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I am in the process of remodeling the 2nd floor of our cape and I am trying to figure our how to insulate it. Here is what the 2nd floor looks like, roughly.

2nd floor

Here is another drawing that shows the venting. You can see that it has a ridge vent, and some sort of small soffit vent. Not shown are the gable vents, the knee walls, and the collar ties, among other things.

roof configuration

I'm in zone 5 and I'd like to have more insulation than the R13 I could fit in the rafters. So because of that I am trying to decide between having spray foam insulation, or using rigid foam between the rafters. I'm hesitant to use spray foam since I know less about it, I don't know if converting to an unventted roof is a good idea, I feel like I might need to change some of my roof sheathing soon, and I am only able to redo half of the second floor at a time.

My thought for the rigid foam would be to do whats shown in the drawing below. Additionally I would use expanding foam to seal foam to the rafters and top plate.

rigid foam

So, besides costing quite a bit of money, is this a good idea? Are my reasons for avoiding spray insulation even remotely valid?

  • Where are you on this planet? – ThreePhaseEel Dec 9 '17 at 18:50
  • North East USA, Zone 5 energystar.gov/… – Jay Dec 10 '17 at 16:54
  • what do the IRC ground snowfall load maps say about your locale? – ThreePhaseEel Dec 10 '17 at 17:04
  • 25-30 psf, depending on the map used – Jay Dec 11 '17 at 10:38
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I for one like the new spray-in foam. It does such a good job because it makes the house so tight. Infiltration is where you loose your heat. It's the cracks that hurt you. Nothing seals the cracks like the spray-in expanded polystyrene. Regarding all that venting business; they are just trying to keep the roof surface cold so the snow won't melt and cause ice dams. Nothing out there can better keep the heat away for the roof surface.

  • Thanks Paul. Regarding the spray-in how should existing soffit and gable vents be handled, and what would I have to do if I needed to replace roof sheathing in the future? Also, while I should look into this myself, would it be cost prohibitive to have a company come in twice (once for this half of the 2nd floor, and again next year for the other half)? – Jay Dec 10 '17 at 16:59
  • I wouldn't use the spray-in product, because of the cost, on anything but top condition sheathing. Then it would be important to keep it that way through the maintenance of the finish roofing. In the process of talking to the insulation contractor, he undoubtedly will tell you what he needs in the way of backing. I can't speak to the cost aspect. – Paul Logan Dec 11 '17 at 5:12

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