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We have very cold floors in the kitchen because the outside air is moving up through the external cantilevered floor.

I'd like to insulate it by cutting 2" foam (probably XPS or Polyurethane) and attaching it between the joists. Note: house is in San Francisco (lowest temp is about 38F) Three questions:

  1. Is this a good approach? (better ideas welcomed)
  2. Do I need to worry about moisture buildup?
  3. What's the best foam for the job?

Photo of my joists: enter image description here

Proposed solution: enter image description here

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Any kind of insulation will help us would consider boxing by in the space also, if in an elevated area we used to have trouble with swallows building their mud nests in areas like that so boxing them was a must, (I don’t see that issue here but we had it just north of you). But insulation will help.

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  • I would suggest closed cell spray foam, it is an Insulation and vapor barrier in one. – Alaska Man Feb 7 at 18:42
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If you insulate as pictured you reduce the cold surface area and prevent some passage of cold air between the boards which could improve things a good bit but you still have a cold bridge from the joists to the floor.

I think you would achieve a greater gain by laying a floating floor with a continuous layer of solid insulation boards over the existing floorboards, topped by a plywood/engineered wood layer, then your flooring. Obviously you lose some height from your kitchen and raising the floor level might mean significant interior work to accomodate. Less disruptively you could use a thinner layer of insulating underlay in addition to your insulation between the joists. Encapsulating the joists in some form of insulation as well as between them would be another way but it would be an awful job to do unless you spray foam insulation on them.

You should have the vapour control layer on the warm side of the floor and a breather membrane on the exterior.

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