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Old 01-27-2020, 07:24 PM   #1
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vapor barrier needed under xps foam on concrete.


I have a house with a converted garage into a living room. It is sunken living room with a 2 steps which is around 13 inches below the rest of the house. I want to raise this up as its a safety hazard with young children. The room currently has carpet right over top of the concrete and the steps are made of wood and has been this way since the early 80's ( we have only lived here 3 years). I will be installing 3/4 oak hardwood on top of the sub-floor.

My plan is to raise the flooring with a 2x10 spanning 11 feet. I will have a 4x6 pressure treated supporting the 2x10's directly in the middle of the 22 foot span. I plan on using advancetec 23/32s sub-floor so the 2x10 and sub-floor should take up 10 inches leaving around 3 inches or air space under the joists. This area has no vents currently and is a concrete floor and cinder block walls.

I plan on installing Owens XPS foam board directly over the concrete and butting up to the 4x6 middle support wall. I read that it is a vapor retarder but Owens recommends a 6 mil vapor barrier under the foam. This foam will not hold any weigh but only act an insulation and prevent moisture from rotting the subfloor.


https://www.homedepot.com/p/Owens-Co...20WE/207179253
Owens instructions for xps over concrete for hardwood flooring.
First it is important to make sure the floor is completely dry, and free any debris, also free from any major cracks or uneven sections.
1. Start by covering the floor with a 4 to 6 mil poly as a vapor barrier. (IF there is not one previously installed under the slab)
2. Lay Foamular and make sure joints are tightly butted.
3. Apply either tongue and grove or 2 layers of 7/16 OSB with off-set seams.
a. (Foamular is not structural and will not prevent punctures from furniture legs/feet, so a subfloor such as OSB is required.)
b. This is to prevent a large amount of weight hitting a seam in the subfloor and adding higher than expected weight through to the Foamular potentially compressing the foam at that seam.
4. Install the final flooring. If you have any further questions, please feel free to contact a representative at 1-800-GET-PINK. Thank you for your inquiry!
Thank you for your inquiry!

Here is the vapor barrier I plan on using. https://www.homedepot.com/p/SimpleSo...B&gclsrc=aw.ds

My questions are this.
1) Do I need to add vents to cinder block? I am treating this as an encapsulated crawlspace.
2) I want to ensure I have no mold issues. Is this the proper way to insulate the floor to ensure the sub-floor has no issues?
3) Would you install both the vapor barrier and the owens xps foam board as I have no clue if under my slab I already have a vapor barrier I plan on using one as its cheap insurance but reading online it seems most people dont recommend the vapor barrier under the xps. https://www.contractortalk.com/f3/fr...ncrete-139786/
4) I plan on using 1 inch xps and not 2 inch as its just mainly to protect my subfloor, Would I be better off with 2 inches or should I save the 150 bucks?
5) Should I take the seams of the Owens xps and spray foam the xps board to the cinder block walls?

Thanks so much for all the input in advance.
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Old 01-27-2020, 08:54 PM   #2
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Re: vapor barrier needed under xps foam on concrete.


Is this slab below ground level outsides? How much ad what is the frost level in your area?
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Old 01-27-2020, 09:05 PM   #3
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Re: vapor barrier needed under xps foam on concrete.


Quote:
Originally Posted by Nealtw View Post
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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Old 01-27-2020, 09:24 PM   #4
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Re: vapor barrier needed under xps foam on concrete.


Quote:
Originally Posted by jonnysteals View Post
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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https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
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Old 01-28-2020, 06:28 AM   #5
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Re: vapor barrier needed under xps foam on concrete.


Quote:
Originally Posted by Nealtw View Post
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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Old 01-28-2020, 07:29 AM   #6
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Re: vapor barrier needed under xps foam on concrete.


Quote:
Originally Posted by jonnysteals View Post
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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https://www.fema.gov/media-library-d...6/chapter7.pdf
figure 7-14
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concrete , subfloor gap , sunken living room , vapor barrier , xps


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