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H. Munster Posted: Jul 16, 2008 12:20 PM	fiberglass with spray foam I have just added an addition to my house with closed cell spray foam in the walls and ceiling (unvented). The spray foam was applied to the bottom of the roof and the roof is a scissor-truss type. I would like to boost the R value of the ceiling before I install the drywall ceiling. Can I add fiberglass (batt or blown)? Will mold be a problem. I live in Missouri - hot in summer, cold in winter.
mason Posted: Jul 16, 2008 02:56 PM	In mixed climates like Missouri, you can use a hybrid (flash and batt) insulation system. though not as thermally efficient as using straight sprayfoam, 1-2 inches of 2 lb density SPF will be sufficient to prevent condensation in your climate. The sprayfoam will make the fiberglass insulation work more efficiently. In colder climates you should use between 2 to 3 inches of 2 lb SPF to prevent condensation when using fiberglass insulation over the foam.
H. Munster Posted: Jul 16, 2008 03:04 PM	Thanks. I wanted to add the fiberglass because even though it the room is air conditioned, it still seems hot near the roof. I have at least four inches of foam. Does it hurt to have an air gap between the foam and fiberglass (ie next to the drywall)? Which is the best secondary insulation to use: fiberglass batt, fiberglass blown, or cellulose? Thanks again.
H. Munster Posted: Jul 18, 2008 09:16 AM	I will be closing in the vaulted scissor-truss ceiling next week. At this point, I plan to have the drywallers leave off a panel so I can blow in extra cellulose. The cc foam is a minimum of 4 inches, but is six or more inches in most spots. I was hoping the foam alone would be adequate, but when I climb up to the trusses, the temperature is ten degrees hotter. Is that because the foam is not thick enough, or the hot air in the house collects there because the addition is vaulted?
mason Posted: Jul 21, 2008 08:15 AM	My preference when doing a hybrid insulation system is cellulose (not the loose blown-in stuff) over the foam. I have seen this system work very well in the hot humid climates of South Texas for over 30 years. The cellulose can adhere very well to the SPF. Don't leave a air gap if you can help it.

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Comments

H. Munster

Posted: Jul 16, 2008 12:20 PM

fiberglass with spray foam

I have just added an addition to my house with closed cell spray foam in the walls and ceiling (unvented). The spray foam was applied to the bottom of the roof and the roof is a scissor-truss type. I would like to boost the R value of the ceiling before I install the drywall ceiling. Can I add fiberglass (batt or blown)? Will mold be a problem. I live in Missouri - hot in summer, cold in winter.

mason

Posted: Jul 16, 2008 02:56 PM

In mixed climates like Missouri, you can use a hybrid (flash and batt) insulation system. though not as thermally efficient as using straight sprayfoam, 1-2 inches of 2 lb density SPF will be sufficient to prevent condensation in your climate. The sprayfoam will make the fiberglass insulation work more efficiently.

In colder climates you should use between 2 to 3 inches of 2 lb SPF to prevent condensation when using fiberglass insulation over the foam.

H. Munster

Posted: Jul 16, 2008 03:04 PM

Thanks. I wanted to add the fiberglass because even though it the room is air conditioned, it still seems hot near the roof. I have at least four inches of foam. Does it hurt to have an air gap between the foam and fiberglass (ie next to the drywall)? Which is the best secondary insulation to use: fiberglass batt, fiberglass blown, or cellulose?
Thanks again.

H. Munster

Posted: Jul 18, 2008 09:16 AM

I will be closing in the vaulted scissor-truss ceiling next week. At this point, I plan to have the drywallers leave off a panel so I can blow in extra cellulose. The cc foam is a minimum of 4 inches, but is six or more inches in most spots. I was hoping the foam alone would be adequate, but when I climb up to the trusses, the temperature is ten degrees hotter. Is that because the foam is not thick enough, or the hot air in the house collects there because the addition is vaulted?

mason

Posted: Jul 21, 2008 08:15 AM

My preference when doing a hybrid insulation system is cellulose (not the loose blown-in stuff) over the foam. I have seen this system work very well in the hot humid climates of South Texas for over 30 years. The cellulose can adhere very well to the SPF. Don't leave a air gap if you can help it.

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