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Curtis Betts | Posted in General Questions on

Apologies if this has been covered many many times…

I am building a garage, with rental apartment above, in Rhode Island.
– Climate zone 5 (or 4-Marine? The humidity-controlled bathroom fan seems to run all summer, windows open)
– 24×36, approximately 800 SF of living area, unheated garage area.
– Glazing: Lots on the South (view) side, minimal on E, N, & W. Mid-range manufacturer.
– Framing 24″ OC throughout: 2×6 walls, 2×10 (or 12?) floor, complex truss roof.
– Siding 50% wood shingles, 50% cardboard-cement clapboards (matching the “big house”)
– Insulation specs are currently minimum code – R-20 FG in walls, etc.
– Heat: a combination of direct-vent gas fireplace, gas-fired heater TBD
– No installed A/C.

As a rental, it’s all about “bang-for-the-buck”, every discretionary penny should go toward attracting an upscale tenant. And efficiency in a small rental is NOT sexy. But here I am, lurking in this forum, so apparently green building is a fun challenge for me!

I am open to suggestions for low-cost improvements. So far, I am thinking:
– Airtight Drywall vapor/air barrier
– Don’t add exterior insulation. Just Tyvek under siding (gotta nail those shingles)
– Rigid insulation – strips, or continuous, between framing and drywall, to minimize thermal bridging (1/2″? 1/4″?)
– Heat – on-demand gas heat/HW combination. Perhaps an undersized radiant system ($$), counting on the fireplace to pick up the slack.
– Windows – Low-E on the East, North and West, perhaps max-solar-gain glass on the South. Subject, of course, to finding something close at the local dent-and-scratch discounter.
– Frame, but don’t cut, an opening for a future window-style A/C

Any suggestions for the pretty-good-rental?

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  1. Richard Patterman | | #1

    I have designed and built several small cottages (700-900sf). Because of the small surface areas it is possible to spend extra on insulation and save more on HVAC. Super-insulated with a ductless mini-split is very cost effective and gives you AC. There is also the savings of no gas line, gas meter or gas venting.

    Check with your utility, but most charge a $15-$25/month meter fee. For small, super-insulated, the meter fee for 12 months is more than the gas purchased. If your roof framing allows, prep the south face for PV. Also 2x10s or 12s in the floor framing will have more thermal bridging than I-Joists.

    Small cottages often look best with a rather simple roof line, another place that could save both lumber, time and energy.

  2. Expert Member
    Dana Dorsett | | #2

    (After kicking this around in the background all afternoon I realized how long it is- warning: Verbose-mode enabled! :-) )

    In a muggy-summer RI location, unless this is at Block Island's power rates you may get more bang/buck out of heating & cooling it with ductless mini-splits. (Mini-splits in no way resemble their wicked-po' PTAC heat pumps that scream "motel" all over them, on looks, noise or comfort.)

    Get rid of the gas-fired fireplace- it's nothing but a low efficiency usually oversized space heater and a big hole in your thermal envelope. Most would have several times the output of your actual heat load at the ~10-12F 99% design condition. Real heat loads on a code-min house that size in a RI location are in the 20BTU per foot of conditioned space range, or 15,000BTU/hr, give or take. With an air sealed insulated floor and reasonable air tightness it won't be more than 20KBTU/hr unless you go overboard on the window area, and could be easily be in the 12KBTU/hr range with judicious window sizing, better windows and lower thermal bridging on the framing.

    A lower output parlor-woodstove-style gas burner like maybe a Hearthstone Tribute or Vermont Castings Intrepid or similar might work if you think it really needs fire-place-like ambiance but is that much charm really worth it in an rental? (I don't know your market that well.) But any gas-burner you install needs to have a min-fire output that won't roast the occupants out in under 2 hours and that's a lower number than most gas-fired fireplaces will run. The only one I can think of with an appropriately low firing range is the euro-modern looking Solas Nua, which isn't exactly a coastal Yankee ambiance. But even the Tribute or Intrepid might be overkill, depending on where your heat load numbers end up.

    Humidistat controls on ventilation doesn't work during RI summers in this climate due to high outdoor dew points/latent loads, but mini-splits run almost continuously in air conditioning mode, drying out the interior quite nicely. Most have a "dehumidify" mode if the humidity ever gets ahead of you.) Operating costs for ductless in heating mode is slightly higher than condensing gas in my neighborhood (Worcester, MA a few 10s of miles north of you- 15-18 cent electricity, $1.10-$1.30/therm gas), and comparable to code-min 82% gas heating.

