Insulate plywood subfloor


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Old 05-31-15, 07:31 PM
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Insulate plywood subfloor

Hi - I have a large room. Has no basement just a 4 foot crawl space that is open on the sides. I don't want to seal off the crawl space and I don't want to insulate the sub-floor from under the house.

I would like to add insulation to the plywood sub-floor internally - but I have only 2" to play with.

What do you think I can do to insulate the floor before putting on the final flooring?

1/2" insulation board?

Looking for some ideas - and cheap. Final flooring might be local cut and seasoned t and g pine 10" boards.

Thanks,

Dean
 
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Old 06-01-15, 03:38 AM
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Why not do it right and only do it once? Insulate from beneath the floor with R19 or so and wire "stays"? Insulating above floor joists will cause anything above it to be mushy and any flooring above it will fail in time. Basically you can't insulate above the floor.
 
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Old 06-01-15, 04:03 AM
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I agree with Larry! You could install strapping and insulate between the straps and install another layer of plywood but the insulation would be minimal - best to address it from below!
 
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Old 06-01-15, 04:35 AM
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Thanks for the replies. There was once fiberglass insulation under the floor - over time, storms and many rodents (this house is in the woods) - this insulation had fallen down and become infested with mice. I removed it. Which is why I started thinking about insulating from inside the room - but I hear your information - so I am open to what is best.
 
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Old 06-01-15, 04:36 AM
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Hi Dean,
"Cheap" is why we have "minimum" codes. Your minimum looks like about R-19 for the floor, but depending upon where in NY you are I would recommend a complete cavity fill and then cover the bottom with 1/2" plywood to keep the critters out.

The big question is, what is down there? Water pipes, heat ducts, and drain lines are very susceptible to freezing. If it is just a big room, no plumbing, then 4' if more than enough room to work under there and a great opportunity to make that room comfortable.

If you search some of the living space over a garage threads on this forum you will see heating a room over a cold space is difficult as the heat in that room is constantly being pushed up to the ceiling and displaced by the cold air at the floor.

Here are your related codes. https://energycode.pnl.gov/EnergyCod...ate=New%20York

Bud
 
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Old 06-01-15, 07:03 AM
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Thanks Bud that is very helpful. I am convinced that insulating from under the floor is best. I was just looking, hoping, to be able to do it from the inside the room - for all the obvious reasons - easier to work inside than under the room, water pipes, electrical, critters getting in the insulation.
 
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Old 06-01-15, 10:05 AM
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How open is open? If this sits on posts, once you fully insulate the floor there will be very little heat escaping downward. I realize that is what you want, but the ground under there has been benefiting from the radiant heat and limiting the frost depth. Whatever is being used to support that big room, are the supports below the frost line?

In addition to insulation and a plywood covering for the critters, you will benefit greatly by air sealing everywhere you can. Any air leaks will pass through your insulation reducing its effectiveness as well as being a source of cold air.

Water pipes will need more than just heat tapes and the soil they pass through will need to be kept above freezing. Rigid foam insulation just below the surface (4' square) will allow the soil below to remain warmer than the surrounding. It might require more than that but you get the idea.

Bud
 
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Old 06-01-15, 10:16 AM
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Room is supported by brick posts on sloping land - 4-5 feet from joist to ground along one side and 2 feet at the other. It gets real cold - this place is at 3,000 feet in upstate NY. So it is quite open. Well - what is supporting the room might or might not be below frost line - I can't be sure - but it sure gets frozen as for months at a time there is no heat at all in that room and it goes below zero sometimes. Actually the water pipes run internal - it is just thew drains that run outside.

I am thinking of enclosing the crawl space between the posts with 2 x 4 panels, insulation in the panel, landscape cloth and lattice as covering - do you think that would make a difference - what kind of difference?
 
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Old 06-01-15, 10:28 AM
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If you review FPSF, frost protected shallow foundations, you will see their approach to protecting footings that are not below the frost depth. Brick posts on sloping land could be a long term issue as frost tends to move everything downhill. Probably not enough slope in your case to be a problem.

I have a camp still on my drawing board where I want it supported above ground similar to yours. I have planned for full cavity fill with insulation and tape or battens to cover all plywood seams. My problem is living long enough to get to building it. I need about 2 years with no new projects

Bud
 
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Old 06-03-15, 04:49 AM
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I am thinking I will enclose the crawls space as a first step. And put a finish floor on top of the plywood sub-floor.

Finish floor - thinking it might be local pine tongue and groove boards. What kind of underlay can I put under the boards on top of the plywood sub-floor? I am thinking a roofing felt #15 or #30 - any thoughts? If I could use an underlay that also gave a little insulation I would use that. . .
 
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Old 06-03-15, 04:53 AM
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15# felt is pretty common under hardwood floors. If you underpin the room you need to have a plan for any moisture/humidity that might get trapped under there. Foundation vents have long been used to accomplish that although there is a trend towards laying/sealing plastic on the ground and insulating the walls [no vents]
 
 

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