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Crawlspace ventilation or not; what is best for insulation?

GBA Editor | Posted in General Questions on

I am building a workshop. The foundation is being built on bedrock (8 inch stepped foundation concrete wall with PT wood kneewalls to bring it up to floor joist level). The granite bedrock is sloped and therefore has surface water draining from the higher ground when it rains. The perimeter of the foundation (29′ x 20′) will have gravel and weeping tile.

There will be moisture present on the granite surface, regardless of my drainage plans.

My questions are :
1. Do I vent the crawl space?
2. What is the best way to insulate the the floor?

I live Halifax, Nova Scotia, very similar to Northern Maine.

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  1. Riversong | | #1

    Yes, vent the crawl space well (1 sf per 150 sf of floor). Insulate the floor joists with whatever batt or blown insulation you choose and then install a thermal break/radiant decoupler/vapor barrier of 2" foil-faced polyisocyanurate rigid board insulation.

    If this is to be a regularly heated space, you should aim for R-38 in the floor.

  2. Christopher Briley | | #2

    I concur with Robert. You will want to seal yourself off from that beautiful granite completely. I would only add a reccommendation to pack yout joist cavities tightly with insulation, refferably a dense packed cellulose. Your assembly cannot breathe to the crawlspace, and will have trouble breathing to the room above (through plywood or other floor sheathing?) so you will want to inhibit the formation of condensation droplets especially against teh polyiso in Robert's assembly.

  3. Riversong | | #3

    With at least 1/3 of the R-value outside (below) the joists, there should be no condensation on the polyiso.

  4. Christopher Briley | | #4

    Robert's card trumps mine.

  5. wjrobinson | | #5

    I would add boat wrap sheet of plastic over wet granite and up sides of crawl.

    Also think better solution would be to build on insulated water barrier protected slab.

    Make sure your crawl is not sitting at 100% rh ever.

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