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Brian Rener Posted: Jun 02, 2009 05:19 AM	Homeowner question on ccSPF ceiling and walls I am a homeowner planning an addition to my house in upper Midwest. I would like the addition to have spray foam and an “unventilated attic” – just use ccSPF on the roof joists and rock over it. Local code requires at least R38 in ceiling. The joists are planned to be 8 inches deep. I would hesitate to call it a cathedral ceiling – at is a low pitch roof with a membrane on it (not shingles) but I want to maximize open space in the family room addition. I have seen this question discussed but for clarity sake will ask again. I see three options 1)Spray to about 6-inch depth to get code R-value and leave 2 inch unventilated gap between drywall. 2)Spray to about 6-inch depth to get code R-value and ventilate the 2-inch gap between drywall. 3)If I can afford it – just go ahead and spray to full 8-inch depth with no gap (super insulate). Thoughts from you experts? PS – Related Dumb question. On the walls (4 inch studs with ccSPF) do I need Tyvek wrap on exterior of the addition? I had planned to do this.
William Gibson Posted: Jun 02, 2009 08:27 AM	For Ceiling: - You may be able to get away with a lesser R-value by completing a "ResCheck" on the home. In the northeast we have this program and we sometimes use it when codes disallows our suggested R-value. I am not sure if this same program is used in the midwest but I'd inquire about it. However, if you need to use the full 6" then go ahead and spray 6" to the underside of your roofdeck and leave the 2" airspace between the foam and the drywall as a dead airspace. Dead air is actually a very good insulator. As for the walls - yes go with the Tyvek. There is a misconception on Tyvek or Housewrap that it is a vapor barrier. It certainly is not a vapor barrier, but serves as a good watershed to prevent water from getting to the sheathing of your home and finding its way into the house.
Posted: Jun 02, 2009 11:14 PM	What would one use to get an R Value of 38 if Spray Foam was not used at all? In your area, would the homeowner just put in cellulose in the rafters or is there something else that they would do to get the R Value that they need to stay within code? Steve
Brian Rener Posted: Jun 03, 2009 04:28 PM	I could go to 10 inch joists and just use Ownens Corning Fiberglass to get the required R38. Which is plan B if the spray foam is too expensive. I just got a call from one GC on the addition saying I should use open cell foam on the ceiling. Which I guess is a debate that has been going on for sometime.

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Brian Rener

Posted: Jun 02, 2009 05:19 AM

Homeowner question on ccSPF ceiling and walls

I am a homeowner planning an addition to my house in upper Midwest. I would like the addition to have spray foam and an “unventilated attic” – just use ccSPF on the roof joists and rock over it. Local code requires at least R38 in ceiling. The joists are planned to be 8 inches deep. I would hesitate to call it a cathedral ceiling – at is a low pitch roof with a membrane on it (not shingles) but I want to maximize open space in the family room addition.

I have seen this question discussed but for clarity sake will ask again. I see three options

1)Spray to about 6-inch depth to get code R-value and leave 2 inch unventilated gap between drywall.

2)Spray to about 6-inch depth to get code R-value and ventilate the 2-inch gap between drywall.

3)If I can afford it – just go ahead and spray to full 8-inch depth with no gap (super insulate).

Thoughts from you experts?

PS – Related Dumb question. On the walls (4 inch studs with ccSPF) do I need Tyvek wrap on exterior of the addition? I had planned to do this.

William Gibson

Posted: Jun 02, 2009 08:27 AM

For Ceiling:
- You may be able to get away with a lesser R-value by completing a "ResCheck" on the home. In the northeast we have this program and we sometimes use it when codes disallows our suggested R-value. I am not sure if this same program is used in the midwest but I'd inquire about it.

However, if you need to use the full 6" then go ahead and spray 6" to the underside of your roofdeck and leave the 2" airspace between the foam and the drywall as a dead airspace. Dead air is actually a very good insulator.

As for the walls - yes go with the Tyvek. There is a misconception on Tyvek or Housewrap that it is a vapor barrier. It certainly is not a vapor barrier, but serves as a good watershed to prevent water from getting to the sheathing of your home and finding its way into the house.

Posted: Jun 02, 2009 11:14 PM

What would one use to get an R Value of 38 if Spray Foam was not used at all?

In your area, would the homeowner just put in cellulose in the rafters or is there something else that they would do to get the R Value that they need to stay within code?

Steve

Brian Rener

Posted: Jun 03, 2009 04:28 PM

I could go to 10 inch joists and just use Ownens Corning Fiberglass to get the required R38. Which is plan B if the spray foam is too expensive.

I just got a call from one GC on the addition saying I should use open cell foam on the ceiling. Which I guess is a debate that has been going on for sometime.

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