Spray foam insulation in the attic?
I have a 3/12 pitch roof with inadequate ventilation and have mold issues in my attic. I live in Syracuse, NY. A contractor suggested they remove all attic insulation, spray the mold to kill it and then add 4 inches of spray foam to the underside of the roof sheathing and close off all ventilation. He said this would make the attic conditioned space and would be the best, albeit expensive, fix for my mold problems.
The idea of sealing off all ventilation in the attic seems wrong to me.
But is this approach valid and is 4 inches of R-7 closed cell foam sufficient for my upstate NY home? Should I put vents in the ceiling of my second floor to allow air circulation to the attic? The idea of completely sealing off the attic doesn’t sound right.
Thanks.
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According to here:
https://up.codes/viewer/new_york/iecc-2015/chapter/RE_4/re-residential-energy-efficiency#R402.1.2
NY State uses IECC2015 with R49 for ceiling. 4x7=28, doesn't sound like enough. Conditioned attics are more and more common now. There should be plenty of articles here on that.
If they remove all the attic insulation, they’ll also be exposing everything on the attic floor. That’s a great time to be air sealing everything. Chances are if you air seal everything properly, you can use blown cellulose insulation and solve the mold issue. The mold is most likely due to excessive moisture, and that moisture is likely leaking up from your house. Proper air sealing work should stop that, or at least reduce it substantially. Once the moisture problem is solved, the attic ventilation becomes less critical.
You don’t really need to use spray foam here, and 4” isn’t going to be enough anyway (4” is around R24-R30 depending on exactly what they’re using). If you do go the spray foam route, be sure they’re using closed cell spray foam if they’re applying it directly to the roof sheathing. Open cell is a problem in this application.
Bill
Walsh,
Installing closed-cell spray foam insulation against the underside of your roof sheathing is, indeed, a valid approach to solving your problem. But as K. Chiu correctly noted, 4 inches of closed-cell spray foam is only about R-26 (or R-28 at most) -- and you really need a minimum of R-49 insulation.
Bill is correct that the cause of your moisture and mold problem is that your ceiling is leaky, allowing conditioned interior air to enter your attic and condense. The most straightforward solution is to seal the air leaks. Here are links to two relevant articles:
"Air Sealing an Attic"
"Blower-Door-Directed Air Sealing"
How deep are the rafters?
Syracuse is in US climate zone 5A. In that climate zone as long a at least 40% of the total-R is air-impermeable foam you can combine it with much cheaper fiber insulation on the interior side.
A 2x8 rafter is 7.25" deep. Three inches of HFO blown closed cell foam comes in at about R21. Compressing an R23 rock wool or R21 fiberglass batt into the remaining 4.25" yields about R17-R19, for R38-R40 total, and huge dew point margin at the foam/fiber interface. That would even have met code for R value under IRC 2009, if not 2018, and isn't bad.
There are other ways of getting there too, including the full R49, depending on your actual rafter depth. But rather than walking you through a dozen possibilities, please share your framing depth information.
Closing off all ventilation to the attic isn't any different from closing all your windows downstairs. Once the attic is fully within the pressure & insulation boundary of the house it's technically "indoors", and the humidity will track that of the rest of the house.
Thanks all for your responses. I was hoping that the correct answer would be that R-28 of closed cell foam completely sealing the attic would magically be equivalent to R-49 of blown in cellulose. Apparently not.
Dana, the roof is supported by trusses made of 2x4s. And with the shallow roof I think it would be very difficult install a uniform, fully covering layer of batt insulation.
Sounds like the best solution is to remove the existing insulation, kill/clean the mold, seal the floor and blow in R-49 cellulose.