Faced or unfaced fiberglass with under-floor radiant heating?

AlaskaDick · August 30

What are the thoughts on using kraft faced vs. unfaced fiberglass insulation to insulate the bottom side of under floor radiant heating? The trade-offs I can think of are that there would be less air infiltration and possibly easier installation with the craft faced, but it would come at a higher cost. I spent about an hour looking for old posts, but but didn't see in any of the discussions on insualtion where faced and unfaced were compared.

The living portion of the house is on a crawl space and I'm installaing radiant under-floor using .016" X 4" omega heat transfer plates in about 75% of the house and .022" X 6" omega plates in the rest, which has a greater heating load. Joist spacing is 16" in about half of the house and 24" in the other half. Pex spacing is/will be 8" in most of the house and 12" in some areas with lower heat loss.

The garage and shop non-living areas portion are on a concrete slab. The garage (about half the space on a slab) has pex in the concrete and the other half uses a radiant heated wall. This section has been in use for about a year with the current boiler via a mixing valve.

Current heating source is a 25-year-old cast iron boiler with baseboard. The replacement is a Lochinvar Noble. DHW is via an indirect water heater. When I cut over to the under-floor radiant, everything will be low temperature except for the water heater.

The crawl space is conditioned. There is blue board insulation on the inside and outside of the foundation with an R value 8. It's higher at the rim joist. I plan to keep its temperature relatively close to the living space.

Design temperature is -15f.

Before I take the insurance off of my truck for the winter I want to lay in a supply of insulation I will need before next spring.

HydronicMike · 12:05AM

The usual adage is “No face, no space”.

hot_rod · 1:25AM

I think you are talking brown kraft paper, not the foil faced stuff? Seems the Kraft would keep any crawl space moisture out of the batts?

DCContrarian · 2:08AM

If the crawl space is conditioned it shouldn't matter. There's no vapor drive between two conditioned spaces.

AlaskaDick · 2:59AM

Kraft, no foil. The outside walls of the house have poly sheet and the blue board against the inside of foundation serves as a vapor barrier there. The ground is also lined with poly. The rim joist is insulated with eps and xps insulation and sealed with canned spray foam so the ends of the fiberglass won't be against a cold face. Also, we are not in a high humidity area and the poly on the floor does a good job of blocking any water vapor coming from in the ground. I also have an HRV for the house.

My thoughts go along with those of DCContrarian, the temperatures will be relatively close. I'm not attempting to keep the crawl space cold, I just don't want it to be warmer than the house.

My experience with kraft faced is that it's less effective as a vapor barrier compared with poly and that unfaced is subject to air infiltration. Infilitration may not be an issue in this application since the bottom of the plywood floors and joists would prevent most air movement if there is no air space above the fiberglass.

Thanks for the inputs to help me think this though.

Faced or unfaced fiberglass with under-floor radiant heating?

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