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Community and Q&A

How to insulate rim joist that doesn’t breathe on the exterior side?

slow2decide | Posted in Green Building Techniques on

The home was built is 2012 we are in Zone 5b just north of Indianapolis. We are the original owners and have pics during construction phase (unfortunately they were not custom builders and would not let us help with design details. e.g. we couldn’t get them to insulate exterior basement walls and below the slab with rigid foam- which is now MADDENING).We are about to finish the unfinished basement. What is the correct way to insulate a rim joist on a modern house? I know that rigid foam board with spray foam is suggested since its impermeable, but our rim joist appears to be covered in impermeable materials on the outside as well with no vapor or moisture barrier between it and the concrete wall. Wouldn’t both sides being air impermeable be asking for wood rot? Your answer will likely depend on other construction details of the basement so I am providing current construction and the future plan.

Current construction: ~ Unfinished Basement poured 10″ thick concrete walls and poured slab (poly sheeting under slab, gravel under the poly). ~ Basement dug deep enough to accommodate 9 ft finished ceilings. ~ French drain, Exterior drain AND interior perimeter drain around basement wall footer. Both drain to air sealed sump. ~ Waterproofing spray on exterior concrete walls. non-functioning radon pipe installed. It is capped in the attic and goes to gravel from under slab (within 3 feet of sump that attached to french drain – ugh). Exterior there is good sloping away from house in all directions and gutters drain ~10 feet away (except 1 which is 5 due to property line) . No water issues, but very humid in summer- would be well above 70% if no dehumidifier is running. ~ Sill and Rim Joist Construction: Double studded Rim Joist. Under sill plate directly on top of the concrete wall is a sill gasket for thermal barrier (but no a water OR vapor barrier). The exterior face of sill and the rim joist is covered with a therm faced foam board (which used as the on the whole house) then a black membrane (I can see it in pics before brick and mortar was installed. I think the black might be rubber.)

Construction plans:
RADON: We will likely have to have the radon fan installed in the attic. Many of our neighbors do when they resell the home it doesn’t pass. Ours did pass at 1 year post-build, but we have’t tested it since. I am assuming we will have to fill the perimeter gap with urethane if we install radon fan in attic. I would hate to need to do that AFTER the basement is finished- but if filled the gap between wall and floor is useless. Sealing the perimeter with the radon vent stack so close to the sump may not be smart since it could short circuit and pull air from the sump/exterior french drain according to a post I read here- not sure which to do. Open would draw from around the room. I will consult with Radon expert.

WALL: Paint Drylock from TOP exposed area of of wall down the poured concrete wall to 1-2 feet ABOVE the slab. Attach 2-3″ EPS (edges sealed and fronts taped) to wall and leave bottom foot of concrete with no drylock or EPS. Stud wall no insulation (that sits on insulated floor described below). ~ DensArmor Plus 5/8 in. Fiberglass-Mat Gypsum Drywall
~ seal french drain gap (maybe?)
~ 6 mil poly on floor take it up on wall yet not sure to where to end… OPTION 1: 6 inches up the wall. tape to wall then gap of 1-2 feet of exposed concrete before the wall Drylock/EPS starts. OPTION 2: Tape to bottom of interior side of Wall EPS which starts 1-2 feet up the wall- leaving exposed concrete to promote base wall drying so moisture/air-vapor won’t go up to the sill plate and saturate it with no where to dry.)
~ EPS on poly ~ I will seal the seal the exposed edges on floor EPS and tape the top seams.
~ Advatech osb tongue and groove subfloor (overlap seams of EPS by 1 foot to prevent compression issues on high traffic areas) ~ pre drill for tapcon ( vacuum and spray foam prior to screw) ~ final flooring will be something with air permeability of 1 or higher.
not sure but I probably will insulate the duct work before closing.

