (2 passes totaling 2-3 inches) and on the joists and brick where the joists are pocketed (to seal it properly/prevent air and water penetration, etc.) Next I’m planning to put Roxul against the spray foam in the cavities between the joists. Baltimore code requires R-38 total, I believe, so this combo should be sufficient.
So then with the ceiling framing in place, do I put a vapor barrier on the room side of that framing before installing the drywall? I have a sense that adding another vapor barrier there would trap moisture in the space between the Roxul and the dry wall, so I want to get some guidance on this. If not, do I need to add anything else or are the spray foam and Roxul the end of the story?
Thanks in advance.
Tim Werner
Washington, DC
P.S. Photo attached. Pocket still needs some cement.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Tim,
There are lots of issues here. I assume that you have read my article, Insulating Old Brick Buildings, discussing the risks to embedded joists when the joists are surrounded by insulation, and discussing the risks to old bricks when old brick walls are insulated on the interior.
I'm going to assume that you have assessed these risks and are confident that you want to go forward with your insulation plan.
1. Froth-Pak kits are an expensive way to proceed for a job that requires a lot of closed-cell spray foam. I urge you to contact one or more spray foam contractors to get bids.
2. Closed-cell spray foam is a Class II vapor retarder; 2.5 inches of closed-cell spray foam has a permeance of only 0.8 perm. So your spray foam meets the code requirement for a vapor retarder -- you certainly don't need any interior polyethylene. For more information on this issue, see these articles:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
-- Martin Holladay
Thanks very much, Martin. Yes, I've read that article. My understanding from your various responses is that there isn't much of a freeze-thaw concern about brick in the Philadelphia-DC area. I'm basically assuming that evaluation applies to my place since the brick is in generally good shape (for a 100-year old structure). In terms of the concern about joist damage, I have been thinking of using a high-build paint on the exterior, such as BM Regal Select, to protect the surface while allowing it to breath. Since the house shares the side walls with neighboring houses, the main concern regarding the joist issue is in the breezeway area toward the back (and only on one side). Would you recommend not spraying that area? Thanks again.
Froth Pak is only 1.75lbs density, and would be about 1.2 perms @ 2.5", still Class-III vapor retardency. Most 2-lb foams test between 0.8-1.2 perms @ 1" and would be under 0.5 perms @ 2.5".
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0946/0901b8038094669c.pdf?filepath=styrofoam/pdfs/noreg/179-05044.pdf&fromPage=GetDoc
If you need more than 500 board-feet of foam it's almost always cheaper to hire a pro. If you used a water-blown 2lb foam you could still be in the same permeance range (or higher) of Froth Pak: eg Icynene MD-R-200 is rated 1.3 perms @ 3". MD-R-210 comes in bit under 1 perm @ 2.4".
http://www.jsjfoaminsulation.co.uk/docs/ICYNENE-MD-R-200-Specification-Sheets.pdf
http://www.icynene.com/sites/default/files/downloads/ProSeal%20Eco%20MD-R-210%20Technical%20Data%20Sheet_August%202013.pdf
They both run about R5/inch- don't confuse it with their HFC245fa blown MD-C-200, which is a low-permeance high global warming impact product.
There are other 1.5lb perm foams out there that would still be a class-III vapor retarder @ 3", but you have to be picky, and pull the data sheets.
As long as the ceiling is reasonably air tight on the interior side you only need ~30% of the total R to be closed cell foam for dewt point control at the foam/fiber boundary. If your goal is R38 you'll need at least 2.5" of closed cell foam to clear that mark with any margin at all, and you would be limited to ~R25-R27 of the fiber layer. With 3" closed cell you'd be good for as much as R30-R34 in the fiber layer. If you cheat the ratio by a small amount you can maintain resilience by using a Class-II vapor retarder such as "vapor barrier latex primer", or a "smart" vapor retarder such as 2-mil nylon (Certainteed MemBrain) on the interior side of the rock wool.
Timothy,
I'm not going to make any recommendations concerning brick damage or embedded joist damage. There are too many variables. Only you (or a consultant who makes a site visit) can assess the risk and make the necessary decisions.
Certainly, wide roof overhangs help preserve brick. If you are worried about your embedded joists, you can cut off the embedded portion with a Sawzall and support the joists on posts.
Thanks very much to the three of you for your generous, helpful, and detailed responses. I will definitely get a couple of bids on the spray foam job. I was under the impression that bringing in a contractor would be far more expensive than the Froth-Pak route. The vapor barrier latex primer also sounds like a good approach. It's unlikely that I'll go so far as to cut off the joists. Ensuring that I have good flashing, drip edges, and gutters along with using high-build exterior paint is probably how I'll approach it. Really appreciate your feedback. Thanks again.
Typical 2lb foam quotes in my area come in at about a buck a board-foot for jobs than need more than 2000 board feet, to maybe $1.20-$1.30 for 1000 board-foot jobs, and ridiculously expensive (or no-bid) for 500 board-foot jobs, since it's all set-up and break-down time, tying up a truck & crew for the afternoon for a piddling amount of foam.
Typical 600 board foot DIY kits are in the buck a board-foot range, so for very small jobs it can be cheaper, but when you add in extra tips and protective clothing/breathing equipment etc it adds up, and it's hard to make the financial case for a DIY job in the 1000 board-foot size range, let alone larger sizes. When you throw in the experience factor and the precision temperature controlled spraying equipment it's hands-down a better deal to use a pro, even if it's 15% more expensive.
No DIY kits I'm aware of use climate-friendly blowing agents. There are a very few pros using low impact HFO1234ze for a blowing agent (, and only a couple of water-blown options (Icynene being the only one with a large national presence, though not all installers have experience with MD-R-200 or MD-R210). A short list of HFO blown closed cell foams can be found in this bit o' bloggery:
https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/next-generation-spray-foams-trickle-market