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Old 03-23-2019, 07:29 PM   #1
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Floor insulation, no subfloor


I have a home that was built in the 1950's that has no floor insulation in a vented crawl space. Flooring is all hardwood, with tile in the kitchen and bathroom installed over the hardwood. As was common at the time, the hardwood floor was installed directly onto the floor joists, meaning no subfloor. 15lb felt paper was layed onto the floor joists before the flooring was installed. The felt paper is what you see when you look up in the crawl space.

I would like to insulate between the floors with bat insulation rolls rather than encapsulate the crawl space, so I have a couple of questions that need to be answered. 1) Can I even consider insulating between the floor joists given that I have no subfloor 2) If I can insulate this way, do I need to remove the felt paper to ensure contact between the paper side of the insulation and the flooring above? 3) would removing the felt paper create a moisture issue for the flooring above?
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Old 03-23-2019, 08:14 PM   #2
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Re: Floor insulation, no subfloor


The subfloor is considered a vapour barrier, in your case I would think the paper is doing that job. I would fill the space with rock wool, fiberglass would be second choice.
I would also back the up with a window bug screen to keep insects out and back that up with a wire mesh to help hold the insulation in place and keep critters out.

The dirt floor should be covered with 10 mil poly and seal to foundation and any other opining like pipe.
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Old 03-23-2019, 08:34 PM   #3
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Re: Floor insulation, no subfloor


Would you keep the felt paper in place? I was under the impression that the insulation had to make contact with the subfloor. While the felt acts as a subfloor/vapor barrier, it does not hang tightly against the flooring above. There is space between the felt paper and the flooring.
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Old 03-23-2019, 08:40 PM   #4
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Re: Floor insulation, no subfloor


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Would you keep the felt paper in place? I was under the impression that the insulation had to make contact with the subfloor. While the felt acts as a subfloor/vapor barrier, it does not hang tightly against the flooring above. There is space between the felt paper and the flooring.
I would leave it some people leave a space. something allowing floor to stay warmer, not sure where the heat comes from. I doubt a space there will hurt anything. You want to be careful doing anything with the hard wood, what we don't know can bite us.
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Old 03-23-2019, 08:57 PM   #5
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Re: Floor insulation, no subfloor


Forgot to ask recommended R value. I live in northern Kentucky, top end of zone 4.
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Old 03-23-2019, 09:03 PM   #6
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Re: Floor insulation, no subfloor


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Forgot to ask recommended R value. I live in northern Kentucky, top end of zone 4.
How many inches do you have to fill? that much.
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Old 03-24-2019, 06:50 AM   #7
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Re: Floor insulation, no subfloor


To answer the heat question, a floor is heated by radiant transfer. All surfaces exchange heat based upon their different temperatures, house to floor and floor to basement soil.

As for the felt, it is a poor vapor barrier, actually classed as a class II or class III vapor retarder depending on the weight of the felt. Plastic sheeting is a class I.

Some good reading here.

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Old 03-24-2019, 11:01 AM   #8
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Re: Floor insulation, no subfloor


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To answer the heat question, a floor is heated by radiant transfer. All surfaces exchange heat based upon their different temperatures, house to floor and floor to basement soil.

As for the felt, it is a poor vapor barrier, actually classed as a class II or class III vapor retarder depending on the weight of the felt. Plastic sheeting is a class I.

Some good reading here.

Bud
So what I gathered from reading this article is do not do fiberglass roll insulation unless you put a rigid barrier under it. This is a catch22 for me. My crawl space entrance is so small that getting foamboard in is not really practical.
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Old 03-24-2019, 11:24 AM   #9
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Re: Floor insulation, no subfloor


Save me going back and reading the entire thread, but "why". What is your objective, warmer floors, lower heating costs, other?

I do energy auditing way up here in Maine and heat loss through our floors is only part of the energy costs, of course few homes up here are over a crawl space and NONE of them are unvented. So I suspect that insulating those floors will do little to reduce your heating costs.

If cold floors are the problem, air leakage is probably the major issue and that needs to be taken care of even if you go ahead with some form of insulation. Any cold air that leaks in drop right to the floor thus drafty and cold.

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Old 03-24-2019, 11:37 AM   #10
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Re: Floor insulation, no subfloor


Warmer floors during the winter is the objective. I have hardwood floors nailed directly onto the the joists, and so I imagine cold air gets through the gaps between the hardwood planks, or the heat escapes the same way.
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Old 03-24-2019, 12:06 PM   #11
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Re: Floor insulation, no subfloor


AND there is no wall insulation yet--going to hire a pro to blow in cellulose.
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Old 03-24-2019, 08:22 PM   #12
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Re: Floor insulation, no subfloor


I would consider installing foam board up between the floor joists and against your felt paper. You can rip it to width in the yard so that you can fit it through your opening. Once the foam is in place against the felt, I would use cans of spray foam to air seal between the foam and the joists. What you do next depends on your priorities. You could fill the rest of the space between the joists with fiberglass or rock wool. Or see how the one layer of foam works.
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Old 03-25-2019, 11:54 AM   #13
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Re: Floor insulation, no subfloor


Why don't you want to encapsulate the crawl space?

Floor insulation doesn't work well due to thermal bridging and air movement.

In NY you have humid summers, vented crawl spaces have moisture problems, the venting makes it worse.

If you have ducts in the crawl space you can see very significant energy savings making it a semi-conditioned space.
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