Replies

1. 
   MattJF | Mar 05, 2020 09:37am | #1

   Open cell foam on roof decks is generally not considered safe in zone 5. So your best path is to implement as many protective measures as possible.

   The ideal option would be to put 3-4" of polysio foam on top of the roof deck. Consider reclaimed foam for this.

   If that is not an option, it might make sense to convert to a diffusion vent. https://www.buildingscience.com/documents/insights/bsi-088-venting-vapor

   On the inside you should provide some conditioned air. This is easy to do if you have have ducts up there for heating. https://www.buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics

   If possible, an interior smart vapor barrier such as Membrain would be good as well. Depending on how the foam was put in, this could be tricky. Making sure to seal up the ceiling is good so that acts as a vapor barrier. Take extra care with the ceiling above the bathroom.

1. 
   johns3km | Mar 05, 2020 10:08am | #2

   It was recommended 5/6 years ago as safe but here we are. I have a minisplit in the attic so I can keep it dry up there. No other HVAC, just steam radiators. 75% of the attic is drywalled, a storage area is not.

2. 
   johns3km | Mar 05, 2020 10:20am | #3

   Is there a certain humidity I can aim for in the attic that would provide protection without doing the 4" of polyiso?

2. Expert Member
   Akos | Mar 05, 2020 10:33am | #4

   I would suggest a more pragmatic apparoch.

   If you pull the shingles and there is no moisture issues, the deck is fine, then don't mess with it. Put it back exactly the way it was.

   If you see rot near the ridge, install the diffusion port.

   If you see moisture damage on other parts of the roof, go for the rigid insulation.

   If the R30 is between 2x10 rafters, the exterior rigid insulation would bump up your whole roof R value by a fair bit since it reduces thermal briding from the rafters. With 3", it would almost double the effective R value of the roof (R24 to R46). If your fuel is expensive, or if you have ice damming issues, this might be worth it.

1. 
   johns3km | Mar 05, 2020 07:33pm | #6

   I like your approach.

   It's OC 8.25" depth with strapping, R value 3.9/inch, so almost R-32 without thermal bridging.

   I've got my roof at around 9% of my heatloss, with my uninsulated basement at 30%. From an insulating perspective I think the money would be best served down there. No ice damning issues either.

3. GBA Editor
   Brian Pontolilo | Mar 05, 2020 10:34am | #5

   Hi Kevin.

   Everything that Matt wrote is right on, and at the same time, that doesn't guarantee that you'll have a problem. Adding exterior insulation during the re-roof would be ideal. You'd take care of the damp sheathing concern and get to a better R-value (current IRC minimum in your areas is R-49)

   The re-roof work will also offer an opportunity to inspect the sheathing to get an idea of its current condition, as Akos said.

   If you decide not to add exterior insulation, keeping the attic space conditioned and relative humidity levels reasonable will help (I believe that "reasonable" may be in the 30% to 40% range in the winter and under 50% in the summer in your area).

   Also, you should have a good air barrier between the conditioned space and the insulation. Your drywall ceiling can be detailed as an air barrier or you could detail a smart vapor retarder as an air barrier and get some vapor protection that will also be helpful. Here are some GBA articles to read:

   Open Cell Spray Foam and Damp Roof Sheathing
   How to Build an Insulated Cathedral Ceiling
   How to Install Rigid Foam on Top of Roof Sheathing
   Creating a Conditioned Attic

1. 
   johns3km | Mar 05, 2020 07:40pm | #7

   Thanks Brian. Are there specifics on what are appropriate dew points for inside/outside? Am I comparing inside dew point with exterior temperature, or dew point vs dew point? Which should be higher or lower?

   Like today, my attic is 69F/35% humidity, for a DP of 40F.
   Outside, it is 46F/44% humidity, DP of 21F.