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In-Floor Radiant Heat in Tight House

rkymtnoffgrd | Posted in Energy Efficiency and Durability on

I’m about to pour the slab, time to decide if I imbed 1/2″ Pex…  I don’t know if I’m too insulated to make good use of it?  My 1000sf house is in zone 7, all heating day, 10k in Colorado Rockies,  south-facing, passive solar slab on grade, 4″ foam under-slab, 2×6, 24OC framing, high-density fiberglass with zip then 2″ exterior foam, thermalbuck + Alpen triple pane Argon filled windows and energystar doors, R50 blown fiberglass in attic.
My original plan, based on research here, was a single low-ambient temp heat pump.
However, I love the thought of the warmth coming from the floor.  Considering imbedding 600LF of 1/2 pex in two zones.
I fear that by the time the floor is warm to the touch, the house will be well over my 70 degree comfort level….
Thoughts?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Rkymtnoffgrd,

    If you use the GBA search function you will see this gets debated here at least twice a month, with lots of interesting arguments and no demonstrable advantage to one side or the other. You won't get appreciably warm to the touch floors, but some advocates say the gain of several degrees over an unheated slab is still a significant increase in comfort - but that increase in comfort comes at cost, in both complexity and money. There simply isn't some knock out punch one side of this debate can land over the other. I think it comes down individual preferences.

  2. paul_wiedefeld | | #2

    Heating with water scales down terribly: the equipment is expensive and the labor barely changes even if your house is 1000 sqft and well-insulated.

    Clearly define what is warmth means to you and then compare that to what your heat loss is. If a 73 degree floor is warm but a 70 degree one isn't, you might have a case for it.

  3. onslow | | #3

    Rckymtgoffgrd,

    I am at 8000 ft in SW CO, so similar context. I put pex in my basement and garage slabs when I had the chance. The slabs are on 3" reclaimed XPS which itself sits on 6" of washed rock with embedded radon pipes. I have not activated either, but if I should find loose cash I would be motivated to put some heat into my shop slab. It currently is stable at 60F drifting to 55F at the perimeter. Mind that this is below grade and my foundation is fully insulated on the exterior with more 3" XPS. I don't wear slippers in the shop, so it has not been a big problem, but it is an endless heat sink which makes keeping the shop temperature at 67F a year round heating condition.

    You will only have one shot at placing PEX, so if you are willing to do the work, have a few hundred extra dollars AND have agreement with the person doing the cement work, I would go for it. I stress having an agreement with your cement contractor as the placement depth for PEX and wire mesh need to be planned for. My cement contractor warned me that if the PEX gets too close to the surface of the slab your chances of cracking is greatly increased. He related a tale of someone that ended up with cracking that exactly echoed the pipe pattern. He also let me know that the mesh needed to be as near the middle of the slab as possible. A four inch slab pour means the PEX is going to be close and any bending will make it closer.

    My work around was to pin the PEX to the foam using lawn staples before his crew placed the mesh a few days in advance of the pour. (which I would not do this early at your altitude) Once the mesh was there I unpinned the PEX from the foam and zip tied it to the underside of the mesh. Hopefully your back is younger than mine. The zip tails were aimed down and trimmed. The mesh sat on cradles to maintain proper setting and all came out very nicely with no cracks.

    You will need to plan on how you run out the infeed outfeed sides of the PEX runs. Keep the run lengths similar and preferably a bit under the 300 ft. The radiant floor sites can advise better on why and such. You can buy special radius clips to hold the PEX in a 90 degree turn where they come up out of the slab or you can feed the PEX through PVC elbows. Just strap 1" PVC conduit elbows to a board which can be mounted on a stake or (if there) the wall at the slab edge. Leave a few feet sticking up so you can set any distribution pieces at a work friendly height.

    Don't worry about the PEX being a bit lower in the slab. The foam under means the heat will still flow to the room side far faster than toward ground. As for hot water sources, some inventive types have rigged up reservoir tanks behind woodstoves that feed the loops with low head recirculator pumps. PEX isn't rated for very hot water, but at your elevation boiling is around 194F so plan accordingly.

    If you want to know instantly if someone nicks your tubing while the pour goes on, you a can rig up gauges and valves to reveal pressure loss. As there will be very little to do about it during the pour, you can just opt to test after the fact as I did. One thing you can do is cap the ends of the PEX to keep them clean. Rubber crutch tips work quite well.

    The mythical warm to your feet floor situation, as so frequently spoken of here on GBA, is quite a real problem. It does take about 82F to feel warm though some will say higher/lower. The floor materials conductivity will play into this more directly. If you are staining or stamping the concrete as a finished floor then 70F won't feel particularly warm. Still it will be above what my ceramic tile floors appear to be with my thermostat at 70F. I just measured three areas around the house and all are showing 65F which is closer to the shop temperature in the basement below.

    Deep in the winter you will likely find that even keeping the floors at 72+ won't over drive your envelope. My Alpens are R6.6 and while quite wonderful they do represent a major hole in my R36 walls. You can run the BTUs lost and needed for your particular conditions and decide. The shoulder seasons will be where the capacity to overrun the need will show most.

    If you are putting wood flooring over the slab the main effect will be to reduce the rapid heat loss out of your feet. Cork is even better. Mies van der Rohe did all the floors in the residential tower I lived in with cork over floor slab and it is pretty effective. Best of all, cork tiles come pre-finished now so the need for re-waxing is much less frequent. His steel window frames were another matter.

    Last thing, have you got plans in place for the slab to be isolated from the perimeter walls? Even an inch of foam will help a lot.

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