# vapor barrier needed under xps foam on concrete.



## Nealtw (Jun 22, 2017)

Is this slab below ground level outsides? How much ad what is the frost level in your area?


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## jonnysteals (Jan 31, 2018)

Nealtw said:


> Is this slab below ground level outsides? How much ad what is the frost level in your area?


Since it was a converted garage I believe the slab is around 1 to 2 
inch higher than the exterior area. However my house is on the highest part of my lot and water drains very fast aw
ay from the house. I am in southern nj and from what i googled frost seems to be 30 to 36 inches but not positive.


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## Nealtw (Jun 22, 2017)

jonnysteals said:


> Since it was a converted garage I believe the slab is around 1 to 2
> inch higher than the exterior area. However my house is on the highest part of my lot and water drains very fast aw
> ay from the house. I am in southern nj and from what i googled frost seems to be 30 to 36 inches but not positive.


 If there is a perimeter drain it is at the footing or otherwise good drainage. 

How do you intend to support the ends of the joists, you talked about the middle.


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## jonnysteals (Jan 31, 2018)

Nealtw said:


> If there is a perimeter drain it is at the footing or otherwise good drainage.
> 
> How do you intend to support the ends of the joists, you talked about the middle.


I plan on essentially building a deck. I will be attaching a 2x10 ledger board to the framing and or block wherever it lands. If the ledger overhangs the concrete I will use pressure treated and the foam roll to ensure concrete and the wood doesnt touch. Than I will be using joist hangers for attaching the 2x10 joists. I am anticipating having to level each joist to the 4x6 because I am sure the concrete is not level. 

Any insight on the insulation strategy?


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## Nealtw (Jun 22, 2017)

jonnysteals said:


> I plan on essentially building a deck. I will be attaching a 2x10 ledger board to the framing and or block wherever it lands. If the ledger overhangs the concrete I will use pressure treated and the foam roll to ensure concrete and the wood doesnt touch. Than I will be using joist hangers for attaching the 2x10 joists. I am anticipating having to level each joist to the 4x6 because I am sure the concrete is not level.
> 
> Any insight on the insulation strategy?


The ground temp. is a constant around the mid 50s. Except in the winter when we we do a slab on grade, we have a choice 4" foam inside the foundation as deep as the frost depth Or the same foam laying flat under the slab next to the foundation. So frost depth width next to the foundation that is facing the exterior would be where to spend the money. Unless you are heating down there, the space will be in the mid 50s. May be after that out side edge insulating the joist would be better protection from the mid 50s for the floor. I wouldn't be afraid of putting down a vapour barrier right up the floor level and sealing the edges, no different than a basement wall.


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