Architectural dormer meeting hot roof – insulate roof or floor?
You guys have been a help so far and I need to ask another question which I haven’t been able to answer via web searches.
We have a 1.5 story ranch built ~1950, with masonry walls on the 1st story. The upstairs has two bedrooms, one of which we are replacing the insulation in right now. The room is almost completely gutted and I’m planning a hot roof approach with rigid foam hacked and stacked between the 2×6 rafters with 2″ foil polyiso under the rafters. Right now, we’re chasing down some recycled foam to go between the rafters to lessen the overall cost.
After we tore out the ceiling, we discovered that the neighboring attic spaces had old/little insulation also (suprise, suprise). So, now we’re trying to figure out the appropriate/effective way to deal with them.
The roof line of the house is typical in the front, minus a small extension (~10 ft) of the living room which has a dormer built into the roof to provide overhand. This portion is unvented and the inside is rather caverneous. I’m concerned about air sealing, mostly because of the masonry walls, but I don’t know if it is easier to install roxul batts (or loose cellulose or a combo) to the dormer floor joists or insulate the rafters instead (which seems to be a waste).
On the back side of the house, a second lower sloped roof line was built off the primary about halfway down. There is insulation laying on “most” of the floor joists in this tight area in the attic. I’m not quite sure how to insulate this space from the attic, other than blowing in loose cellulose.
Any help would be appreciated.
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Replies
Nick,
I'm having a hard time visualizing some of your descriptions and I'm not sure of your climate zone...
But considering that it sounds like you will be doing a "hot roof" assembly, I would choose an approach that allows you to install the most effective air-barrier you can manage.
If possible, first start by thinking of how you might get a continuous air barrier and then move on from there.
I'm a bit stuck on the air barrier. I'm sure we could squeeze into the dormer in the front and put up some sheets of foam/wood and seal it up. The back of the house is where I'm really struggling for a solution that doesn't involve tearing out ceilings. I was thinking about pulling down the bottom of eaves for access.
Here is a link to a picture of the house. http://t2.gstatic.com/images?q=tbn:ANd9GcS_zuyujXlFYf6omQUuW0G1QAmhJxre_kErZ8VGHIkdADmaeXNx
The left side of the picture has the living room with dormer.
I don't have a picture of the side of the house. But basically it looks like they made an addition to the house at one point in time to extend the backside by ~15 ft (but I don't think it was built during the original build.
Nick,
It's tough to say what your best options are...
If I understand you correctly, you are saying that portions of the roof have an attic-type space while other portions do not?
Are the attic-type spaces vented?
What climate zone are you in?
It sounds like the roof geometry and structure are pretty complex, so there are probably no easy answers for a good air-sealing strategy...
Is there any existing air barrier (ie poly "VB")?
Nick,
It's almost always better to insulate the floor of an attic than to try to insulate a sloping roof. If you have access, get in there and seal any air leaks in the attic floor with canned spray foam and rigid foam board. Then install a deep layer of cellulose insulation.
If you can't figure out how to gain access to these areas, you may need to call in an insulation contractor.
As Lucas pointed out, you need to pay attention to air barrier continuity. Make sure that the air barrier of your hot roof is continuous with the air barrier of your insulated attic floor.