Insulating an Exposed Floor

Insulating an Exposed Floor

Insulating an Exposed Floor

Why are there signs of condensation in the insulated joist bays of this house on piers?

With cavity insulation pulled aside, Mr. Mike notices some condensation has collected in an exposed floor system of an addition to this house. Is this a problem worth worrying about?

A GBA reader who calls himself “Mr. Mike” is working on an 11-ft. by 14-ft. addition to his house in central New York that sits some 5 feet off the ground. The space beneath the addition is a great place to park a lawnmower, but it’s also open to the cold.

The floor joists are supported by two beams, each a tripled 2×10. To insulate the floor, Mr. Mike put two layers of R-13 unfaced fiberglass batts in the joist cavities, ran a 3/4-in. layer of Cellofoam foil-faced rigid insulation over the bottoms of the joists, then covered the insulation with 1/2-in. oriented strand board (OSB) to keep out pests. The subflooring is 3/4-in. tongue-and-groove plywood.

All seemed to be well until Mr. Mike got ready to install strip flooring in the addition.

«When I pulled the existing carpet up to redo the floor, I made a couple of access holes to inspect the cavity and found signs of frost and condensation on the foam insulation — but only in certain areas, closer to where the addition is attached to the house,» Mr. Mike writes in a post at Green Building Advisor’s Q&A forum .


The flooring manufacturer has recommended a layer of #15 asphalt felt between the strip flooring and the subfloor, but Mr. Mike is concerned this may trap moisture in the floor system, making the situation worse than it already is.

Should he remove the foam and re-install the OSB? Add more foam to the bottom of the floor system? Insulate the tops of the 2×10 beams with foam?

Those questions are the start of this installment of Q&A Spotlight.

Make the foam thicker and seal the air leaks

To David Meiland, this problem seems to be caused either by the insufficient R-value Measure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the rigid foam insulation on the underside of the joists, or by air leaks. If there are air leaks between the house and the addition, they could be the source of the moisture that seems to be condensing inside the joist cavities. The problem could be made worse by a forced-air heating system that’s slightly pressurizing the house interior.

As to the foam, Meiland says, «it should either be omitted or made much thicker.»

And this makes sense to GBA senior editor Martin Holladay. «This is either an air sealing issue, or perhaps the home is being pressurized,» Holladay says. «In any case, it’s possible that the condensation is a seasonal phenomenon, and that the moisture will dry harmlessly as the weather changes. (The floor system will have some air exchange due to fluctuations in outdoor temperature.)»

Holladay suggests that Mr. Mike read an article he wrote for Fine Homebuilding magazine about insulating floors over cold spaces.

Mr. Mike replies that he doesn’t have a forced-air heating system. «I think I have air leakage on the sill/rim joist from where the addition is attached to the house where the warm air from the basement [can] enter the floor system,» he adds. «The moisture does seem to be more in that area, and I was considering installing rigid foam against it on the addition side.»

He has sealed some of the OSB seams with caulk, but he suspects cold air may be leaking into floor system in the area of the 2×10 beams.

Holladay notes, «When you have wintertime condensation in a cold area of your thermal envelope, the source of the moisture is usually interior air, not exterior air. (Exterior air is dry; interior air is moist.) It is unlikely but possible that there is a path for warm, moist, interior air to reach the cold rigid foam. However, the stack effect Also referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. usually prevents this kind of air leakage. It is even possible that there was no condensation until you opened up the floor. Once you opened up the floor, interior air had access to the cold foam (for the first time).»

Use felt under the floor

Mr. Mike’s fears that the asphalt felt will trap moisture in the floor system are probably unfounded, Holladay says, and on this issue Mr. Mike is probably «overthinking» the problem.

«Go ahead and put rosin paper or asphalt felt under your new oak flooring,» Holladay tells him. «(Some people prefer rosin paper to asphalt felt because they are worried about asphalt odors. However, as long as you don’t have hydronic tubing in your floor system, I would use either one without worrying.)»