    A decent 1-ton mini-split will have a heating output capacity between 12,000-16,000BTU/hr @ +5F, and a nominal cooling capacity of 12,000BTU/hr @ 110F, but read the specs closely on any model you're really looking at. There are 1.25 and 1.5 ton models out there too, if you think you're cutting it too close on the heating end. A mini-split is far cheaper to implement than any hydronic solution- you'll be under 5 grand, and it covers both heating and cooling with fairly high efficiency, but more than efficiency, high COMFORT, and very low noise.

    Going with 2x4 16" o.c. on the framing and buying reclaimed roofing foam from any of the southern New England vendors buys you an R20 whole wall at the same thickness as an code-min R14 2x6 approach. If it means you need to add an exterior OSB nailer (through screwed to the studs 24" o.c.) for the shingles, factor that into your cost equations, but using reclaimed goods 2"/R11-12 iso runs 30-45cents per square foot, f.o.b. the recycler's yard. Staggering the seams of the foam with the sheathing layer's adds a lot to the air-tightness too.

    For sanity checking while designg, set up an I=B=R type heat load spreadsheet on wall, roof, and window areas & U factors while adjusting the design. Assume a (70F indoors, 10F outdoors) 60F temperature delta for an RI location. A code-max U0.35 window loses 0.35BTU/hr per degree delta per square foot,so at a 60F delta it's 21BTU/R per ft. If you have 100' of window (a 12% window/floor fraction) thats about 2100 BTU/hr.. (If you go with 0.28 windows, it'll be about 1700BTU/hr.)

    The R20 24" o.c. wall with shingles runs about U0.07, or 4.2 BTU/hr-ft. With 120' of perimeter and 9' walls, you're looking at about 1000 square feet of wall after subtracting out windows, so that's about 4200BTU/hr . (If you go 2x4 +2" foam call it U0.05, which would yield about 3000BTU/hr.)

    A code-min R30 cathedral ceiling runs about U0.04, for 2.4 BTU/hr-ft. With a 24x 36 exterior footprint call it 800 square feet, for 1920BTU/hr. (If you add 3" of reclaimed roofing foam on top of the R-30 cathedralized ceiling roof deck your U-factor drops to ~0.025, and a heat loss of 1200BTU/hr out the roof.)

    With the garage door closed you can mostly ignore the heat loss out the floor, or add a 30% fudge factor to cover ventilation and floor losses.

    That s still only 1.3 x (2100 + 4200 +1920)= ~11,000BTU/hr. If you doubled the window size for better views you might hit 13K (or not).

    Internal heat sources like plug loads and body heat reduce the real heat load from those numbers- subtract 250BTU/hr per sleeping human, and another ~1500-1700BTU/hr for things like refrigerators, water heater standby loss, etc. maybe as much 2000BTU/hr total if you also subtract for curtains & shades, etc. This is definitely mini-split territory, but run the real numbers on the real components, and run them room-by-room. A high-loss room that is doored off from the main area with a mini-split head or gas-fired parlor stove might need supplemental heat with the doors closed when it's below 20F outside, or you can design-down the heat loads in those rooms with smaller/better windows, etc.

    The 3/4 ton mini-splits can typically deliver ~10,000BTU/hr @ 5F, and may ultimately prove a better fit, if your spreadsheet comes in under that, but for now I'm thinking 1-ton to have a bit of margin and a quicker ramp up to temp if you would normally park it at some low temp (or off) when unoccupied. Either way, budget $3.5-4.5K, installed cost. A DIY-install would run about half that. Look for HSPF ratings of 10+ and SEER of 16+ if you go internet-shopping. The big three that hold the lion's share of the New England market are Mitsubishi, Fujitsu, and Daikin, in about that order, but LG and Sanyo et al have some decent offerings, if less local sales & tech support.

  3. Kevin Dickson, MSME | | #3

    In an 800 sq. ft. low energy apartment, you can definitely consider saving $1000-$2500 with a PTHP instead of a minisplit. Here's one success story:

    Here's another idea for a "green" fireplace:
    Ventless, but supposedly has no carbon monoxide output, they might be OK if used with an HRV.

  4. GBA Editor
    Martin Holladay | | #4

    Right now, I'm staying in a hotel in Atlanta. (I'm attending the Greenprints conference.) The hotel room has a PTHP -- the first hotel PTHP I've ever experienced that is quiet. It really seems like a good unit.

    It is a Model LP076CD2A unit manufactured by LG. It's rated at 7,200 Btuh. You can buy one online for only $715.

  5. Richard Patterman | | #5

    Martin, I clicked on that site and it is labelled a "packaged terminal air conditioner" with heat & cooling.
    I assume this is a one piece system, no separate compressor. Is this just a window air conditioner with resistant electric heat or is it a heat pump?

    Kevin, I would be very concerned with a "ventless" gas appliance especially in a very small, very tight home. To me, one of the bid advantages of small and tight is the ability to remove gas and go all electric. Often the meter fees are more than the fuel purchased.

