The Exterior Rigid Foam is Too Thin!
The Exterior Rigid Foam is Too Thin!
You bought a house with exterior rigid foam, and you just discovered that the foam is too thin — so now what?
Let’s say you’ve owned your house for several years. Your growing interest in energy efficiency brought you to the GBAGreenBuildingAdvisor.com web site. GBA taught you that it’s a great idea to install rigid foam on the exterior side of your wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , as long as (a) the rigid foam is thick enough to keep your sheathing above the dew point during the winter, and (b) your walls don’t have any interior polyethylene.
You think to yourself, “That’s interesting. I wonder if the walls of my house have any exterior foam.” So you poke around. Maybe you lift some vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding. Maybe you make an inspection hole in a closet wall. And you discover that (a) your walls have R-5 exterior foam, even though you need at least R-11.25 rigid foam for your Zone 6 walls, and (b) your walls include a layer of interior polyethylene.
Three possible approaches
If you’re planning to build a new house with exterior rigid foam, you should follow the rules outlined in a GBA article titled Calculating the Minimum Thickness of Rigid Foam Sheathing. That way, your foam will be thick enough, and you won’t have any interior polyethylene. Then you’re all set.
If you’ve inherited or purchased a house with too-thin foam, though, the fix isn’t easy.
You can take one of three approaches:
The “fix everything” approach is so expensive that few homeowners will go this route unless they are already planning a gut rehab job and new siding.
To fix the problem of the interior polyethylene, you have to remove the interior drywall. Once the polyethylene is in the dumpster, you can install new drywall. That’s a lot of work.
To fix the problem of too-thin exterior rigid foam, you have to remove the siding. In most cases, you’ll have to put the siding in a dumpster (although some types of siding can be recycled or re-installed). Once the siding is off, another layer of rigid foam can be installed over the existing rigid foam — staggering the seams, of course. Make sure that the total R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the two rigid foam layers is enough to keep your wall out of trouble. This work will probably require new exterior jamb extensions and sill extensions for the windows, along with new window flashing to limit water entry.
Once the new rigid foam is installed, you can install new vertical furring strips and new siding.
All of this work is possible, but it will probably cost tens of thousands of dollars. Although I've described the work involved, I'm not advocating that any readers adopt this approach.
New foam without new polyethylene
Most people don’t want to open up their walls from the inside just to remove a layer of poly. But it’s possible that you are planning to replace your siding. If so, it’s a good opportunity to add enough extra rigid foam to make your wall safer.
As noted in the section above, you’ll need to install a new layer of rigid foam, new exterior jamb extensions and sill extensions for the windows, new window flashing, and new furring strips.
Once your siding job is complete, you have to consider whether the polyethylene is a problem.
First, it’s worth noting that the exterior work that you just performed — the window flashing work, the creation of a ventilated rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap, and the new siding — should ensure that wind-driven rain won’t enter the wall cavity. (Assuming, of course, that you did a good job with this exterior work.) That greatly reduces the chance that the sheathing will get damp from the exterior side.
While the interior polyethylene layer stops inward drying during the summer, it performs a beneficial effect during the winter (reducing outward vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. ). In any case, the thick rigid foam ensures that the sheathing stays warm, and therefore dry, all winter long. So the risk of moisture problems in this type of wall is fairly low.
Although it would be better if the polyethylene weren’t there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven’t been any reports of widespread problems. According to building scientist John Straube, all signs indicate that these foam retrofit jobs are “not so risky as most people think. These homes will probably be fine.”
So it’s probably OK to relax now.
The harm reduction approach
What if you are forced by economic circumstances to take the third approach: leaving the walls as they are? If the too-thin foam and the interior polyethylene have to stay in place for the next few years, are there any steps you can take to lower the risk?
There are. Among the steps worth considering:
- Keep the interior relative humidity (RH) low during the winter. The lower the interior humidity, the lower the chance that exfiltrating air or outward vapor diffusion will introduce dangerous quantities of moisture to your wall cavities. So if you have a humidifier, unplug it or disable it. Install a few hygrometers around your house and check them regularly. During the winter, strive to keep your indoor RH at 30% or less. If the RH creeps above that point, operate your bathroom exhaust fans for 24 hours a day (or turn up your HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) until the indoor RH drops to 30% or below.
- Seal any obvious air leaks in your walls. The most likely way for interior moisture to contribute to damp wall sheathing is via exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. — that is, via interior air that escapes through cracks in your wall. Exfiltrating air carries interior moisture, and that moisture can condense on cold surfaces in your wall cavity. Sealing air leaks in your exterior walls reduces the chance that interior moisture will contribute to damp sheathing. It’s a good idea to caulk the joint between your subfloor or your finish flooring and your drywall. (In some cases, it makes sense to remove your baseboards for this work; otherwise, leave the baseboards in place and use clear caulk.) It also makes sense to seal air leaks at your electrical boxes. (Needless to say, turn off the circuit breaker before performing this work.)
- Inspect your exterior window sills for signs of water damage, and correct any flaws you discover. Most wet-wall problems involve water entry from the exterior — in other words, we're talking about rain. To keep rain out of your wall cavities, you need good roof overhangs and proper flashing details. A regular inspection of your home’s exterior might allow you to nip some of these problems in the bud.
Will my walls start to rot?
The guidelines for minimum rigid foam thickness make sense, and builders who ignore them are running a big risk. That said, a real wet-wall disaster usually takes more than one mistake. In addition to foam that is too thin, sheathing rot usually requires high indoor humidity and walls with air leaks.
Unless your house has all of these problems, your walls will probably survive. So if you follow the advice in this article, there’s no reason to panic.
Martin Holladay’s previous blog: “Does a Passive House Use 90 Percent Less Energy?”
- Owens Corning
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