properly insulating/finishing an attic


Old 05-23-12, 11:47 AM
Thread Starter
Join Date: May 2012
Posts: 7
properly insulating/finishing an attic

Hi all,

I have a bit of a situation with a half-finished attic. The previous owners tried to insulate with batts with up against the underside of the roof deck, which apparently is a big no-no. I've heard there either needs to be an air gap between the deck and the insulation or that a closed cell spray foam needs to be used.

I would just rip out the batts, seal it up, and blow cellulose on the floor, but they went to the trouble of ripping a big hole between the stories and building a proper set of stairs, so going back in time won't be that easy.

The house is likely balloon framed and, strangely, has stuccoed soffits.

Here is a gallery of the attic:

I'm curious to know what my options are, both for continuing to finish the space and for returning the space to an attic.

Thanks so much for your help!-c

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Old 05-23-12, 01:38 PM
Join Date: Jan 2009
Location: PA
Posts: 132
Are you going to be using the attic for anything other than storage?

You should have baffles on the underside of the roof with insulation against that.
Old 05-23-12, 01:51 PM
Join Date: Aug 2009
Location: USA
Posts: 4,941
Well. Do you want to finish the attic, or return it back to attic status?

You should go to building and read about unvented roofs. You can use the search feature instead of wading through all the other stuff to find it. The other stuff is very useful as well. You will want to learn all about water vapor and vapor barriers.

Here is the concern when insulating the roof. You need to keep water vapor from coming in contact with the roof deck and condensing. In the winter, your roof is cold and the air in the home is warm and moist. When that warm moist air hits the cold roof you get condensation. You can deal with that a couple of ways. You can insulate in a traditional manner that provides ventilation under the roof deck that removes that water vapor, or you can go with an unvented assembly.

To provide ventilation, you need to have vents at your soffit and at your ridge. You then have to provide and airspace between the insulation and the roof deck. This is going to be a challenge for you because you have a hip roof and those rafters bays on the ends won't be ventilated.

My suggestion is to use closed cell spray foam. CCSF becomes a vapor retarder at 2 inches of thickness. It also air seals and prevents any type of wind from blowing through. Fiberglass batts cannot do this. And when the wind blows through those batts, you lose most of the R-value. Because of the air sealing, you won't need much more than 3 inches of foam. That will block 95% of conductive heat loss. Now people have concerns about a roof leak. Reading through one the papers on, they did talk about that. They said that the CCSF localizes the leak to a much smaller area. So yes, you can have some rot occur, but it isn't a large part of your roof that needs to be repaired.
Old 05-23-12, 02:13 PM
Thread Starter
Join Date: May 2012
Posts: 7
I think I want to use it for storage but have the option of finishing it in the future. Would the spray foam be appropriate for that?

I guess it isn't clear to me what I would do differently, storage versus finishing, in terms of insulation.
Old 05-23-12, 02:54 PM
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Join Date: Oct 2004
Location: WI/MN
Posts: 18,651
If this is going to be living space, you insulate the ceiling of the attic. If this is not going to be conditioned space, you put the insulation in the floor of the attic.
Old 05-23-12, 03:05 PM
Thread Starter
Join Date: May 2012
Posts: 7
It is going to be living space then. I am not interested in blocking the stairs and having cellulose on the floor again.

From what I'm hearing, then, it sounds like CCSF is probably my best choice.

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