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flyboy 09-28-2009 09:04 AM

Vapor Barrier Question
My house is on stilts, leaving the underside of the house exposed to the weather here in Central MA. The previous owner and builder did some pretty unorthodox things. I am just beginning to understand the magnitude of the problem.

The floor construction is as follows from top to bottom:

Pine boards
Plywood Underlayment
R30 Fiberglass between 2x10 joists
1" Polyiso foam insulation board

Except for the foam, there is no vapor barrier of any kind. The fiberglass has kraft paper on one side but is otherwise unfaced. The foam insulation board is fully taped with tyvek at every joint making is basically airtight over the entire lower surface of the house.

I started taking the insulation down (pulling foam board and fiberglass down from the bottom of the house) because of some other work I am doing. I found that most of the floor joists and the plywood subfloor had mildew and water stains. I will post pictures when i get a chance but the majority of the stains seemed to start at the plywood subfloor joints (i.e. where two panels of subfloor but up against each other). The further down the side of the joist you go, the fewer stains. The fiberglass itself was dry and showed no signs of water damage or mildew.

A few questions:

1. Are the water/mildew stains the result of a lack of vapor barrier ? If so, why are the stains concentrated at the top of the underfloor cavity ?
2. What can I do to solve this problem that does not involve ripping up all my pine floors. In other words, how can I solve this problem from the bottom side of the house. Will a poly barrier on the lower side of the subfloor do the job ?
3. I plan to add plywood or chipboard on the bottom of the joists so that I have something that looks like this:

Pine boards
Plywood Underlayment
R30 Fiberglass between 2x10 joists
Plywood or chipboard
1" Polyiso foam insulation board

Is this going to make the problem worse ?

Scuba_Dave 09-28-2009 09:09 AM

The kraft paper acts as a vapor retarder (sometimes called a vapor barrier)

Sounds like cool air is getting up & condensing aginst the warm floor
I had the same problem at my last house- crawl space
I sealed & insulated the perimiter & put heavy duty plastic (6 mil) on the floor

How far off the ground on stilts is the house?
Closing in the area will help on keeping the house warmer

Daniel Holzman 09-28-2009 10:17 AM

Scuba, I don't follow your post at all. Cool air condensing against a warm floor? Warm air would condense against a cool floor, not the other way around.

As for the OPS question, you need to verify that the moisture is not coming down from the floor above, due to leaks on the floor, a dripping pipe, or some other source of moisture. Since you indicate that the fiberglass is dry, that suggests that you are not getting condensation from below in the insulation.

Consider the four seasons. In winter, the air inside your house is warm, but probably dry, unless you have a humidifier. If no humidifier, the air will be around 65 degrees, with a low relative humidity. The temperature of the floor at the top of plywood will be around 60 degrees, and will transition down to outside air temperature at the bottom of the insulation. Little moisture, no condensation.

In the summer, the outside air is likely to be warm and moist, while the inside air in the house is likely to be cooler, and dryer if you have an air conditioner. If your house is air conditioned, moisture will "flow" from outside air to inside the house, and there may be some condensation on the underside of the floor against the insulation if the floor is below the dewpoint. If the house is not air conditioned, this should not occur frequently, if at all.

In the fall and spring, the floor of the house is likely to be warmer than the outside air, and condensation is unlikely. In sum, I don't really see how the current setup is causing condensation due to outside air moisture, I would start by examining inside moisture as the possible culprit.

flyboy 09-28-2009 11:52 AM


I am fairly certain that there is no leak involved. I can not rule out the possibility of a leak before I bought the house but it seems highly unlikely because there is no sign of water damage anywhere except under the floor and there this no plumbing anywhere near the area I am looking at.

I understand your line of reasoning regarding the thermal differential in various seasons. I had the same thoughts myself. Nonetheless, I can not think of any other explanation for what I am seeing. The wood under the floor clearly has been wet off and on for some time even though the interior of the house is bone dry and is not airconditioned.

The house is unusual in that the foam insulation on the outer layer along with the tyvek tape creates an almost completely airtight space. Even a small amount of condensation would have nowhere to go. The other thing worth mentioning is that the house goes through pretty big temperature swings in the winter time. We heat with wood and it is not unusual to go to bed with the house at 70 degrees and wake up with the house at 50 degrees F.

