garage converted to family room vapor barrier


Old 06-02-05, 08:13 PM
edsjr's Avatar
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Join Date: Sep 2001
Location: Springfield IL
Posts: 76
garage converted to family room vapor barrier

I bought a home that at one time had a 1 car garage, several years ago it was converted to a family room.

I have mostly gutted the room and want to do something to eliminate the moisture smell in the house during the summer. The whole house has always had a "strange" smell when the A/C was on. Last summer when I started gutting this room (I move slow) it got considerably worse. I am assuming the subflooring was acting as somewhat of a vapor also had tar paper between the subfloor and the floor.

I will be adding a laundry room to one end or the family room and making the other end a small home theater. The water supply is run up and in the floor joists for the upstairs to prevent freezing.

What all do I need to do to seal this? Here is what I had planned....

1. Add a foundation vent.....any tips on this would be appreciated, the foundation is block.

2. Covering the concrete with thick plastic and stapling it up the sides as much as I can, probably holding it down with concrete blocks.

3. Paper faced insullation in the floor joists

4. Black paper between the subfloor and floor.

The original subfloor was 1/2" sawdust board. I will be using OSB for most, there will be a walkway to the back door of the house which will be 3/4" hardwood so I will use 1/4" plywood under it and the laundry room will be ceramic tile.

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Old 06-04-05, 04:28 AM
Join Date: Oct 2004
Location: New Jersey
Posts: 1,132
I cannot tell you what to do about the entire area, but as for the tiled bathroom, use either 3/4 ply or osb, though ply would be better, and with ply, use bc or better graded ply, no cdx or sheating rated ply. Then go over it with cement board set in unmodified thinset. You don't want any 1/4" plywood anywhere under ceramic. I'm assuming this garage floor (slab) had joists setting on it, then a subfloor installed, so if height is a concern, you can cut down the size of the joist under the bathroom area to gain some additional available height for the flooring. Deflection of the joists is not an issue if it is resting on a slab. As for the rest of the questions, you will have to wait for someone else to come along for that.
Old 06-04-05, 06:35 PM
Join Date: Aug 2000
Location: USA
Posts: 15,834
Although the contractor may have installed vapor retarder beneath concrete for basement or slab in structure, many skip this vapor retarder in the garage because it is not intended for a finished area for living space. Many garage conversions take place and the vapor issues are a problem. Vapor emissions can cause mold/mildew problems and associated odors.

You state that you had a 'whole house odor problem when AC was running.' Please have the AC serviced and cleaned to assure the odor is not originating from the unit. This is a concern because the odor problem is not isolated to the garage conversion area.

Your post is not clear as to the construction of the garage, whether built up above crawl space area or what. You mention joists? Most garages are built on slabs.

If garage floor is elevated with block foundation beneath, then access to the area is a must for exterminator inspection and inspection for moisture and other issues. Soil should be covered with plastic ground cover to prevent moisture emissions. Ventilation with no dead air spaces is also recommended.

Are you describing a raised floor over concrete and asking about insulation between what you are describing as joists? Polypropylene vapor retarder is recommended to cover concrete to prevent vapor emissions before installing sleepers for raised floor.

If you are planning on installing solid hardwood, go to for installation instructions. There are two options: installing 3/4" exterior grade plywood over vapor retarder or sleeper system over vapor retarder. Concrete slabs below grade are not candidates for solid hardwood installation. Engineered wood (laminate) products can be installed there.

You need to do a moisture test of the concrete. To test any slab for excess moisture vacuum a small area, then tape a 15 inch square piece of clear polyethylene film to the slab using two strips of duct tape along each of the four edges. If no condensation or fog develops under the plastic after 48 hours, the slab is sufficiently dry. If the slab is wet, then heat the space and open windows or use a dehumidifier to draw moisture from the slab. Retest the floor for moisture content once you think it has dried. Concrete must be flat. Use a 6 foot long straightedge to check for high and low spots. Standard asphalt shingles make excellent shims to fill in low spots. The vapor barrier can be installed over the shingles and then the wood on top of that. Fasten plywood or 2x4s to concrete with concrete screws in pre-drilled holes with powerful screw gun.

Temperature and humidity should be under normal occupancy levels for at least a week before flooring is delivered. Temperature should be around 70 degrees. Humidity between 35-55% (measured with hygrometer & sold where they sell thermometers). Wood can then be delivered and place in room where it is going to be installed, no higher than 3 boxes, to acclimate for 5-7 days.

If installing solid hardwood, go to for instructions. If installing engineered wood (laminate) then follow the manufacturer's installation instructions. Each manufacturer tends to have its own very specific instructions in order to maintain warranty. Even so, temperature, humidity, and acclimation recommendations still apply.
Old 06-05-05, 08:41 AM
edsjr's Avatar
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Join Date: Sep 2001
Location: Springfield IL
Posts: 76
I guess I should have explained the concrete better.

The garage floor sits about 14" below the floor of the house and has a crawl space next to it. the family room floor is on joists that sit off of the concrete. So there wont be any flooring directly on the concrete and if I put a vent in there should be some air flow to keep it dry.

The foundation for the house is block. The concrete is just about level with the top of the second block down on the foundation and the top block can be seen from outside the house.

The insulation I was speaking about is going between the joists that sit above the concrete floor.

hope this clears it up a little


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