SO, after all that I am confused on 4 items-
Fill perimeter?
Where to end the floor poly on the wall? If I tape it below the insulated area to the wall (~6 inches up from slab) the wall drys to the stud wall area with organic materials. My drywall and paint is air permeable though. If I tape it to the interior side of the wall EPS (usual recommendation when EPS goes to floor), the poly t will hang straight down from wall EPS and leave about of 2 inches of air between poly and wall for 1-2 feet of height until the floor- not sure this will allow the concrete wall to dry out- would this just force air vapor up the concrete wall to the sill.
Concerned that the exterior rubber membrane will inhibit the rim joist to breathe and dry out any water vapor or moisture that comes up the concrete wall.
4) AM I OVERTHINKING THIS? I have no construction experience other than some DIY remodeling projects and watching This Old House too much as a kid. I want to do things the most mold-resistant yet healthiest way possible so I am not a huge fan of extensive spray foam. I also like that you can pop rigid foam off to take a peak at the joist once every few years to check for moisture.

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  1. Expert Member
    Dana Dorsett | | #1

    Indianapolis is zone 5A, not 5B, not that it matters a whole lot here.

    In Zone 5A you can use 1" of closed cell foam against the rim joists with up to R15 of rock wool on the interior side without risk of wet fiber insulation. At 1" most 2lb spray polyurethane runs ~0.8-1.2 perms, on the Class-II/Class-III vapor retardency boundary. That's sufficiently vapor permeable to provide a drying path, but sufficiently vapor retardent that the peak moisture levels in the rim joist from wintertime moisture drives will be well bounded. Using 3" of EPS would have comparable vapor permeance to 1" of 2lb polyurethane, and would allow you to install even more interior fiber, but you have to be meticulous about air sealing it to avoid humid basement air convection around the foam to a cold band joist.

    You're going to have to seal the sumps and slab perimeter for a slab depressurization fan to be effective, and if you don't have any bulk water moisture issues in the basement you don't really need a gap a the slab edge to to the perimeter drain. Any house built in 2012 SHOULD have had a vapor barrier under the slab, and putting one on top would be an unnecessary complication to deal with. You may have to drill a small hole in the slab to verify that if you can't quite remember...

    Don't put ANY poly on the wall- an inch of EPS is already a sufficient vapor retarder for managing ground moisture in the foundation wall from getting into the studwall, and 3" would be getting to Class-II vapor retardency territory. It's fine to stop the insulation a foot from the floor to allow moisture that might otherwise wick up the foundation to the foundation sill to dry toward the interior. But go ahead and seal the seam between the slab & foundation wall with self-leveling polyurethane caulk- it'll make the house more air tight, and you really DON'T want any air coming into the house when the house is depressurized (from exhaust fans, etc.) to be coming in via the soil.

    Painted drywall isn't AIR permeable, but it IS water vapor permeable- about 3-5 perms if using standard latex paint, which is fine.

    Put an inch of EPS under the bottom plate of the studwall as a capillary & thermal break. It's not structural, not holding up the house, so you don't have to engineer the compressive strength of the EPS at all. TapCon the bottom plate to the slab through the foam every few stud bays and call it a day.

  2. user-2310254 | | #2

    In addition to memorizing Dana's great advice, also read this article:

  3. slow2decide | | #3

    Dana, Thank you! I was hoping you would be one to answer. Thanks for the correction on air/vapor permeable. (I am an engineer, so I do know the difference- or I did 20 years ago anyway. Now I only know enough about building to be dangerous without following expert advice or hiring it out.) I tried to pour through previous recommendations that had been made on GBA first. I was just going to attach the poly of the floor up the bottom edge of the wall, but it sounds like there is no need since there is poly under the slab (I can see it when I lift the sump cover). Can I skip the Drylock on the walls as well since there is a coating on the exterior of the foundation wall and the EPS will be on the interior side.
    Thanks again for the advice- I have a spray foam contractor coming next Tuesday (but I already know what his recommendation will be). It does look like I am leaning that way on the band joist area since my registers and some wires/plumbing would make a perfect job on the EPS tricky. Kara Rice (my sign in isn't working here today?)

  4. slow2decide | | #4

    Steve, Thanks for that link. I just read it and had previously read the one on Insulating Rim Joists (although, admittedly, I was still confused about mine not drying to the outside, which is why I posted). I am going to have to dig into the article that you linked on the walls and also a flooring one before deciding on the EPS thickness. After reading some of Martin's responses back to questions posted on the wall article and others, I think we would get along splendidly. He seems rather direct or has a dry sense of humor, possibly both. :) Kara Rice

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