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Insulating under a hydronic radiant heat under subfloor between joists system

I have hydronic radiant heat set up attached to the underside of subfloor and between floor joists spaced at 16". This set up sits above 1400 sq.ft. of unheated basement and needs to be re-insulated. Above it is the traditional subfloor with 3/4" engineered oak flooring immediately on top. I have been told many different methods to insulate this ranging from reflective bubble foil to standard fiberglass batting with a face to rigid insulation to any combination of 2 or more of these. How do I know how much R value I need and how best to achieve it efficiently (current and future costs) and safely (not prone to mold)?

Without having a basis to calculate, I was considering using a double bubble/double reflective foil stapled into the insides of the joists 4" or so below the radiant tubing (reported to be reflective and an R value of nearly 17) plus some rigid foam beneath that but still between the joists.

Asked by Doug Adler
Posted Wed, 11/28/2012 - 14:17
Edited Wed, 11/28/2012 - 14:19

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Foil & bubblepack is nearly WORTHLESS in this application, and the air-gap you'd have to leave provides a thermal-bypass channel. If the tubing is in aluminum heat spreaders, it's already a low-emissivity surface, and the fraction of heat radiating rather than convecting downward is already miniscule, so the foil would already be reflecting back next-to-nothing. Foil only works well with high temperature differences and highly emmissive surfaces, and here you would have neither.

If it isn't already in aluminum heat spreaders (preferably extrusions rather than sheet metal), spend the money on the aluminum heat spreaders rather than overpriced reflective bubbles and rigid foam.

This is one of the very few applications where cheap fiberglass batting is appropriate. If the basement is conditioned or semi-conditioned, never dropping below 50F, cheap R11or R13 roll batting snugged right up to the tubing and subfloor is enough to providegood zone-isolation. If it comes with a facer (like the loss-leader long roll R13s at box stores) it doesn't matter a bit which side the facer is on, so use the facer as the bottom-side air barrier for the batts. If it's kept 50F or even cooler, low-density R19s are appropriate. If it's a drafty vented crawlspace that drops below freezing you'll need to go higher (or better yet, air seal an insulate the crawlspace/basement walls converting it to a semi-conditioned space.)

For a mold hazard to exist with the batt facer on the cool side of the assembly the facer has to spend a substantial amount of time every year below the dew point of the air in the room above. A typical cool-climate mid-winter humidity is ~35% @ 70F, which correlates to a dew point of 40F. If your basement is averaging 40F you have bigger problems than mold- you're at high risk of freezing up your plumbing (or even frost-heaving and cracking the basement slab during a particularly cold winter.)

Answered by Dana Dorsett
Posted Wed, 11/28/2012 - 17:04

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