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Old 01-14-2009, 12:01 PM   #1
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Insulate between roofing joists?


Hello,

I have an older house, about 80 years, with no insulation. I hope to be able to use low expansion foam to insulate the walls.

The upstairs is two finished rooms, dormer style. The ceiling in a portion of the upstairs is actually the bottom of the roof rafters.

Can I put foam insulation here, or am I asking for trouble because it will no longer be ventilated?

--Robert in CT

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Old 01-14-2009, 12:57 PM   #2
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Insulate between roofing joists?


foam is a bit too expensive. Use unfaced R30 batts. do not stuff along the edge of the roof to avoid blocking the soffit vents. or get someone to blow in insulation, which is mouse resistance. These critters will love to live in fiberglass.

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Old 01-14-2009, 01:14 PM   #3
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Insulate between roofing joists?


You are right about the cost of foam, but it isn't completely outrageous.

The main advantage is that I can pump it in, by cutting smaller access points in the plastic ceiling. This is what I'm considering and asking about.

This stuff really covers and fills in every nook and cranny, which is the reason I like it, as it won't leave gaps somewhere.

But, I'm worried about moisture, as the foam would not only be touching the back of the roofing deck, but also the plaster ceiling.

--Robert in CT
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Old 01-14-2009, 01:50 PM   #4
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Insulate between roofing joists?


You need to maintain a minimum of 1-1/2 inches of clearance between the top of your insulation and the bottom of the roof deck for air flow. That will help to prevent any condensation issues on the under side of the roof.
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Old 01-14-2009, 01:53 PM   #5
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Insulate between roofing joists?


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Originally Posted by Maintenance 6 View Post
You need to maintain a minimum of 1-1/2 inches of clearance between the top of your insulation and the bottom of the roof deck for air flow. That will help to prevent any condensation issues on the under side of the roof.
the use of foam avoids this need. Check out specs of sites like icynene
Can foam will not do it. You cannot control the expansion rate. Some kits for icynene type are available and will work for this application. I did not realize you had not attic access.
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Old 01-14-2009, 01:54 PM   #6
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Insulate between roofing joists?


Aha! ... so it is space between the bottom of the roof decking and the ceiling, or insulated ceiling, that i need to deal with. I guess I wasn't sure where air needed to be.

What you say makes perfect sense.

How about those styrofoam/insulation panels? I could cut an access point and slide them down between the rafters. I'm thinking that could work.

So, what physics are at work here where we need the air space at the roof, but it's OK to fill a wall cavity?

--Robert in CT
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Old 01-14-2009, 02:03 PM   #7
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Insulate between roofing joists?


no neither cavity is filled. Everything in a house needs to breath, but at the same time you need to control water moisture. Insulation is used to diminish not block air flow. The vapor barrier is used to stop the flow of water moisture but not air. When air moves between cold and hot surfaces condensation happens. This is one of the many issues you need to deal with. (many others) The not blocking at the wall is because the soffit is vented so air can pass over the insulation and under the roofing to "vent it" Thus the moist air is carried out the ridge vent or gable vents and does not sit there rotting the rafters. For insulation to be effective, a tight seal must be made. sliding foam insulation will not cut it. What exactly is the ceiling and roof structure. Flat, access to attic What?
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Old 01-14-2009, 07:01 PM   #8
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Insulate between roofing joists?


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You need to maintain a minimum of 1-1/2 inches of clearance between the top of your insulation and the bottom of the roof deck for air flow. That will help to prevent any condensation issues on the under side of the roof.
Unless you incorporate a Hot Roof System. My brother-in-law did this to his new home a few years ago and he's completely satisfied with his choice and how energy efficient his home is.
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Old 01-15-2009, 06:44 AM   #9
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Insulate between roofing joists?


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Unless you incorporate a Hot Roof System. My brother-in-law did this to his new home a few years ago and he's completely satisfied with his choice and how energy efficient his home is.
There is a whole different set of physics at work on a low slope hot/cold applied membrane roof. The insulating factors are applied differently than a shingled roof. And improperly installed, low slope roofs can have condensation issues too. To the OP, warm air holds more moisture than cold air. If warm air strikes a surface that is below dew point, such as the underside of a cold roof, then the moisture will condense on that surface. Proper insulation with an effective vapor retarder will keep warm moisture laden air from reaching the cold surface, however there is always a small percentage of vapor that can reach the underside of the roof, through the vapor retarder or through cracks, etc. Air flow under the roof will keep both the top and bottom surface of the roof at relatively the same temperature. If both sides of the roof are the same temperature, moisture cannot condense.

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