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Community and Q&A

Vapor Barrier Location with Radiant Floors

Pauley93 | Posted in Green Building Techniques on

Hi everyone! Hoping someone on here can point me in the right direction. I am in zone 5 and am building a 12×16 four season room off the back of my existing house. Concrete pier and beam construction with plenty of airflow beneath. I am looking to run radiant heat in the floor to maximize the room footprint and, well, just nice to have radiant heat in the winter when the floors are usually cold. 

I am looking to see where, if any, a vapor barrier would need to get installed. Attached is a rough layout of the floor cross section. I was looking to run subfloor, then radiant heat under the sub floor, foil faced insulation beneath the radiant heat with an air gap, right into batt insulation, and finally capped with foam board (or comftorboard is) and enclosed by sheathing. Does this insulation method seem adequate? And would the foil faced insulation serve as a vapor barrier? I don’t want to build moisture up and have it get into the rest of the insulation. 

Thank you!

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  1. gusfhb | | #1

    I would put the tube on top of the subfloor. They make products for this, or you can site build sleepers which I did and worked well
    Having lived in a house on piers with fiberglass above foam, it makes a great squirrel house.
    Consider something rodent proof as a bottom layer.
    Unless the rest of the house is radiant, I think the cost of a proper radiant install is pretty high for one room.

  2. Expert Member
    DCcontrarian | | #2

    If it's on piers and you have good airflow I would make it vapor-open to the bottom. The floor heat will create a good vapor drive toward the exterior, you want it to be able to dry down. So put the foam board on top, then the batt insulation, and wood sheathing on the botom.

    I agree that rodents are an issue, if you make sure the wood sheathing has no gaps it should do a pretty good job.

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