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New Basement Heating: Radiant Slab? Comfort and Efficiency

CDG01 | Posted in Green Building Techniques on

Hello GBA Team – looking for some input on whether or not I should plan to include radiant slab in a new basement addition. Home is in at the very northern edge of Zone 4A, close to Zone 5A. Hot & humid summers, cold dry winters.

Finalizing plans for a 800 sqft. addition with a 9′ non-walkout basement and a 4″ slab. 2″ XPS or EPS rigid foam below slab.

Basement will initially be un-finished. But 2/3 of it will eventually be finished as a game/entertainment room that I expect will get frequent periodic use, especially in the winters. i.e. 4x a week in the evenings/nights. Floors will likley be ceramic or vinyl tile. Basement walls will be framed 2×4 or 2×6 with R-15 or R-23 Roxul. Home has an existing forced-air furnace w/ capacity for the addition. No current hydronic heating in the home.

I’m considering roughing in PEX for a radiant slab, while I have the opportuinty to do so. But don’t want to waste money on the PEX if there are better heating options.

My primary concern is comfort in the winter, secondary is system efficiency. My thought is to use the radiant slab primarily to keep the ‘chill’ off the room throughout the winter 24/7, say 60*. And then augment it with forced-air or hydronic baseboard heating to quickly warm up the room when in use. I’d add a dedicated boiler for this purpose, which may also support radiant in a bathroom above the basement.

Is radiant slab a good option for this scenario, making it worth adding PEX to the slab pour?

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Replies

  1. Andrew_C | | #1

    Several thoughts come to mind.
    - IF you put radiant heat into the slab, you need to increase the amount of sub-slab insulation.
    - You mention Roxul for the basement walls. You are aware that you shouldn't use air permeable insulation directly on the basement walls, right? There are a lot of articles and Q&As on this topic here at GBA, but basically you can't allow room air to get through the insulation and touch a cold wall without causing moisture problems. So your first layer of insulation is probably going to be foam.
    - If you put in sub-slab insulation and insulate the walls and rim joist correctly, you should have a dry comfortable basement without the radiant floor heat. Save your money and put it elsewhere.
    - If you use XPS insulation, choose recycled. The environmental hit of XPS vs EPS for commonly available brands is orders of magnitude worse.

    Dry basements are great. If you properly do the sub-slab prep (including radon abatement and drainage), you're close to a good solution and everything else can be done later if necessary.

    1. CDG01 | | #2

      Thank you for your input, and apologies on the confusion. Using mineral wool in the upper walls and eventually basement ceiling joists (sound) and confused things. Code requires that the unfinished basement walls be insulated (and then fire-blanketed), so that will be done with a non-permeable insulator long before I finish it off.

      In your opinion, is 2” rigid adequate? I’ve never owned a house with a well-insulated basement (owned plenty with poorly-insulated ones). And want to make that downstairs space as pleasant as possible.

      Rim joists will be well-sealed and insulated, and a proper drain tile/sump will be installed and tied into my current radon system. Have a whole-home dehumidifier as well. Mostly just worried about temps.

      1. Andrew_C | | #3

        IMO, R10 is probably close to what you want, so 2" is probably not enough.

        If you have a significant project, you should definitely invest in a GBA membership so that you can read articles like this one: https://www.greenbuildingadvisor.com/article/determining-sub-slab-rigid-foam-thickness. GBA membership will be your best value for money on your project. IMO.

  2. Expert Member
    Akos | | #4

    There are a number of threads about radiant for basement:

    https://www.greenbuildingadvisor.com/question/radiant-floor-heating-logistics-and-insulation-materials

    Generally it is not worth it unless you already have hot water heating. The heat loss in a well insulated basement is very small and it will never be hot enough to feel warm under your feet.

    Andrew's suggestions are spot on. There is really no point for 2x6 studs in a basement, you are just loosing space without making the space any more energy efficient or comfortable.

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