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Community and Q&A

Mudroom crawl space insulation

BrendonR | Posted in General Questions on

Hi ,I have a mudroom approximately 6’x8′ built using a CMU foundation and attached to the side of my homes foundation. It looks to be an addition. the concrete block is about 36″ above grade at the top. There is no access to the space but it does have a vent in one wall of the concrete block. The mudroom above it is very cold in the winter. So I would like to insulate the crawl space. I was planning to cut an access in the CMU big enough to get into the space and see what is there. There are no mechanicals etc in the space and no way I can see to make it conditioned space. Would it be best to keep the vent, air seal the floor from underneath and then put foam board insulation between the joist bays OR put poly on the ground and walls ,seal the vent and then insulate the walls and make an insulated hatch/access. Thanks for any help on this I want to do this the best way I can working with the building I have.

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  1. Expert Member
    Dana Dorsett | | #1

    Got a location/climate-zone? What makes sense in Fond du Lac, Wisconsin doesn't always make sense in Fort Worth, Texas, and conversely.

    Putting foam board BETWEEN the joist bays is a waste, and puts the joist edges outside the conditioned space. Continuous foam board attached to the bottom of the joists is easier to air seal, and puts the susceptible wood at least partially inside of the conditioned space.

    Sealing the sub-floor & joists to the band joist, and the band joist to the foundation sill all the way around makes sense in all climates.

    How deep are the floor joists?

    The amount of foam board (if any) makes sense, and how much attention is paid to the total R and to the band joist will vary by climate.

  2. GBA Editor
    Martin Holladay | | #2

    Creating an access door is a good idea. Once you have cut the opening, you want to install a door frame and a custom-made door that is (a) insulated and (b) weatherstripped.

    You can either insulate the crawl space walls or you can install continuous rigid foam under the floor joists, as Dana suggested. For more information, see "Building an Unvented Crawl Space."

  3. Expert Member
    Peter Engle | | #3

    Another possibility is that the space can be accessed from an adjacent basement, if there is one. if so, opening the crawl space to the basement, sealing the vent, and sealing and insulating the walls may be a good option. Though with a space that small, insulation the underside of the floor may be easier & cheaper.

  4. BrendonR | | #4

    Thanks for the replies
    . Dana, I am located in Connecticut. I don't know the size of the floor joists as yet, I need to open the wall to find out. I agree with your idea to cover the joists with foam board and seal down to the sill plate.
    Martin , if I insulate the walls of the crawl space how can I get the space to dry adequately.
    Peter, I cant really open the space to the house basement without some serious concrete cutting, but I have considered it as it would be the only way to run a heating duct to the mudroom. I was hoping that insulating the room would be good enough, the mudroom is open to the kitchen but because it is cold I loose a lot of heat.

    It sounds like the best way is to use continuous foam board under the joists and seal down to the sill plate. I may need to remove mold by soda blasting before sealing depending on what I find who knows. I wanted to understand better before I start cutting open the block foundation wall.

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