Rigid foam in eaves w/vapor barrier


Old 09-10-10, 03:39 PM
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Location: Annandale, VA
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Rigid foam in eaves w/vapor barrier

We have a Cape Cod w/Eave access (about 5 ft out from the rooms upstairs). We saw most of our bills evaporate through the r-11 in the walls and the r-13 in the ceiling last winter. We're planning on putting a layer of r-30 in the ceiling next week to bulk it up before winter.

In the walls, we're going to pull out the existing r-11 (that has obviously endured some water damage in the past) and put in 3" mineral wool (partly for the sound insulation) and add 2" of rigid foam on the exterior wall in the eave to bulk the whole thing upto an r-24. If you follow me, the roof line continues to the floor of the eaves and we are insulating the exterior of the wall that seperates our rooms from the interior of the eaves.

My question is in reference to vapor barrier. I know that rigid foam at these densities can act as a vapor barrier, so I'm concerned with whether I should put 6 mil poly against the drywall, install mineral wool in the stud bay's and put on the rigid foam (sealed with sheathing tape and spray foamed around exterior gaps). Or just leave out the vapor barrier if I would be creating a wet space where moisture could not escape. Thoughts?

We live in Northern Virginia. Mild winters (normally not lower then teens) w/hot and humid summers.

Before I forget, Yes I will indeed install rafter vents to maintain airflow up to my ridge vent around the new insulation.

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Old 09-10-10, 05:21 PM
Join Date: Oct 2008
Location: New England
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Hi Scott,
When a thick layer of rigid foam is used, it is doubtful that the dew point will reach the other side. I would omit the vapor barrier but d a good job of air sealing.

Those side attics on a Cape have another problem area, it is the floor, or ceiling from below. Those cavities need to be well insulated and an air block added beneath the knee walls. Lacking the air block, cold air can pass from one side of the house to the other or simply circulate under the floors. I'll add a related link that may help.
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Old 09-11-10, 05:44 AM
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Thanks for the info. (and confirming my suspicion's about the vapor retardency of the foam) Good point about the ceilings. We just finished a total gut job on the first floor w/all new insulation and we were planning to insulate the floor as well in the eaves once we got the walls done.

As an interesting side question, the previous owner partially finished the eaves w/plywood floors, and I'm thinking we can just roll fiberglass out over top of it. Do you think there would be any problems w/that approach?

Old 09-11-10, 06:12 AM
Join Date: Aug 2009
Location: USA
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Yes. Bud's p[oint about the floor is that air will flow between the joists and under the floor of your living area. You need to air seal it with somthing like the foam you are using. Fiberglas batts alone will not stop air flow.
Old 09-15-10, 04:52 AM
Join Date: Jun 2007
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The eaves I found have to be sealed to stop air movement.My son had forced hot air removes and hot water baseboard put in.The next winter his pipes started to freeze.He had 6 in insulation in the 1 1/2 foot eaves when the new heat was put in.Air was getting by the insulation when ever there was a breeze of any kind.We had to remove the soffets (?) and use 1 in rigid foam and sealed with silicone to stop all air migration. the temp in the floor went from low teens to mid forty's.No more freezing pipes.The only problem now is the cold water it's that cold any more,need ice now.

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