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Mark_S Posted: Nov 28, 2016 03:00 AM	Shallow pitch roof with shallow rafters Greetings Mason- I have a 16' by 20' three-season room addition that I am converting to year round living. It has a cathedral ceiling with 2X6" rafters on a 3/12 pitch with no room for ventilation. My contractor recommends an application of open-cell LD-C-70 foam. What would you recommend as a minimum foam depth for this application? I live in central Indiana on the boundary line between climate zones five and six. Thanks for any insight!
mason Posted: Nov 28, 2016 01:16 PM	I don't believe that would provide sufficient insulation. IECC Building codes in climate zones 5 & 6 require R-49 in the ceiling or attic space. While I believe that is overkill, still 5.5 inches of open cell foam would only provide R-3.7 to R 4.1 per inch which would give you only from R 20 to R 22. In order to be code compliant, you would need from 12 to 14 inches of open cell SPF. Since SPF provides more effective insulation by air sealing the space, you could use less foam to achieve a practical energy efficiency, but even so, I would not recommend less than 10 inches in your area. You can fill the space with closed cell SPF. That has an R value ranging from 6 to 6.8 depending on the foam would have an R value ranging from R 33 to 37. From my experience, that would be sufficient insulation for your region. Email me masonknowles@aol.com I will send an article on Determining Insulation Thickness, by Roger Morrison. It discusses the factors used to calculate the right amount of insulation for your application including: moisture condensation prevention, energy efficiency and building code compliance.

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Comments

Mark_S

Posted: Nov 28, 2016 03:00 AM

Shallow pitch roof with shallow rafters

Greetings Mason-

I have a 16' by 20' three-season room addition that I am converting to year round living. It has a cathedral ceiling with 2X6" rafters on a 3/12 pitch with no room for ventilation. My contractor recommends an application of open-cell LD-C-70 foam.

What would you recommend as a minimum foam depth for this application? I live in central Indiana on the boundary line between climate zones five and six.

Thanks for any insight!

mason

Posted: Nov 28, 2016 01:16 PM

I don't believe that would provide sufficient insulation. IECC Building codes in climate zones 5 & 6 require R-49 in the ceiling or attic space. While I believe that is overkill, still 5.5 inches of open cell foam would only provide R-3.7 to R 4.1 per inch which would give you only from R 20 to R 22.

In order to be code compliant, you would need from 12 to 14 inches of open cell SPF. Since SPF provides more effective insulation by air sealing the space, you could use less foam to achieve a practical energy efficiency, but even so, I would not recommend less than 10 inches in your area.

You can fill the space with closed cell SPF. That has an R value ranging from 6 to 6.8 depending on the foam would have an R value ranging from R 33 to 37. From my experience, that would be sufficient insulation for your region.

Email me masonknowles@aol.com

I will send an article on Determining Insulation Thickness, by Roger Morrison. It discusses the factors used to calculate the right amount of insulation for your application including: moisture condensation prevention, energy efficiency and building code compliance.

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