  6. GBA Editor
    Martin Holladay | | #6

    Looks like you're right -- it's cooling only. But maybe LG makes a similar unit that is a heating/cooling heat pump.

  7. Curtis Betts | | #7

    Thank you, Richard, Dana, Kevin, Martin

    A lot of food for thought. First, a preliminary discussion with an HVAC guy left me with the impression that the heating requirement would be much much higher. Time to sharpen my pencil, and work it out myself, see if it can be in the 10-20 BTU/SF range. (Is the RESCheck code calculator of any use in this process?)

    A/C? In the "big house" (currently 1400 SF, will grow to 1800) the rationale is that we have a lovely spot, on the water, with extensive birds and wildlife. We are happy with the many many windows wide open all summer. The loudest noise at night is the neighbors' A/C. Yes humidity is a bit high, but why live on the coast if you can't handle that? Perhaps there should be A/C in the apartment if our goal is to market it as "premium". If so, machinery will have to be either very quiet to the exterior, or located on the street- or far- side. Those two sides are currently packed with closet, bath, utility, kitchen, and open staircase, so not a lot of room for a standard wall unit. A mini-split seems a good idea, or maybe I can find a space for a PTHP, but this A/C stuff is all new to me.

    Gas vs. Electric: Our utility rates are about $1.50/therm for gas, and $.14 /KWH; using a conversion factor of 29.3, gas is about 1/3 the cost. Even with a fireplace that's 75-80% efficient, still less than half. Or am I missing something? Seems to me gas makes sense for DHW, cooking, and at least some heat. But then, this IS a rental...

    Fireplace: Our direct-vent gas fireplace in "the big house" is great. Timed setback thermostat provides scheduled ambience, a blower circulates the heat, battery back up gives protection from power outages. For the apartment, small units range up to 15,000 BTU - cozy indeed! But no thanks on the ventless, even (especially?) in a rental.

    Framing: My engineer specified 24OC "stacked" framing, so floor wall and roof framing all lines up. Also, heavier structural pieces fit into the 6" wall. I'll price out I-joists vs rigid insulation above the garage ceiling, Roof line is a simple hip, except the south side, where a big hip dormer allows a 10' ceiling in the center third (the sides are code-minimum ceiling height). Plenty of room for additional loose-fill insulation up there.

    Sheathing: I don't love the idea of sheathing, foam, more sheathing, then siding. Hence the insulation under drywall idea; I'll run the numbers.

    Fenestration: Currently showing 164 SF windows and doors - 20% of FA, more than half on the south (view) side. Yes it's a lot!

    Thanks again

  8. Kevin Dickson, MSME | | #8

    Here's a $500 Amana PTHP:

    The fireplaces use a bio-ethanol gel, pretty safe. But I agree with you about not providing it for a renter. It looks like a good option for taking an old masonry woodburning fireplace out of service and still having the ambiance of a fire.

    Remember you probably don't want any sort of natural gas service to this apartment. Most utilities will charge $12-$25 per month "service fee" which will be more than the cost of energy used. That makes a propane direct vent fireplace or even an electric fireplace more attractive. Gas is OK if the apartment isn't separately metered.

    " preliminary discussion with an HVAC guy left me with the impression that the heating requirement would be much much higher." This is the industry's biggest problem right now. The folks who should understand this stuff better than anyone else actually don't. Last month I had an experienced technician tell me "air to air heat pumps just don't work in Denver".

  9. John Semmelhack | | #9

    Kevin has probably looked into this a bit more than me, but for the units that I've looked at, the heat pump function switches off at a fairly high temperature (~35-40F) because many of the units don't have a defrost function, I presume. For the one that I'm aware of that does have defrost (GE Zoneline 6100), the heat pump cuts off when outdoor temperature is below 25F.

  10. Expert Member
    Dana Dorsett | | #10

    For the quiet comfort of variable speed blowers & scroll compressors I'd take a $500 no-name mini-split over a noisy $500 Amana PTHP. A bottom of the line solution doesn't say "premium" to me, as much as it says "motel". YMMV.

    Some newer-better PTHPs have scroll compressors and variable speed blowers, most don't. Whatever you buy dig up the actual noise output data (usually expressed in dBA, sometimes sones). Most 1-ton minisplits at low to mid blower speed (where they will spend MOST of their time) are quieter than most refrigerators.

