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Community and Q&A

Insulating Floor System Over Conditioned Crawlspace

jmend | Posted in General Questions on

I’ve read that in conditioned crawlspaces, the floor (above the crawlspace) should not be insulated.  However, I’m planning to install hydronic heat in portions of the floor and Siegenthaler recommends in Modern Hydronic Heating that hydronic floors above partially heated basements be insulated between the floor joists to R-19.

Which is correct in this situation?

I’m planning to ventilate the crawlspace by connecting it to the exhaust side of the HRV, pulling air in through floor vents on one side of the house and through the crawlspace to the exhaust tubing on the other side of the house.  This would be about 2/3rds of the exhaust flow, with the remainder coming from the bathrooms.

I’m in climate zone 4C (Pacific Northwest) and plan to insulate the interior of the stemwall to R15 and the ground below the vapor barrier to R10.  The heat source will be wood and/or propane boiler(s).  The flooring above the hydronics will be a mix of tile and engineered wood.  I’m still trying to decide between thin slab, gypsum, or a Warmboard type of product to encapsulate the tubing above the subfloor and would welcome any recommendations.

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  1. Expert Member
    Dana Dorsett | | #1

    How many BTU per square foot does the radiant need to emit at your 99% outside design temperature?

    It depends on the type of radiant heating. If the tubing is above the subfloor such as WarmBoard or thin slab, kraft faced R13 snugged up to the underside (facer side down) would usually be enough, but R19 doesn't hurt. If it's a staple-up with heat spreader plates, kraft faced R19 snugged up to the plates & subfloor. If convector fins or suspended tube in an air space above the insulation, R23 rock wool or higher.

    1. jordiem | | #5

      In a situation like this where the crawl space is conditioned, is there any reason why spray foam shouldn’t be used as an insulator for underneath a hydronic slab over sub floor?



  2. jmend | | #2

    Thanks for the reply Dana,

    My BTU/hr/ft^2 at 99% design temp is between 11 and 17 depending on how I tweak my insulation and window parameters.

    The concern I'm trying to address is condensation on the bottom of the floor joists due to insulating between the joists (hence the recommended practice to insulate the walls but NOT between the joists for a conditioned crawlspace). This conflicts with the recommendation to insulate between the joists for radiant floor heating.

  3. Jon_R | | #3

    Do you have a link to this recommended practice? If the dew point is high enough to condense underneath the floor (because of AC use), it's high enough to cause mold problems in the conditioned crawlspace.

  4. Expert Member
    Dana Dorsett | | #4

    With a radiant floor, at your floor temperture, even if you insulated to R50 between the joists the joist edges would stay above the dew point of the conditioned space air in winter from conducted heat alone (unless actively humidifying to some ridiculous level.) The only period of risk would be during the cooling season, and with your ventilation scheme and l0w summertime dew point climate it's not a big risk. Outdoor summertime dew point averages in zone 4C are well below the deep subsoil temperatures.

    Unfaced R15 rock wool or unfaced R13-R15 fiberglass is plenty. Kraft facers don't have the fire ratings to be safe when left exposed, but perforated aluminized fabric radiant barrier does. While R15 rock wool is dense enough can be installed with friction fit and still hold, in seismic regions like the PNW with use oeven a small earthquake it would bring it down. With 16" o.c. joists, take a 48" wide roll of perforated RB and carefully cut it into three 16" wide rolls, which has sufficient width to side-staple it to the joists to hold up the batts. The perforated versions are class-III vapor retarders at about 5 perms, so even seepage from a spill or minor flood would still be able to dry into the crawlspace at a rate many times faster than through a subfloor.

  5. CIFanatic | | #6

    Hi Jeff, what did you end up doing with the joist bays? Did you insulate them or not? I have the exact same scenario in the PNW. I am considering leaving the joist bays uninsulated with a fully sealed and conditioned crawl space with one of two options. First, allow the radiant heat to flow freely into the crawlspace to keep it close to the home temp or installing 2” rigid foam to the subfloor and then stapling the tubing to the insualtion and pouring gypcrete over the system. Option two will direct more heat towards the living space and leave the crawlspace joist bays open. I am using geothermal and a heat exchanger to run the hot water heating so fossil fuel consumption is a non-issue making me wonder if option might provide a more optimal solution. Thoughts?

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