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Community and Q&A

SIP house — Backup heat source?

John_Brown | Posted in Mechanicals on

I am going to be building a 26′ x 32′ SIP enclosed house (6 1/2″ Walls and 12″ Roof) that will be heated with a Ductless MSP (1 outdoor unit and 2 indoor heads). My coldest day requirement will be around 17k BTUs per hour. The design temp for coldest temp is -15. I know I will need a backup system for when the Coefficient of Performance drops down… I have tentatively designed for a woodstove as backup but I really would rather avoid it. Does anyone have any experience with Ductless as their main source and what a long high pressure weather cycle (cold temps) might be like in a SIP house that is well sealed? I am looking at a couple wall convection heaters as an alternative to a woodstove and all the requisite penetrations through the building envelope required for the chimney.

I do like a wood fire but I’ve been putting most of my research into getting the Ductless properly sized, planned, specc’ed and commissioned (so it works well).

If there is a woodstove forum that perhaps might be a good place to look for info, that would be great too…

Thanks in advance. I’ve learned a great deal from all the folks here.

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  1. jkstew | | #1

    Where are you building? Have you looked at a Rinnai gas heater? They're small, compact, cheap, and fairly efficient. You still need electricity to operate one though.

  2. user-2310254 | | #2

    I'm sure you know how important air sealing is with SIP construction. So I imagine your house will be fairly air tight. Are you projecting 17K BTUs for 832 square feet of conditioned space? Is this what you came up with after running an aggressive Manual J?

  3. STEPHEN SHEEHY | | #3

    I'm in Maine, zone 6. The last two winters have been pretty mild, with about -12F as cold as it has been. My two Fujitsu minisplits have never had a problem keeping us warm. I think the key is a robust building envelope. When we've had a winter power outage, in 24 hours the house drifts from 70° to 68°.
    I love a wood fire, but we opted for an efficient envelope and skipped the wood stove. So far, we haven't missed anything.

  4. ohioandy | | #4

    John, you do not need a backup. Fujitsu or Mitsubishi units, properly sized and positioned, are all you need. Their COP drops at low temps, but they'll do the job. For redundancy you might consider putting each head on a separate outdoor unit--it's the most efficient configuration anyway, and if one fails the other keeps going. The only wildcard is extended power outage--a woodstove is your best hedge for that, but if you've got a reliable utlity and outages are rare or short, your house will stay warm.

    Steve's right--17K sounds high for your envelope, unless that 26x32 footprint includes a basement and two floors.

    My new SIP house is 20x28 with three levels, and with four years of data it's clear that my two 9K Fujitsus are significantly oversized for the job here in northern Zone 5.

    Know that your SIP thicknesses do not achieve code minimum R-values for your climate zone. If I were you I would bump those up at least a couple inches. I wish I had gone thicker than 6.5 walls and 10.5 roof. (There's no building department around here to force the issue.)

    Good luck with the build!

  5. GBA Editor
    Martin Holladay | | #5

    I agree with Stephen and Andy: your house should be fine without backup heat. But if you want to install a couple of electric-resistance heaters for peace of mind, they're cheap.

    Operation of a wood stove can be tricky in an airtight house. Some homeowners report that they have to open a window to get a fire started without backdrafting problems.

    The Structural Insulated Panel Association lists the R-value of a 12 1/4-inch-thick SIP with an EPS core at R-42. Most building codes require cold-climate homes to have a minimum roof R-value of R-49.

  6. John_Brown | | #6

    Thanks for the recommendations. Regarding the SIPs R-Values, I asked the SIP company what they recommended for my area and they suggested 6 1/2" and 12" so I guess I better follow up with the AHJ. The house is 2-stories to answer a Steve's question. I am planning on an 8" thick stem wall but I need at 4" of that thickness for structural posts to bear. I'd rather not have a 10" thick stem wall so I will make some inquiries.

  7. user-2310254 | | #7


    Whatever you do with your foundation, consider installing insulation at the perimeter and under the slab. There are several articles on GBA with practical advice on this topic. For example,

  8. Expert Member
    Dana Dorsett | | #8

    With a 99%design temp of -15F you may run into a code-compliance issue with Mitsubishi, since their extended temperature capacity tables only extend down to -13F. They'll keep running and still put out heat below that temperature, but the amount of heat is not specified by the manufacturer, which can be a problem for some code inspectors, who might then require a backup heating system.

    If it ever hit's the -20F & lower range the Mitsubishi units are designed to automatically turn off, then automatically re-start when it warms up to -13F, so it's not insane to have a backup plan if it's -15F is the 99th percentile temperature bin in your location.

    Fujitsu's cold climate single-zone mini-splits DO have a specified output down to -15F (but not lower.) They too will keep running at lower temperatures with real but unspecified output capacity. Unlike Mitsubishi, they have no auto-cutoff feature. Fujitsu's 2-zone muliti-splits only specify the capacity down to -5F which again backup becomes a code-compliance issue for some inspectors. It would be cheaper to specify single zoned Fujitsus (to have a specified capacity @ -15F) instead of a multi-split than it would be to install a complete backup system.

    But if that "...coldest temp is -15..." is -15C, not -15F, there are many multi-split options from multiple vendors that will get you there without a backup system.

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