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Community and Q&A

Are ductless minisplits a viable solution for a 200-year-old timber frame retrofit?

Ryan Wiley | Posted in Mechanicals on

Project type: full house renovation; currently down to the stud.
House type: 200 year old timber frame style farmhouse
Location: Milton, NY, USA

We’re in the midst of a full-house renovation of a building that sat vacant for 20 years. We replaced the roof and applied closed-cell spray foam in the attic and gable wall extensions. The intention is to closed-cell insulate the remained of the building envelop upon completion of MEPs.

We were originally considering two forced air systems (one in the attic for the second floor, and one in the basement for the first floor); however, we are not thrilled about air quality with conventional forced air heat. We then considered a hydronic air application, but have been met by each HVAC contractor with some hesitation due to the complexity of the ductwork needed in the basement. Several contractors have suggested ductless units which we initially opposed due to the appearance of the wall-mounted units throughout the home.

More recently, and with a little more research, we are becoming more turned on to the ductless solution and want to hear from others about their experience.

1. Would a series of mini splits be a viable option for a whole-house application in the northeast?
2. Has anybody had success with the floor mounted units? Can a decorative cover be placed over them, like an old radiator cover, or would that throttle efficiency too much?
3. How is the air quality? Would we still need to install an air recovery ventilator (HRV/ERV to be determined)?
4. What are the general benefits and shortcomings of this solution?
5. Finally, are there any other alternatives that you would suggest?

I know this is a lot to ask for, so any response will be greatly appreciated. Thank you!

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Replies

  1. User avatar
    Dana Dorsett | | #1

    Run an aggressive room-by-room, zone by zone Manual-J heat load calculation. (or have an engineer, RESNET rater or other energy nerd run those numbers, NOT an HVAC contractor.)

    There are many potential cold-climate mini-split/multi-split solutions that would work just fine at your ~+6F outside design temperatures (Poughkeepsie's design temp: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf). It's only a matter of how much.

    Foor units work fine, at least as efficeint as wall coils in heating mode, a bit less efficient in cooling mode, but they're slightly more expensive. A radiator cover is not recommended for floor mounted units- the need unrestricted air movement. (Radiator covers were invented explicitly to reduce convective air motion, limiting the output of the radiator.)

    Ventilation is not a function provided by ductless heat pumps. (Nor SHOULD they be for ducted heating systems, which tend to over-ventilate when it's cold outside, and the air is driest.) Also by not creating room-to-room air pressure differences, ductless systems to not drive outdoor air infiltration the way ductess systems do. Your indoor air quality is not affected either way by ductless heat pumps. Active ventilation is a good idea in any house, but it's better to divorce the ventilation function from the heating & cooling function. If you're having a issue with duct-routing, there are ductless HRVs out there (eg: Lunos e2 & Nexxt.)

    A Google spin-out company called Dandelion is active in your area, doing lower cost ground source heat pumps (GSHP).. I believe they're all ducted systems, but it's worth investigating. GSHP done right can have an efficiency advantage over air source heat pumps like mini-splits.

    https://dandelionenergy.com/

    https://dandelionenergy.com/press-release/mayor-announces-plan-offer-geothermal-homes-village-rhinebeck-dandelion-energy/

  2. User avatar GBA Editor
    Martin Holladay | | #2

    User-6952768,
    First of all, can you tell us your name?

    Your reference to MEPs confused me -- so I'm guessing that other readers may be confused, too. I think that "MEP" stands for "mechanical, electrical, and plumbing."

    Ductless minisplits and ducted minisplits are heating and cooling appliances. They do not provide ventilation. All of the air that comes out of a minisplit unit is indoor air.

    For information on ventilation, see these two articles:

    Designing a Good Ventilation System

    Revisiting Ventilation

    For information on ductless minisplits, see these articles:

    Rules of Thumb for Ductless Minisplits

    How To Buy a Ductless Minisplit

  3. User avatar
    Dana Dorsett | | #3

    It's not clear what the rationale is for:

    "The intention is to closed-cell insulate the remained of the building envelop upon completion of MEPs."

    Closed cell foam is one of the least-green insulation products out there, adding VERY little to the thermal performance of a framed building (despite the high center-cavity R) at a very high cash cost, and a significant environmental hit, especially if blown with industry standard HFC245fa. Even when blown with a low-impact blowing agent, it has a very high polymer per R compared to open cell foam (which is blown with water, not HFCs.)

