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Can I cool an old house with 1 minisplit… in the attic?

Damien Kirrmann | Posted in Energy Efficiency and Durability on

my 1953, 1300 sqf, new to me florida ranch 3bed/2bath has an HVAC issue. The current system is a heat pump split system with ducts, located in a *completely* non-insulated attic. I mean, NO insulation. Not on the floor/ room ceilings. Not under the roof. Nowhere. The attic is super small, which means that not only there is no way to add insulation without removing the ductwork and the air handler (and not putting it back), but also that the ductwork is very poorly done (squeezed everywhere, not straight, sharp bents, etc.) making it a very, very unefficient HVAC.

My plan is to:
1- remove the AC.
2- insulate the attic making it part of the conditioned space (creating a “hot roof”). Having no more ductwork, I should have enough space to insulate it close to R-48 or maybe R-60 without thermal bridging.
3- insulate the crawlspace.
4- install a mini-split in the attic coupled to a good whole house ventilation system.

My idea is by putting the mini-split in the attic, I can use it to full capacity. Putting the AC in a room would mean a lot of units (I have over 10 rooms with doors and the floor plan is not open) and would also mean underutilized units (the rooms are small).
So, can I just treat my house as one big room, and use the attic as a “plenum” (the mini-split cools the attic to an even temperature, and then the attic cools each room through ventilation, thus avoiding to have one cold room where the AC is and hot rooms on the other end of the house).

I would boost the insulation of the attic and the crawlspace (conditioned as well) but I would still have poor wall insulation and poor window insulation (the windows are original single pane). However, the attic would be very well insulated and therefore could serve as a great “plenum” to distribute cold air throughout the rooms.

Does it make sense? is it stupid? Any other idea?

Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Damien,
    Before I address the question about using your attic as a plenum, I'd like to point out that you may want to consider installing rigid foam insulation on the exterior side of your roof sheathing. If you take this approach, it would allow you to create an unvented conditioned attic without removing the HVAC equipment from your attic. Of course, this approach would require you to install new roofing. Here is a link to an article with more information on this approach: "How to Install Rigid Foam On Top of Roof Sheathing."

    Second: concerning your idea about cooling your entire house by using your attic as a plenum, there are a few problems with the idea:

    1. You didn't describe what type of ventilation system you have in mind, but it's worth pointing out that the average residential ventilation system moves tiny amounts of air (often only 50 cfm to 100 cfm), while space heating and cooling requires blowers that move large quantities of air (often 1,000 cfm to 1,200 cfm). That's why a ventilation system usually can't move enough air to affect the temperature of the room served by the ventilation system.

    2. Single-glazed windows, especially west-facing windows, can allow so much solar heat gain that these rooms need a lot of cooling -- and your method probably won't be able to keep rooms that have this type of solar heat gain comfortable.

    3. If you install very large ceiling-mounted grilles in all of your rooms, your method might work (except for the problem of rooms with high solar gain) -- but this approach means that you would have no acoustical privacy in your house.

  2. D Dorsett | | #2

    An R60 roof makes no financial sense or comfort sense in any FL climate, but there may be a case for R40 whole-assembly insulated roof. An overlay of 4-5" polyiso above the roof deck with 5-6" of cellulose or open cell foam on the underside of the roof deck would be "close enough", provided that you verify that the attic is truly air-tight with blower door testing.

    Using an attic or crawlspace as one big plenum becomes a sound-transmission/privacy issue. A 1.5 ton mini-duct type mini-split with a Manual-D duct system would probably be more than sufficient for a 1300' house. It may or may not be cost effective depending on the efficiency and oversizing factor of your existing system though. Sometimes it's better to just re-commission an existing system, fixing it's duct issues. But with a completely uninsulated attic it's likely that the system would be 3-5x oversized for the new lower load, which could impact latent cooling (humidity) comfort. A right-sized mini-ducted mini-split modulates, and would run almost constantly, providing fairly good humidity & temperature control, far better than a ridiculously oversized system running a 10% duty cycle.

    With mini-duct cassettes it's possible to install the unit in a sealed-insulated crawlspace under the floor, which may be easier than in an attic. While this would be an unusual way to go about it in FL it can still work well in a rancher. If installing a new mini-duct mini-split system under the floor, ripping out the old system in the attic provides enough attic space for R30+ with a vented attic, that may be cheaper and better overall.

  3. Damien Kirrmann | | #3

    guys these are by far the 2 most useful answers I have gotten since I asked my question in multiple places. Thank you for taking the time to really answer my question, that was tremendously helpful.
    Considering that my ventilation system would have been at around 150 cfm for each room and that I have large single-pane west facing windows, I will have to give up on the idea of using the attic (or the crawlspace) as a plenum. I like the idea of Insulating the outside of the roof. Fixing up the ductwork of the existing system might be wishful thinking as there is just not enough space to do a better job than what was done... I'm disappointed that my idea doesn't work but hey, now at least I understand why. The point you are making about noise insulation is great, I didn't think it would impact noise insulation much so I'm glad you brought it up. Thanks!

  4. Expert Member
    Dana Dorsett | | #4

    Large single pane west windows drives the cooling load numbers skyward, unless shaded by a very deep porch roof overhang or a local fence, etc. A reflective window film an a low-E exterior storm window can cut that some, but not nearly as much as actual shading.

    What's the tonnage of your existing AC?

    How are you heating the place?

    If using a sealed insulated crawlspace as a plenum it's possible to get at least some sound isolation by using panned-joists as "ducts" to the "plenum", and spacing the open end of the panned joist bays as far apart as possible, rather than an open grilles directly into the crawlspace. That's probably harder to do in an attic, which, which may have much shallower joists too (= higher effective duct impedance)

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