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Insulating a floating floor addition over an uninsulated front porch

kurtz | Posted in Green Building Techniques on

Good morning guys, and happy Monday.
I’ve never had to worry about insulating a floating floor addition over an uninsulated front porch in my 4 short years of building green. So before I do anything stupid I figured I’d make use of your brains.
To keep floor heights inside consistant we would have 5″ airspace between 2×5″ “joists” running the width of the addition, actually resting on the concrete porch we are turning into a living room. 1.55″ thermax cut to fit the joist space and glued in place will not create the perfect air seal but will do a good job. Also using thermax to insulate the band. Do we need to put another layer of thermax in the additional space or could/should we put in an unfaced r-13 batt? Should we put a layer of tyvec or the like beneath the advantech sub-floor we will be installing? and finally, if we don’t want to cut the concrete to run an hvac duct could we insulate and then pan one of the joist bays (only 5″x14.5″) and not have to worry about condensation in that space? I don’t think I’ve missed anyting…but wanted to make sure before I proceed. thanks for your help. -dave

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  1. GBA Editor
    Martin Holladay | | #1

    We need more information to answer this question. The most important missing detail: where is this building? Insulation levels vary with climate.

    Quick observations:
    1. One and a half inches of Thermax (polyiso) has an R-value of about 9.8. That's not much.

    2. You don't need Tyvek under an OSB subfloor. If you use subfloor adhesive, the subfloor will be airtight.

    3. You need to have good drainage details at the exterior of the building to be sure the slab stays dry. If there are any signs of slab dampness, you'll be in trouble quickly.

    4. It's always a bad idea to use a joist bay as a duct. In a pinch, you could surround a duct with polyiso and then foam the duct in place, but the joist bay would still have compromised insulation.

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