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Insulating a Sill Plate

ceeopa | Posted in Energy Efficiency and Durability on

We are redoing an older home in zone 4, dirt crawl space, vented, without  insulation. The sill is untreated, and without sill seal. The plan is to air seal with foam cap nailed to the rim joist and then spray around the foamboard,  but unsure if insulation will compromise the sill in the long run. would like to run eps foam on wall of foundation leaving the crawl space vented after insulating the rim. Any thoughts

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  1. user-2310254 | | #1


    Is this area above grade? What is the wall assembly like (brick, vinyl siding, etc.)?

    Also... Do you plan to install a liner over the existing dirt floor?

  2. Imme | | #2

    I'd recommend sealing off those vents with foam plugs as well. Insulating the rim joist delivers high bang for buck because you're sealing a common location for significant air leakage while insulating the portion that's mostly exposed above grade. Leaving the vents allows outdoor air in to bypass the insulation or move behind it and condense wherever it can. This plus the untreated sill without a gasket or anything to mitigate capillarity is courting a serious failure. If there's already HVAC ducting down there adding a supply and return will help regulate humidity, or you could add a dehumidifier(downside being dumping it often or adding a condensate pump too) If the vents absolutely must stay it's better to insulate under the floor but you'll use a good bit more insulation. Mineral wool is a good pick for this, fiberglass is common but it can get waterlogged and filled with pests-much heavier when you're dragging it back out in a few years. If using batts you could get away with open-cell spray foam to air seal the rim joist and preserve the ability to dry inward, but the floor insulation will still be windwashed by the vents to some degree. Personally, I'd spray the sill plate and any connecting wood elements with borate solution/Timbor several times before sealing the vents, then apply a durable flexible sealant at the junction of sill and foundation and any other visible cracks on the interior only. The foam panels inhibit inward drying so don't plug both sides. Can foam works but may shrink or crack over time so it's best to seal the area before applying any insulation. You can wait a bit between steps that way and use a moisture probe periodically to see if everything looks good. Bulk water management is critical to this plan though. Depending on the site conditions you may need grading, installing or correcting gutters and drainage systems. You'll also definitely want to check at least monthly for a while until you're confident it is performing as planned. If you're going for a quick n' dirty seal job before winter hits this will do the job and everything can easily be reversed if needed. You can cover the dirt and insulate the walls later, a sump pump is always a good idea. If you end up with standing water problems trenching a perimeter drain sloped towards the sump would be helpful. I sealed up my crawlspace in a structural brick home in Zone 5B following this plan and it has worked well through last winter and an unusually rainy spring and summer. The details may change a bit for other wall assemblies but not too much.

    1. ceeopa | | #3

      The crawl space and foundation wall are 18' above grade down to the dirt. This is a block assembly . Getting ready to add 1.5-2 inches EPS and a rain screen to the external side so will be stripping everything else off down to the original wood lap siding. thought about using Vycor self adhesive tape around the outside of house from existing siding, over the rim joist and connection with sill plate down to the block foundation wall prior to insulating. Wondering if lapping the external foam down and over the rim joist area to within 1' of grade after air sealing will be sufficient. Then adhering Eps internal to the crawlspace to finish from ground to top of foundation wall. Not going for Passive house lol but see this as a major improvement.

  3. user-2310254 | | #4


    Just curious if the conditions are right for this type of deep energy retrofit. Does everything need to come off? I ask because you don't really need exterior foam in CZ4 (unless you are in Marine CZ4).

  4. ceeopa | | #5

    We are moving toward a better envelope. Replacing windows, adding new probably fiber cement siding, so the extra for insulation and dudley boxes to adapt the rough opening seems marginal since we are doing so much. The original house was about 1000 sq. ft, then a porch was added on the front and back, 40 years ago 700 sq. ft. addition was added. There is no house warp on structure. 2x4 construction with a little fiberglass in some walls. We estimate the cost of remodel materials to be around 40000 but the house and property were bought dirt cheap. The insulation will aid in both bolstering the lagging r value in the walls and begin to bring the air flow of the house closer to control.

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