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Minisplit heads or slim duct for bedrooms with uninsulated attic?

monkeyman9 | Posted in Mechanicals on

I have a 2 story Colonial with a large room above the garage.  I am looking to mainly add cooling to (heat pump for shoulder seasons).  Based off Clinton, MA data.  The large room above the garage has a Mitsubishi FH15NA head in it already for heating and cooling. Manual J data is below.  There are 3 bedrooms upstairs, 1 being a master suite with large bathroom and closet attached.  The attic is vented, attic floor is well insulated with R60 fiberglass other than the master bath cathedral ceiling with R30.  Underside of the roofdeck is not insulated and not interested in doing so as it is a very steep pitched and tall roof.  The attic gets A LOT of sun the west side of the house (master suite side, as well as family room and table area (linked to kitchen). 1.  First floor I am thinking of a Fujitsu or Mits 15k btu head in the family room blowing 16 ft across to the 6ft wide opening of the kitchen/table area that is the hottest area of the house (12 deep past the 16 ft family room depth).  I also would put a 6k btu head in the office (future spare bedroom) so it can blow through the double doors across to the dining/room across from it (doors open 90% of the time).  That leaves the bathroom on the 1st floor as not getting much cooling being at the far end and around a corner.  I can always add a small duct through a wall with a fan there later.   Other option is using a slim duct unit for floor 1 and lose the basement headroom and efficiency.  Thoughts? 2.  I’m really not sure of what I should do on the 2nd floor.  I either have to put a 6k btu head in each bedroom plus one in the master bath (hottest room in house) or run 2 slim duct units in the attic (mast suite slim duct plus one for the other bedrooms and 2nd bath).  If I run slim duct units in the attic I’d spray foam insulate the ducts then bury them in insulation.  The ducts would unfortunately be perpendicular to the joints, so I can really run them in joist bays.  The room above the garage is next to the 2nd bath and with the doors open the cool air flows into that bathroom well enough.   Looking for expertise on the heads or slim ducts in this situation? 3.  If I do heads in the 1st floor, should I get a dedicated 15k btu unit for efficiency then use a larger multi unit for the other 6k head in the office and the 2nd floor units.  Or a Fujitsu 24k multi unit for the 1st floor and another 24k multi unit for the 2nd floor? EDIT:  Master bathroom cooling loads look low because it has 3.5″ of closed cell spray foam in the walls (partial bump out room) and cathedral ceiling.  The remainder of the house is only R11 unfortunately.

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  1. Expert Member
    Dana Dorsett | | #1

    > Master bathroom cooling loads look low because it has 3.5″ of closed cell spray foam in the walls (partial bump out room) and cathedral ceiling.

    The master bathroom HEATING load seems super-high (isn't there any glass in the windows? :-) )

    The bedroom loads are also unusually high, even for 2x4/R11 construction. Who ran the Manual-J, using what tool?

  2. monkeyman9 | | #2

    Master bath is an 11x13 and has a large cathedral ceiling and 3 exterior walls. Master bedroom big with large windows and 2 side exterior walls. Both rooms are the hottest in summer and coldest in winter as well. Baseboard heat. I can say a 11k BTU portable AC (dual hose) can't keep up with the master bedroom and bath together on hotter summer days. Bath temp just keeps rising and rising. Was sweating to death working in there last summer.

    I ran a hand (or Excel calc with manual J book) and also used cool calc as a verification.

    Edit: Wanted to add the windows are old and from 1996 other than 1 I replaced. A new high end slider Downstairs. Windows are on the list but probably not for another year or more :(. Gotta figure out what double hung to get as well...another thing to research on my quest to make this place more efficient.

  3. Expert Member
    Dana Dorsett | | #3

    >"Master bath is an 11x13 and has a large cathedral ceiling and 3 exterior walls. "

    A heat load of 6199 BTU/hr for 143 square feet of space is a ratio of 43 BTU/hr per square foot, well over twice what a typical 2x4/R11 would be, which implies a large amount of glazed area (?) or something, unusual for a bathroom.

    >"I ran a hand (or Excel calc with manual J book) and also used cool calc as a verification."

    Among the freebie calculators Coolcalc is among those I trust the least for heating loads. It often disagrees with another online freebie Loadcalc by 50% or more (and Loadcalc tends to overshoot by double digit percentages.)

    The ~80K whole house load (not counting the garage) is also suspiciously high unless the place is quite large. Have you run a fuel-use based load calculation as a sanity check?

    The subtotals highlighted in green in the spreadsheet screen shot also don't add up the the floor totals.

    1. monkeyman9 | | #4

      It's around 2500 Sq ft w/o the above garage room.

      I would like an accurate heating calc for future boiler replacement, but wasn't concerned on heating at the time I did the calc as I was trying to size mini split systems for cooling. I could def scrutinize it more for heating I'm sure and I'll definitely try to get to that soon so I can check my inputs again. Was hoping to do that more after window replacement.

      So the fuel check is a great idea. But I used the stove for heat mostly during winter. So I don't have any accurate way of doing it :(

      The house is pretty leaky. The windows and doors were not flashing taped and you can def feel air coming in on frigid days around them. I actually removable caulk the worst offenders in winter til I can replace them all. The slider and one window I did we're a large improvement on the thermal camera.

      I did air seal attic floor though and add the 2nd layer of R30. So that is a start!

      Are slim ducted in an unconditioned attic a bad idea even if sealed with closed cell foam and blanketed over with insulation? My wife is a bit adverse to the split heads

  4. Expert Member
    Dana Dorsett | | #5

    I live in a 1920s vintage 2400' 1.5 story bungalow with 1600' of basement, 150' of crawlspace, kneewalls behind the build out upstairs rooms, a footprint that has 14 corners (lots of extra thermal bridging), and mostly the original wood sashed double hungs with 1980s vintage clear glass storms.

    When I first moved in there was only half-inch horsehair laminated between kraft paper for wall insulation, leaky as hell unintentionally ventilated kneewall attics, 0-8" of sagging rock wool in the mini-attic floor with 2.5" thick rock wool batts on the kneewalls, and 0-6" of rock wool in the cathedralized ceilings. Even in that condition the 99% design heat load at Worcester's +5F design temp was ~50KBTU/hr. After some retrofit air sealing, dense packing cellulose into most of the wall area (still missing in a few spots) putting a bit of foam in the rafters behind the kneewalls and insulating the foundation the heat load is well under 40KBTU/hr @ +5F.

    That experience makes me skeptical that a house in/around Clinton MA with a more efficient shape, and better windows really has a heat load of 80K with the windows & doors. If the house is leaky it might be 50K or even 60K but if it's really that leaky it would make more sense (and provide more comfort) to spend at least some amount on blower door & IR imaging guided air sealing before going for high efficiency heat pumps. If you're in a MassSave ZIP code (is Nat'l Grid your electric provider?) you can get some of that done for free, or at least highly subsidized.

    If heating with a pellet or wood stove it's still possible to estimate the load based on the combined fuels use.

    1. Deleted | | #6


    2. monkeyman9 | | #7

      Good call and I appreciate you looking at this! I took a look and made some fixes. I looked at average and semi-tight for air infiltration. I had also tried to factor ducting in the attic (now removed so we have a clean look). Deleted any loading due to heating piping as well (will all be well insulated in the basement for next winter anyways).

      Let me know what you think of the new pic in the first post replacing the old one. The loads are much lower now.

      Master bath has 20 sq ft of glass.

      I do not have Nat grid. I have local utility.

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