Gary in WA 09-28-2009 02:15 PM

I agree with Daniel. The foam is stopping the joist spaces from drying the moisture they get from the interior. You need more air changes for such a tight house. Your foam should be next to the sub-floor, then batt, then ply. I would rather see ply with vent holes like a soffit, to rid any moisture IF the increased air change doesn't handle the problem.
"Low air change during the heating season due to the construction of tight enclosures can lead to elevated interior levels of moisture. Cold air is not capable of holding as much moisture as warm air. Cold air is therefore typically dryer than warm air. During the heating season, cold, dry air from the exterior infiltrates through random leakage openings in building enclosures or is brought into the building by controlled ventilation. This cold, dry air is subsequently heated by the enclosure's heating system and becomes capable of holding appreciable amounts of moisture. Should moisture be available, it is picked up by this heated, dry air. This heated air, now containing moisture, exfiltrates to the exterior through other random leakage openings or is deliberately exhausted by controlled ventilation.
Air change (infiltration/exfiltration combined with controlled ventilation) removes interior moisture from within building enclosures during the heating season. The greater the air change rate, the greater the removal rate of interior moisture. However, typical construction practice results in building enclosures which have air change rates from random leakage that are inadequate to control interior moisture levels. As such, in heating climates it is desirable to ventilate enclosures in a controlled manner to limit interior moisture levels.
Relative humidity should be maintained at 30% or lower at 70°F (21.1°C) within the conditioned spaces in cold climates during the heating months (the key is to prevent 70% relative humidities from occurring adjacent to surfaces in order to control mold, mildew, and other biological growth). In very cold climates the interior relative humidity should be maintained at even lower levels during the coldest month of the year. Humidity control within conditioned spaces is accomplished by the dilution of interior moisture by air change, facilitated by controlled mechanical ventilation coupled with source control. In the more moderate heating regions with high exterior vapor pressures during the heating season, such as the Pacific Northwest, mechanical dehumidification is also practical." From:
3/8" plywood has a permanence rating of .75 - 1" extruded foam board has a rating of 1. So you have two layers of ply and one of foam, all Class II vapor retarders that are semi-impermeable: "Avoidance of the installation of vapor barriers on both sides of assemblies—i.e. “double vapor barriers” in order to facilitate assembly drying in at least one direction." from same article.

Be safe, Gary

flyboy 09-28-2009 07:47 PM


Great post. Thanks for the info.

While I understand that the problem is moisture in the joist cavity that can not escape, I am still struggling with what to do about this.

The advantage of the original configuration:


Is that the floors are warm and draft free. The house has some elevation and -10F with 40 MPH winds is not that unusual in January (remember also that I am 6ft up on stilts). I am worried that I will loose insulation value and get drafts if I do


I assume that compressing the fiberglass to accomodate the extra thickness of the foam will cut the R value below rated and I will not be able to seal the floor as tightly as possible with the foam sitting on the bottom of the joists. There is also heat loss along the joist to consider.

Ventilating the joist cavity to the outside seems problematic. Any strategy that gives significant air exchange would negate the effect of so much insulation.

Is there some strategy I could use that would keep my floors warm ? It seems like adding a vapor barrier between the subfloor and fiberglass would reduce the amount of moisture that enters the cavity substantially. I understand that this also keeps moisture in the cavity once it is there. Perhaps I could combine the vapor barrier just under the subfloor with small vents in the foam/plywood layer ?

Gary in WA 09-28-2009 10:42 PM

I was referring to the foam is on the outside of the batts when it should be on the inside, next to the sub-flooring plywood. Then the batts, then ply. In a concrete wall basement, when water moisture comes in house, slows at the glued on foam board which lets it dry to the inside, slowly, through batts and drywall and paint. In your scenario, the vapor is stopped at the cavity by the complete sealing of the foam. 1. I would address the moisture created from showers, cooking, wood from the stove? , etc., by more air changes per hour than what you have. 2. Do a blower test on the house to see why moist air is being forced out of heated rooms into floor spaces with a dead air rating. Something is not balanced. At the very least, I would caulk the sides of joists at moisture (joints in ply) areas with canned foam, possibly installing foam board up against the sub-floor to make up for the lack of a tar paper under the pine. Are you getting any condensation on the windows, inside?
Be safe, Gary

flyboy 09-29-2009 09:10 AM

Hi Gary,

The odd thing is that the house is generally bone dry in all seasons. We never get condensation on any interior surface. It is uncomfortably dry in the winter. Towels dry faster than any place I have ever lived.

After looking into permeability numbers for the materials involved, I am focused on two things:

1. Foil facing (the polyiso is foil faced on both sides) has the lowest permeability of any commonly used building material.
2. The joist cavity is not well isolated from the room above. There are gaps enough that air movement in addition to moisture diffusion thru material is probably significant.

The other thing I had not realized is that the thin kraft paper on the fiberglass (which currently is on the cold side of the batt) is intended as a vapor retarder. I thought the kraft paper was just a mounting feature.

I found one suggestion online that a spiked roller could be used to make many small holes in the polyiso foil facing, increasing the permeability. Has anyone heard of this before ?

