Ray,

We call the combination of spay foam and air-permeable insulation, hybrid. Hybrid installations are possible by one of two methods:

1. Install a 0.5-1” flash spray of foam then fill the remaining cavity with your air-impermeable insulation. You will definitely need a vapor retarder on the interior side of this assembly. This is not the best option, because if the vapor retarder fails, there is a good chance that condensation could occur on the cold spray foam layer.

2. Install enough spray foam so that by adding the layer of air-permeable insulation, you will not have condensation at that juncture. In your area you’d need around an R-10 of closed-cell spray foam with an R-12 batt of air-impermeable foam.

For the roof deck, you may need around an R-30 of the closed-cell SPF and an R-19 amount of air-permeable insulation to finish the cavity.

It is possible, but you need to be careful. Also, these recommendations are assuming you are in a climate zone 7 per ICC-IECC building codes. Please consult with your contractor and/or the foam manufacturer that you choose to ensure you will not have any condensation issues.

Feel free to click on the "Contact the FAQ Administrator" to ask me a follow-up question.

Copyright © 2011 NCFI Polyurethanes