    There are times when closed cell foam is the only cost effective way to manage water vapor diffusion issues, but those are pretty limited in scope. It's almost always possible to design it out of wall stack-ups.

    Milton NY is on the very warmest edge of US climate zone 6- skipping a stone to other side of the river it would land on the cool edge of zone 5, where interior side vapor retarders aren't even necessary if there is back-ventilated siding, and as little as R5 of exterior insulation provides sufficient dew point control at the sheathing for up to 3.5" of fiber insulation in the walls, and R7.5 is sufficient for up to 5.5" of fiber insulation. The fact that Ulster County is technically zone 6 based on the county-wide temperature averages doesn't mean Milton's local climate isn't really zone 5. (Is Milton really any colder than Poughkeepsie? I doubt it!)

    Suffice to say, there are greener (and cheaper) ways to hit your thermal performance goals here than closed cell polyurethane without risking the integrity of this antique. More information about the wall's material stack up, and the plans for any re-siding or refinishing of the interior would be needed to advise on the alternatives.

  4. Ryan Wiley | | #4

    Dana, your response is much appreciated. I have been in contact with 475 regarding the Lumos units and are leaning in that direction; the concern is the potential number of penetrations to the exterior--we'd like to minimize that and preserve the appearance of the house as much as possible. Based on how airtight the house will be may allow us to reduce the number of units--which leads to the appropriate insulation type...

    The plan is/was to apply closed cell spray foam. This is already done in the attic, as originally mentioned. We will be keeping the original clapboard siding, so it was suggested by all the spray foam contractors that we apply this directly to the interior side of the siding. Originally I had a number of concerns about this (breathability, areas for moisture to condense, the longterm health of the siding, adhesion to substrate, etc.) but over time the concerns were somewhat put to ease. Now, I'm back to questioning SPF and revisiting other alternatives (Havelock Wool?).

    To clarify "The intention is to closed-cell insulate the remained of the building envelop upon completion of MEPs." what I mean is that after mechanicals (TBD), electric, and plumbing are roughed in, we would insulate the remainder of the building envelope--I provided this detail with the hope it would help others better understand the airtightness/insulation of the home with respect to the mini splits, and controlling the home's interior climate.

    Really appreciate your input.

    Thanks,
    Ryan

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Ryan,
    If your spray-foam contractor suggested that you install spray foam on the interior side of your clapboard siding, I'm guessing that your walls have no sheathing.

    Your spray foam contractor is giving you bad advice. I urge you to read this article: Insulating Walls in an Old House With No Sheathing.

    In that article, I wrote:

    One “solution” to this situation is to install spray polyurethane foam insulation directly against the back side of the siding. Why is this is a bad idea?
    • Spray polyurethane foam — especially closed-cell spray foam — will cling tenaciously to the siding, gluing everything together. This will make future siding repairs — for example, replacing a cupped or split piece of siding — almost impossible.
    • The spray foam may ooze out of cracks between the siding as it expands, forcing the siding pieces apart. If this happens, you’ll end up with an unsightly, hard-to-clean-up mess.
    • The spray foam will limit the ability of the back side of the siding to dry. When a rainy day is followed by a sunny day, the exterior side of each piece of siding will be dry, while the interior side will be wet. This condition leads to cupping and splitting.

  6. Ryan Wiley | | #6

    Martin, thank you. This was my concern and the night before they were scheduled to work, I lined the stud bays on the gable ends with Tigerpaw. The installer thought this was unnecessary and removed the material. If there is any silver lining, I still have the remainder of the envelope to approach differently.

  7. Bob Irving | | #7

    We build "net zero ready" homes in NH, and use minisplits (ASHP) consistently. We've done two upgrade to older homes - my own home being one - where we changed the heating system to ASHP's which work great. The most critical part of sucess with them is to make the house very tight with good insulation values. In my many years building I've used many types of heating and the ASHP minisplits are the best system by far - quiet, effective, affordable. And if you do make the house tight, install a ventilation system!

  8. Ryan Wiley | | #8

    Bob, I appreciate this input. We closed-cell spray foamed the roof and gable walls, and intended to do the remainder of the house in the similar fashion; however, I'm really turned on to mineral wool (thermafiber or roxul) for the remainder of the envelope, and air sealing the interior. In your old house projects, what insulation did you go with?

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