«The felt under the flooring is a non-issue,» Meiland adds. «Assuming no radiant tubing in the floor, or other source of heat that will bake it, install the felt. I put felt under my wood floors and have done it on many jobs for others over the years.»

What if we’re starting from scratch?

Part of the problem is the two 2×10 beams that support the joists. Because of the way the addition is framed, the beams interrupt the layer of rigid foam insulation.

«A flush beam, on the other hand, supports the joists with joist hangers,» Holladay says. «If you had installed a flush beam, it would have been easier to install a continuous layer of rigid foam on the underside of the floor system.»

Insulating an Exposed Floor

In that article, Lstiburek writes that it’s possible to build a warm floor over a cold space by using a layer of rigid insulation below but also by leaving an air space between the cavity insulation and the subfloor. In other words, don’t completely fill the joist cavity with insulation.

Although it may seem counterintuitive, floors are warmer this way because the warm air keeps the tops of the joists warmer and reduces thermal bridging Heat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. somewhat, Holladay says. The airspace will keep the floor warmer

«Lstiburek’s approach — leaving an air space between the top of the insulation and the subfloor — is dangerous to follow if you are a builder with average skills, because air leaks in floor assemblies routinely allow cold exterior air to infiltrate the joist bays,» Holladay adds. «Once that happens, this detail is a disaster. The detail only works if the builder has impeccable air sealing skills. That’s rare, but possible.

«Here’s the bottom line: The insulation that is doing most of the work in this assembly is the continuous layer of rigid foam under the joists. If you want a warmer floor, beef up the thickness of the rigid foam — and do an impeccable job of air sealing at the perimeter of the floor assembly (the rim joist area). Then it hardly matters how much fluffy stuff you put between the floor joists. You can put in a little (the Lstiburek approach) or a lot (my way).»

Our expert’s opinion

We asked GBA technical director Peter Yost for his opinion. Here’s what he had to say:

Without a vapor retarder between the conditioned space Insulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. and the first condensing surface, and with the first condensing surface only warmed by 3/4 in. of rigid foam, it sure sounds as though the air-permeable cavity insulation is cooling the cavity enough for condensation to occur. But if this were simply diffusion of interior moisture into the floor cavity, we should see condensation throughout, rather than just in some areas of the floor. So, that makes one think that the problem is due to air leakage.

I would confirm the air leakage, either with a blower-door test Test used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. or, if one is not readily available, by investigating with a smoke stick after you have turned on every exhaust fan in the house. I have induced an interior/exterior pressure difference of 30 pascals (60% of a typical blower door test) by just turning on every exhaust fan (baths, kitchen hood, and clothes dryer) and a pretty powerful window box fan.

What makes me nervous is to go with just the information you currently have, thinking we know where the moisture is coming from, but not making sure. Before you close up the floor system, it would be a good idea to take full advantage of having the floor system open for inspection.

Why not tape the seams of the subfloor before you put down the finish floor, to add an interior air barrier Building assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. Two air barriers are always better than one, and it sounds as though it is quite possible to make the interior air barrier continuous.

Concerning the temperature of the finish floor during the winter: I built a kitchen addition on piers in southern Vermont using R-38 SIPs for the floor, walls, and roof. During the first really extended cold snap, my wife walked across the kitchen floor, moving from the old floor (over the heated basement) to the new floor. “Why is this floor so cold if you used such a high performance building system?” she asked. I explained that the temperature of the floor of the addition was exactly the same temperature as the addition’s walls and roof. She looked at me and wryly said, “Well, now I feel so much better.”

I sure wish I had thought of Joe Lstiburek’s approach — decoupling that finished floor with an airtight air space beneath it. I can add more rigid insulation under the R-38 SIPs floor panels, but since insulation just slows conductive heat loss rather than stopping it, I can’t add enough to beat the longest cold periods. That addition’s floor will still be noticeably colder than the floor over the old basement many times throughout the winter.

So, in this case, adding more rigid foam underneath the floor framing system (warming the first condensing surface) and providing an airtight space above the cavity insulation will result in better energy performance, moisture management, and thermal comfort.

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