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Community and Q&A

Best way to insulate for staple-up radiant heat?

Jack Roberts | Posted in Mechanicals on

Hello,

I have 1/2″ PEX w/ O2 barrier w/ aluminum heat transfer plates stapled up to bottom of 3/4″ subfloor w/ 5/8″ engineered hardwoord and 3/4″ tile above. I also have 1/2″ pex installed in the concrete basement slab and have 3 zones. One is the basement (which is currently unfinished 10″ block walls w/ 2″ XPS on exterior side. I currently have no insulation under the staple up (main floor). My basement and house stay warm but the basement zone rarely runs and the main floor runs a lot. My heat source is a Thermolec electric boiler and my electric bill jumps $100-500/month depending on the temps. I live in Climate zone 5A (northwestern part of PA). My thoughts for not insulating were that the heat would rise and continue to heat whole home, however, I’m wondering if I insulated w/ R-13 or R-19 faced fiberglass my electric bill might come down and boiler run less by separating the zones?

The main floor is separated into 2 zones, one is main living space (family/dining/kitchen/foyer) the other beds and baths. Main floor is 2050 sq ft. I believe my water temp is around 115-120 degrees F. I, eventually will finish 65% of basement and install drop ceiling. Does anyone have any input or experience with this situation? Should I insulate under the staple up or leave alone? I will also say I ran my 2″ XPS up over my rim joist and I insulated the ends of the joist cavities w/ R-23 Roxul. If the recommendation is to insulate, should I snug it up tight to the bottom of the PEX or leave a 1″ or more air space?

Thanks for your help in advance.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jack,
    The main advantage to installing insulation in your basement ceiling is that you will gain better zone control. Right now, you are heating your basement -- even on days when you may not need to heat your basement.

    Ordinary fiberglass batts, snugged up to the PEX tubing, will be adequate insulation for this purpose. For more information, see Is a radiant barrier necessary for staple-up radiant heat?

  2. Jack Roberts | | #2

    Martin,

    Would you recommend R-13 or R-19? faced or unfaced?

  3. GBA Editor
    Martin Holladay | | #3

    Jack,
    In the thread I linked to, Dana Dorsett wrote, "The R-value that make sense varies -- ‘value rolls’ of kraft-faced R-13 is sufficient for zone-isolation over a conditioned basement."

    My answer would be similar. I think that unfaced batts are preferable -- there is a small risk that when kraft facing is installed close to warm PEX tubing, the kraft facing could emit an asphalt smell. Fiberglass is cheap; use R-19 if you want to feel like you're doing a really good job.

  4. Jack Roberts | | #4

    Ok I'll take all of that into consideration. Thanks Martin for your input.

  5. Expert Member
    Dana Dorsett | | #5

    Assuming this is a DIY...

    For this application it's better to use rolls rather than the pre-cut batts, since the cut batts are designed for standard wall framing heights, and your joist bays are probably a very different (and longer) dimension.

    Contractor-roll R13s are dirt cheap typically 25-35 cents per square foot at box store pricing, and usually have asphalted kraft facers. With the facer on the basement side smell isn't an issue, but fire codes may require you to install a ceiling if the kraft facer is exposed to the basement (check with your local inspector.)

    Unfaced 3.5 inch thick "sound control" batts are good enough for this application- but check pricing, which varies quite a bit. Unfaced 3.5" fiber glass typically runs 35-45 cents per square foot, but sometimes it's 50+ cents. Unfaced R19s in contractor rolls are typically 45-55 cents/per square foot.

    R19s and R13s contain the same amount of fiberglass- it's only a matter of the manufactured loft/density. Per-square foot they weigh the same. It seems silly that R13s with facers are less expensive than R13s or R19s without facers, but the manufacturing volumes are much higher for kraft R13s than unfaced batts. Shop around for what's on sale- any manufacturer's product will be fine.

    Insulation contractors buying in volume get steep discounts, and for standard walls & attics the installed price is less than what a DIYer can do at box store pricing. The quality of installation varies by quite a bit too, but you can always fix any errors later.

  6. Expert Member
    Dana Dorsett | | #6

    BTW: If you're really seeing $500 wintertime heating bills, a mini-split heat pump may be a very good investment. You can still retain the cushy warm floors by using a floor thermostat, but control the room temperature with the mini-split. Properly sized & placed it should cut your winter power bill by roughly half, even with the radiant floor.

    A 73F floor is pretty cushy for bare feet and will be delivering about 2 BTU/hr per square foot into a 72F room, nearly nothing into a 73F room. Keeping the room temp at 73F with a mini-split costs less than half of what it costs to heat the room to 68F with the electric boiler. So when it feels too cool indoors even with the 73F floor, crank up the mini-split temp, and only bump the floor temp if the mini-split isn't keeping up.

    A 1-ton cold-climate mini-split costs about $3500-4000 in my neighborhood, and has enough cooling capacity to cool most of the house, enough heating capacity to cover more than half your 99% design heating load, over 80% of your average mid-winter heating load, and all of your shoulder season load. Done right it'll pay for itself in less than 5 years, and you'd still have your cushy warm floors.

  7. Jack Roberts | | #7

    Dana - Thanks for the information... When I decided to go electric propane was through the roof $$ and there were shortages... about 2-3 years ago.. now propane is way down and I was considering adding a propane boiler to my heating panel to use instead of the electric... I can fill a 500 gallon tank for about $600-700 bucks right now... However, I will look into the mini-split system, I am really not familiar with them. Also is the 3500-$4000 price installed? I have no duct work in the house right now but after this hot summer I was contemplating adding some type of central air system. As far as the insulation goes I will definetly check w/ some insulation contractors and compare to box store DIY pricing. Thanks for your help.

  8. Expert Member
    Dana Dorsett | | #8

    A 1-ton cold climate mini-split fully installed, no subsidy runs a bit under $4K in my neighborhood (central MA) during the busy season, cheaper when the contractors are a bit hungrier. A few years ago a relative of mine in WA installed a 1.5 tonner for about $4300, and qualified for a rebate subsidy from her utility (Puget Sound Energy) for $1000. That price point was comparable to what it was running in my area at the time, but it's slightly cheaper now (with even better equipment!) The Mitsubishi FHxxNA series or the Fujitsu xxRLS3H would be the likely choices, but it would be worth running some heat load analysis to be able to size it correctly. Oversizing it for the rooms/zones by too much, or undersizing by too much results in lower efficiency- you need to figure out the "Goldilocks zone" of mini-split output that works best for your house & climate.

    With some summertime and wintertime power bills (with exact meter reading dates) it's possible to assess the whole-house heat load with some precision, but a room-by-room load calculation would be more useful. Doing both gives you some sanity checking on the room-by-room numbers, which may need scaled a bit to match the power-use measured reality. See:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

    and

    https://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1

    A propane boiler installation is likely to be quite a bit more expensive than a mini-split. In most of the northeast propane is a mini-monopoly, where the tank is owned and maintained by the propane supplier, and stories of low introductory pricing followed by price gouging of low-volume users are rampant. Hopefully that isn't the case in your neighborhood. Either way, the money for the propane boiler retrofit may be better spent on rooftop PV solar to offset electricity use.

    Rather than a boiler, if you already have a propane fired hot water heater it would be cheap enough to use it to supply the 115-120F water for the radiant floor, for the cost of a bronze pump and a heat exchanger and some controls.

  9. Jon R | | #9

    See here for more discussion of floor temperature vs comfort. But I'd put on some slippers and seriously consider an inverter mini-split.

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