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eric himmer Posted: Mar 11, 2008 07:00 PM	Attic Retrofit question Will be insulating an attic of a house with vaulted ceiling below. On each side of the house are 45 degree diagonal sloped sides that lead to the flat ceiling. Each sloped side is 6' long and has prop-a-vent with fiberglass inside. Currently fiberglass is laid across the top of the flat ceiling section. Without pulling out the fiberglass from the sloped sides, should I continue the pro-a-vent to the peak and foam over with CC 2 lb., cap the prop-a-vent and spray right to the roof sheathing, or just spray across the top of the flat ceiling sheetrock from above. Worried about moisture issues and feel insulating roof deck will be best place for the Foam. Located in NE. Any suggestions would be appreciated
mason Posted: Mar 12, 2008 07:31 AM	You do not want to foam over fiberglass insulation. If you leave the fiberglass, you have to vent, if you spray foam to the underside of the roof deck, you do not require venting. If you spray the top of the ceiling (floor of the attic) of the flat section, you would need to vent.
eric himmer Posted: Mar 12, 2008 01:10 PM	I'm planning on spraying 4-5" of 2 lb foam. If I leave the fiberglass in the sloped (vaulted)sections and decide to spray the roof deck up to the ridge, I assume I should continue the prop-a-vent to the ridge vent and spray CC foam over the prop-a-vent (to let the fiberglass section breathe). If I do this, will there be any negative effect of having that small airspace between the prop-a-vent and the roof deck plywood ?
mason Posted: Mar 15, 2008 08:59 AM	Outside of losing some R value, this would be the approach to take. The fiberglass needs to breath.
eric himmer Posted: Aug 21, 2008 06:51 AM	Would removing the R-30 fiberglass on the diagonals and completely filling the diagonals with 12" open cell, then continue spraying the roof deck to the ridge with no venting be a better bet. 1) Do you think that replacing the fiberglass on the diagonals with open cell will be far more efficient for the amount of effort to remove the fiberglass? 2)The fiberglass in diagonals has kraft paper vaopr barrier and this will probably come out with the removal process. Therefore, the proposed 12" open cell will not have a vapor retarder. What is you guess on moisture concerns ?
mason Posted: Aug 24, 2008 09:56 AM	Question 1, Yes the open cell foam would be much more energy efficient than the fiberglass Question 2. Yes, the open cell foam would require a vapor retarder element in colder climates per code and per hygrothermal modeling. Check with your supplier for their recommendations. In many cases a coating can be used over the foam to reduce the moisture vapor transmission and in attics may also double as an ignition barrier. (must have code approvals on the specific foam and coating)

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Comments

eric himmer

Posted: Mar 11, 2008 07:00 PM

Attic Retrofit question

Will be insulating an attic of a house with vaulted ceiling below. On each side of the house are 45 degree diagonal sloped sides that lead to the flat ceiling. Each sloped side is 6' long and has prop-a-vent with fiberglass inside. Currently fiberglass is laid across the top of the flat ceiling section.

Without pulling out the fiberglass from the sloped sides, should I continue the pro-a-vent to the peak and foam over with CC 2 lb., cap the prop-a-vent and spray right to the roof sheathing, or just spray across the top of the flat ceiling sheetrock from above. Worried about moisture issues and feel insulating roof deck will be best place for the Foam. Located in NE. Any suggestions would be appreciated

mason

Posted: Mar 12, 2008 07:31 AM

You do not want to foam over fiberglass insulation. If you leave the fiberglass, you have to vent, if you spray foam to the underside of the roof deck, you do not require venting. If you spray the top of the ceiling (floor of the attic) of the flat section, you would need to vent.

eric himmer

Posted: Mar 12, 2008 01:10 PM

I'm planning on spraying 4-5" of 2 lb foam. If I leave the fiberglass in the sloped (vaulted)sections and decide to spray the roof deck up to the ridge, I assume I should continue the prop-a-vent to the ridge vent and spray CC foam over the prop-a-vent (to let the fiberglass section breathe). If I do this, will there be any negative effect of having that small airspace between the prop-a-vent and the roof deck plywood ?

mason

Posted: Mar 15, 2008 08:59 AM

Outside of losing some R value, this would be the approach to take. The fiberglass needs to breath.

eric himmer

Posted: Aug 21, 2008 06:51 AM

Would removing the R-30 fiberglass on the diagonals and completely filling the diagonals with 12" open cell, then continue spraying the roof deck to the ridge with no venting be a better bet.
1) Do you think that replacing the fiberglass on the diagonals with open cell will be far more efficient for the amount of effort to remove the fiberglass?
2)The fiberglass in diagonals has kraft paper vaopr barrier and this will probably come out with the removal process. Therefore, the proposed 12" open cell will not have a vapor retarder. What is you guess on moisture concerns ?

mason

Posted: Aug 24, 2008 09:56 AM

Question 1, Yes the open cell foam would be much more energy efficient than the fiberglass
Question 2. Yes, the open cell foam would require a vapor retarder element in colder climates per code and per hygrothermal modeling. Check with your supplier for their recommendations. In many cases a coating can be used over the foam to reduce the moisture vapor transmission and in attics may also double as an ignition barrier. (must have code approvals on the specific foam and coating)

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