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Old 11-27-2010, 10:28 PM   #1
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How do I install insulation to floor joist setting on a concrete slab


I had a new addition put on a few years back 24' X 24' sits on a concrete slab and the other 24' x 24' is over a crawl space. They didn't insulate either except for the walls of the crawl space. It is very hard to access the slab portion from the crawl space. The qustion is how do I know go back and try to insulate that floor on the slab. Can I spray foam both ends or is that accomplishing nothing. I would like to get the slab done before I start on the crawl space. Help!!!! They did form the inside of the slab with 2" blue foam board if that helps any. They told me that it would stop the cold from transferring through.

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Old 11-28-2010, 01:54 AM   #2
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How do I install insulation to floor joist setting on a concrete slab


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I had a new addition put on a few years back 24' X 24' sits on a concrete slab and the other 24' x 24' is over a crawl space. They didn't insulate either except for the walls of the crawl space. Crawl spaces should be insulated that way, ie, make them conditioned space, generally. Is there a good vapor barrier on the ground of the crawl space, taped to the walls and sealed w/ something like Tremco Acoustical sealant? That is important.

They did form the inside of the slab with 2" blue foam board if that helps any. That will help w/ the cold a lot, if by "form" you mean under and at the edges of the slab. Can you condition this space, too, like the other area? I assume there is a vapor barrier under the concrete? I hope so.
See after the bullets. john

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Old 11-28-2010, 10:16 AM   #3
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How do I install insulation to floor joist setting on a concrete slab


They did not put down plastic and that is something I'm looking at doing in the next few weeks. I am also looking at adding insulation to the bottom of the floor in the crawl space. I did go back in and put 2" foam at the end of each floor joist on the outer edges. I still need to go back and seal around the edges of those.

The concrete slab had form around the inside foundation but I do not think they stuck any under the slab itself. They were supposed to use a vapor barrier but I was not home during the pour and I have my doubts. It is really hard to get to the area where the slab and crawl space meet but I was wondering if I should try and petition off that section from the crawl space. The floor above the concrete isn't to bad but above the crawl space it gets very cold.
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Old 11-28-2010, 11:24 AM   #4
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How do I install insulation to floor joist setting on a concrete slab


You neglected to say where you live and this has a bearing on how to insulate.
In my area R10 (2" foam) isn't adequate. You should add another 2" of foam on top of this, if your temperature go below 45F.
Your crawl space needs to have a heat supply and a cold air return as well.
As for the section on the slab, assuming there is a vapor barrier under the slab, I would have cellulose insulation blown in there.
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Old 11-28-2010, 03:46 PM   #5
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How do I install insulation to floor joist setting on a concrete slab


I live in Southern Indiana. I thought about doing the cellulose but was not sure if it would mildew. The concrete was originally put in for a large covered patio but I had them pour it as if I was going to build it in because that was my future plans. I have never had my pipes freeze (knock on wood) but it is a ventilated crawl space.
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Old 11-28-2010, 11:35 PM   #6
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How do I install insulation to floor joist setting on a concrete slab


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I live in Southern Indiana. I thought about doing the cellulose but was not sure if it would mildew. The concrete was originally put in for a large covered patio but I had them pour it as if I was going to build it in because that was my future plans. I have never had my pipes freeze (knock on wood) but it is a ventilated crawl space.
I know your area! In fact I spent some time snowed in, in a truck stop outside of Indianapolis.
Your weather isn't much different than mine.
Here, we don't vent crawl spaces, we insulate the foundation walls and condition the space.

Heat loss can never be completely stopped. The term 'R value' is a measure of the rate of loss. The rate of loss can be reduced, by adding insulation, but is never stopped.
So, if heat is not added to replace this loss, eventually an enclosed area will eventually drop to the temperature of the outside air. In spite of the insulation.
To have warm floors you must have a heat supply to the crawl space. The wall insulation merely retards the heat loss and doesn't add anything to warming the floors.

Because you don't have a vapor barrier under the slab insulation over it could be a problem.
In this case it may be best to bury vertical slabs of 4" foam all the way around the outside of the perimeter. You can buy 24" sheets so dig a 2 foot trench, place the foam in and back fill.
The foam should cover the edge of the slab and as its exposed to the sun, it should be covered with an aluminum cap.
Is ther anyway that some heat can be allowed to circulate into the slab area? If so, this should be down also.
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Old 01-25-2011, 11:53 AM   #7
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How do I install insulation to floor joist setting on a concrete slab


Timmy, how much space do you have between your actual floor and the concrete slab?

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