Regardless of whatever else I do, it seems like I need to calk up all the gaps in the subfloor. Perhaps if I combine this with holes in the polyiso foil facing or removing the foil entirely, and poly against the subfloor ?

Gary in WA 09-29-2009 11:34 AM

I would want to research the hole thing (no pun intended). I once read, while researching the permeability of kraft paper on insulation batts, that cutting slits in the asphalt paper does not affect the retarder at all. So I checked my files, nope, nothing. You know how you read something but can't remember where you read it...... If I find it, I will let you know. I would definitely move the poly to the sub-floor.
Be safe, Gary

flyboy 09-30-2009 07:46 PM

I exchanged emails with DOW, the manufacturer of my polyiso panels. Not surprisingly they were not willing to offer any advice on tinkering with the foil facing on the panels they manufacture. They were able to confirm that their foil faced panels are <0.05 perm.

They suggested I throw all the existing polyiso panels in the garbage and replace them with brand new DOW styrofoam panels which apparently have much high vapor permeability.

flyboy 11-19-2009 11:18 AM

A quick update. I came by some additional info that has some bearing on this problem.

Apparently, the floor and subfloor of the house were built long before the house had walls or a roof.

From what I was able to determine, the subfloor and joists were exposed to the rain and snow for a year or more.

It seems likely that the moisture stains were partly or mostly due to lack of protection during construction. This would explain why the joists and subfloor showed signs of moisture but the fiberglass bats did not.

Gary in WA 11-19-2009 12:16 PM

Thank you for the update. I will return one to you that I recently found:
Sounds a lot like what you have............

Be safe, Gary

colinp 11-07-2011 06:23 PM

Vapor Barrier Questions (New Related Subject)
Hi Folks,

I just came across this topic searching for info about the cottage I am renting. IT seems like you all have a lot of knowledge, so hopefully you are available to lend some advice! I hope that Flyboy got your situation figured out.

I too live in MA, but western, and my house is also on stilts. I am in the CT River valley flood plane. During Hurricane Irene, there was 3 ft of water under the house, which was about one foot from being to the floor joists. There has been a lot of rain and mositure this summer and now that the place is closed up, there is some mold growing in the house. I think that the week of no power from the October storm here allowed more mold to grow.

I am renting this space, but I would like to find out what my options are. Right now, mold appears on various wooden and leather surfaces in the space. There is also black/grey mold on the vinyl double pane windows, which also have condensation on them in the morning.

The underside of the house is 1/2 foam board, and the other 1/2 is fiberglass battens with tyvek covering it.

There are no gutters (which I just convinced the owner to install). Everytime it rains (or snow melts like in Oct) there is a pool of water under the house.) Hopefully gutter will help.

There are no fans in kitchen or bathroom.

Vented Rinnai propane heater w/electric back-up baseboard

The space was remodeled two years ago and is somewhat well insulated and somewhat tight (not sure how well insulated or sealed actually).

Humidity levels are currently around 50-60. (with heat on-windows closed)

My main question is: Should I put up a vapor barrier right under the house to help prevent mositure from under the house from coming in? If so what would it be?

Any other ideas are appreciated!


Arky217 11-07-2011 08:35 PM


I'm glad you dredged up this old post as I also have a similar situation.
Rather in Maine, I'm in western Arkansas, but my house is also on posts (3') and the crawlspace is completely open.

So far, I only have my subfloor installed which consists of 3/4" T&G Avantech which is glued (including the T&G joint) and screwed to the joists.

My plan is to install Roxul batts between the joists and then cover them with 7/16" OSB.

With the Avantech glued at all edges, the floor should be fairly airtight, but I was wondering if I should paint the subfloor with a good vapor retarding paint before I install the flooring, which will be vinyl in the kitchen and bath, and composite 'wood' flooring everywhere else. (I know the flooring also requires 6 mil poly under it as well if installing over concrete, but I don't remember whether it is required if installing over a OSB subfloor.)

Maybe I should put 6 mil poly over all the subfloor, but I don't know about installing the vinyl flooring yet, that is, rather or not it is supposed to be glued. If so, the poly would present a problem.

On the bottom, I know plywood would be better to enclose the Roxul than OSB since it's not as much of a vapor barrier as OSB, but it is twice the price, plus I already have the OSB.

As to foam panels instead of Roxul, or in addition to Roxul, I really don't want to go there for financial reasons. (even the Roxul alone is really exceeding my budget)

Any opinions welcome.

Michael Thomas 11-08-2011 12:32 PM

1 Attachment(s)
You want to be very leery of installing anything that can act an a effective vapor retarder on the cold side of the floor assembly, as condensed moisture can do a lot of damage to the floor structure:

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