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Community and Q&A

Sealing PT sill plate to concrete slab ?

Mark_McFarlane | Posted in Energy Efficiency and Durability on

I am a homeowner in the midst of a 2000 sq ft addition. Houston TX, foundation is a concrete slab. Walls (from inside out) are drywall, stick 2*4 framing with open cell foam, OSB, tyvek, air gap, brick veneer.

The builder used pressure treated 2*4s for the sill plate but there is no (capillary) barrier between the concrete and the sill plate. 3.5″ of open cell spray foam is going in the exterior walls today.

I am wondering if I should put a bead of silicon or poly caulk on the sill plate/slab boundary (on the inside) or if, since there will be some absorption of moisture into the sill plate due to the lack of a barrier beneath the PT sill, if I should just leave that junction uncaulked to aid in drying the sill plate. Since the sill is foamed on top, it seems the only way for it to dry is through the edges.



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  1. user-6184358 | | #1

    Use polyurethane caulking for the slab to sill. Home Depot in the concrete area Skita flex brand is good. Do the outside also.

  2. Mark_McFarlane | | #2

    Thanks Tim.

  3. GBA Editor
    Martin Holladay | | #3

    Installing sill seal (usually made of closed-cell foam) between the concrete and the sill plate is a code requirement. Your builder is guilty of a code violation, which should increase your level of vigilance concerning other details of the builder's work.

  4. Mark_McFarlane | | #4

    Thanks Martin, I have been watching closely and made them redo a lot of things. Do you think I should add polyurethane caulk to the sill plate - slab boundary at this time?

  5. GBA Editor
    Martin Holladay | | #5

    The polyurethane caulk is a good idea. A less forgiving owner would have required the builder to disassemble the building and do the work correctly -- in compliance with the building code.

  6. Anon3 | | #6

    The builder needs to redo it.

  7. Mark_McFarlane | | #7

    Thanks Martin.

  8. JC72 | | #8

    With the exterior walls up and sheathed I wish you the best of luck OP !

    Just remember when you talk to your contractor that your concern is that even though PT lumber won't rot it will still take on moisture and you're concerned about the end grain of the studs sitting on the pressure-treated sill plate taking on this moisture.

  9. Anon3 | | #9

    Caulk is going to trap water inside in this case, either redo it or don't caulk. I hope the builder got good insurance.

  10. Expert Member
    Dana Dorsett | | #10

    Caulking the seam will help air seal it, and won't appreciably affect the drying capacity of the sill plate.

    The pressure treated sill plate should be able to tolerate some moisture wicking, and unless you are in the lowest swamp on the gulf coast it won't become saturated. The sill plate and the studs mounted to it will be able dry into the open cell foam toward the interior as long as there isn't any interior side vapor barriers (such as vinyl or foil wallpaper). It's less than ideal, but it's not an automatic disaster.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Peter Yost wrote a blog on this several years ago.

    The lack of a gasket isn't ideal, but i don't think it is against the IRC requirements which appear to mandate either PT or a sill gasket, and I don't see how air sealing with caulking would make much difference to the capillary action taking place.

    - R317.1 Location required.

    - - Protection of wood and wood based products from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1 for the species, product, preservative and end use. Preservatives shall be listed in Section 4 of AWPA U1...

    - - - 2. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.
    - - - 3. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier.

  12. user-6184358 | | #12

    I think the caulking will at least keep ants from coming in. I am retro sealing my house now, due to ants.

  13. GBA Editor
    Martin Holladay | | #13

    A few years ago I looked up the question of whether the IRC required sill seal and I convinced myself that it was a code requirement. Now I can't find it.

    Here's what is required by the 2015 IRC: Table N1102.4.1.1 requires that "The junction of the foundation and the sill plate shall be sealed." I'll admit, however, that most inspectors would probably conclude that caulk would work for this requirement.

  14. Mark_McFarlane | | #14

    FWIW, my builder said he used to use sill seals but stopped after having reports of 'moisture problems' (not necessarily in his homes). I didn't dig deeper since it is too late anyway to apply sill seals. He believes they are not required by local code, and indicated that the requirement for these seals has changed over the years.

    He also asked me to consider how water would get out of the building should I have a flood or failed water heater, plumbing leak,... if I caulked the sill-slab interface. He was happy to apply the caulk, or have me do it, he was just sharing his experience and explaining why he doesn't do it. Apparently this is a common local problem. 4 of my neighbors took on some water last week (6-7 inches of rain in 2 days).

    Texas is a weird place in terms of building codes (it is an anti-regulation state). Very few zoning restrictions also. Some codes are applied at the neighborhood level (HOA). Even inspections aren't required, or are limited, in most cities/counties. We hired for 5 inspectors for this build, but it was beyond any local requirements, just something my builder does to cover his a** for future liability.

    We do have an impervious barrier beneath the slab, so taking on additional moisture through the slab shouldn't be a problem.

    Interestingly, the flooring supplier had me sign a contract that discussed a bunch of items including moisture in the slab and how that would effect their installation mechanism. (e.g. glue down versus floating) They test the slabs for moisture before installing the flooring. Without prompting by me, he said I didn't need to worry about any of the slab moisture problems, that my particular builder's slabs were always dry and in good condition, unlike other builders in the area. They have been working together for 30 or 40 years.

    Thanks again everyone for your assistance in this discussion. I truly appreciate everyones concern and help.

  15. GBA Editor
    Martin Holladay | | #15

    Any house that is designed to let water out after a flooding incident can't possibly be airtight.

    You want to make your house as airtight as possible.

    If your house gets flooded, and you have standing water in your house, I advise you to open the door.

  16. brendanalbano | | #16

    While it's a moot point, as the sill plates are already installed, it seems odd to me that the plan for a big interior water leak or spill is to let the water slowly trickle out under your sill plate.

    Shouldn't the plan be to suck up the water in a shop vac, run a big dehumidifier for a while, etc?

    Or better yet, if your water heater leaks, the water should drain through the floor drain in the room where the water heater is, no?

    Edit: I like Martin's flood solution of just opening the door! ;)

  17. JC72 | | #17

    Houston (Having lived there in a former life) is in desperate need of updating their flood maps. Flash flooding is a constant issue in that town.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    "Just open the door". Sometimes the obvious solution eludes us.

    Martin, you might be right about sill seals. I don't know your code that well. Either way the absence, and the justification for it, do bear out your observation that the builder is a bit shaky in their some of their building knowledge.

    Peter Yost's blog posited that the moisture in the sill plate could affect the joists above - not something I'd like - but if I was the OP I wouldn't lose sleep about the situation.

  19. Mark_McFarlane | | #19

    FWIW, opening the door works if you are home, not so much if you are on an extended vacation. I had a house flood before due to a busted hot water heater. I was 9,000 miles away. Luckily someone called me and we remedied the problem remotely.

    Also, water heaters in my area are typically put in the attic. No floor drain, just a small drain pan that can only handle a small leak. They are a mess when they go, enough so that some people replace them every 8 years before there is a chance of failure.

    Thanks everyone again for the help, I am applying the poly caulk now (starting in 5 minutes).

  20. Mark_McFarlane | | #20

    Caulking is completed, thanks everyone.

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