    You don't have to break the bank to go with a high-efficiency 1-ton LG:

    DIY installation isn't rocket science, doesn't even take refrigerant line flaring tools if you use a pre-packaged lineset of the right length for how/where you intend to install it. Even if you pay a tech to fully test it and adjust the refrigerant charge (not always necessary with pre-charged line kits), it's a relatively short service-call, not a huge expense. Read up about it online before diving in, but the industry has bent over backward on KISS installation requirements strategies so that untrained 3rd-world customers are be able to install them successfully, no refrigerant-tech training necessary. Of course it's always better to have the right pressure measurement and vacuum pump tools to dial it in just-so and get the best efficiency/capacity out of it, but most of the work involved is in the electrical wiring to the unit and the mounting issues.

    BTW: The humidity on the coast isn't dramatically higher than 50-100 miles inland- summertime dew points in Worcester track those of Newport or Westerly within 5 degrees. The mean mid-summer dew point in Westerly is about 65F, in Worcester it's about 61F, and peak humidity days are usually closer than that, according to datasets. All of New England is just plain sticky during some midsummer weeks, but never as torrid as gulf-coast state peaks.

    Kevin: "air to air heat pumps just don't work in Denver"

    NICE! No wonder the Asian ductless vendors are eating US-vendors' lunch, eh? :-) If the mean fleet efficiencies run with seasonal average COPs north of 2.8 in some of the cooler eastern WA & ID locations in the NEEA field test data reports, they will SURELY do as well or better in Denver, where the capacity hits from defrost are lower and the average mid-winter temps higher:

    Coastal RI has comparable mean mid-winter temps to Denver, (about 30-32F) but somewhat higher winter dew points (for a slightly higher defrost hit), but a decent high efficiency mini-split will still average a COP of around 3.0 in that climate if oversized only 1.25x for the true 99% heat load.

    165' of U0.35 code-max windows represents a 60F-delta heat load of about 3500BTU/hr- definitely not nuthin', but not enough to blow the heat load budget. A 20% window/floor ratio of mostly south-facing windows could lead to some SERIOUS overheating issues though, even in winter.

    Adding 1/2" XPS strips to the interior side of the studs and other framing does nothing to break the bridging at the joists & subfloor but it will reduce the whole-wall U-factor from about 0.070 to about 0.064- it's not nothing. Of course if you thermally bridge that with nails and sheet-rock screws every 6-8" a lot of that goes away, due to the high thermal conductivity of the steel, but it'll still be something. For estimation purposes using U 0.066 for the walls with the 1/2" XPS strips would probably be closer to reality.

    With buck-fifty gas and 14 cent electricity heating with mini-splits will be slightly cheaper, even if you found a condensing gas unit appropriately sized (SFAIK they don' exist.)

    Net heating from a 95% burner is 95,000BTU/therm. At $1.50/therm that's 1.58 cents per 1000 BTU delivered.

    Net heating even at an average COP of 2.7 out of the mini-split would be 2.7 x 3412=9212 BTU/kwh, for which you paid 14 cents, or 1.52 cents per 1000BTU delivered.

    And that's without getting into the inherent distribution losses of ducted systems vs ductless, or the fact that you'll likely average better 2.7 with the mini-split. (WAY better in the spring & fall seasons.)

  11. Kevin Dickson, MSME | | #11


    Thanks for the additional minisplit information, it's important because there are many advantages to minisplits:

    1. The manufacturers seem to be concentrating more on minisplit efficiency than PTHP efficiency.
    2. A minisplit violates the conditioned envelope to a much lesser extent. (needs a smaller hole)
    3. The metal sleeve for a PTHP is a significant thermal short circuit, and hard to air seal well.
    4. PTHPs are noisier.
    5. John is right, they turn off at 25F. I ran a simulation model for Denver which didn't show much of a penalty due to that, however.

    But in general, the installed cost is 2-3 times as much, and I'm always trying to tunnel through the cost barriers to energy efficient construction:

    "Of course it's always better to have the right pressure measurement and vacuum pump tools to dial it in just-so and get the best efficiency/capacity out of it, but most of the work involved is in the electrical wiring to the unit and the mounting issues."

    So, in your experience, what is the labor content of minisplit installation? We know that a PTHP is only $200-$300 for the outlet.

  12. Richard Patterman | | #12

    When I was designing and building 3000 sf single family homes I would have never considered an all electric home. The energy needs were great enough that natural gas was the logical choice.
    But now designing and building small, super insulated cottages the energy needs are so small that
    keeping it simple pays off. The install costs and meter fees of a second fuel (natural gas) often are more than the fuel costs. And in a small space there is a lot to be said for "no combustion".

    When I started designing small spaces the first thing I dropped was the fireplace. Small spaces work best with an open floor plan and that means very few walls in the typical living room. If you have few walls in the living room and you add a fireplace to one of them, you really limit furniture placement options. My guess is especially in a rental, that tenants would value flexibility of layout over a fireplace that gets used occasionally in the winter only. Remember that it takes up sf and wall space the other three seasons also.

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