Electric Baseboards or Central LP Furnace?

Discussion in 'HVAC Heating & Cooling' started by chuck b, Oct 20, 2014.

  1. chuck b

    chuck b sea-bee

    Joined:
    Jan 19, 2011
    Location:
    levering, michigan
    RE: NORTHERN MICHIGAN 700 SQUARE FOOT LAKESIDE COTTAGE


    Structure:

    Built on Post, Beam, and Joist hardwood - very solid (circa 1939 - true dimensional saw mill timber). About an 18" free air space underneath with only removable lattice around perimeter. All timber and underside of subfloor is dry due to great airflow.

    Floors:

    Subfloor is 5/8" tongue and groove hardwood nailed to joists. I have added a new layer of 5/8" exterior grade plywood inside for under carpet.

    Exterior Walls:

    Tongue and groove original 5/8" wood siding, with a 1/4" folding blue foamboard under vinyl siding.

    Windows & Doors:

    New Super Efficient Andersen 100 "fiber" casement windows with upgraded Argon filled "E" Glass.
    Two new Fiberglass exterior doors with minimal glass and storm doors.

    Insulation:

    Walls: 3 1/2" fiberglass batts (with aforementioned wood siding, blueboard, vinyl siding). 5/8" tongue and groove Knotty Pine on walls and ceilings. No drywall underneath.

    Attic: 6" fiberglass batts with kraft paper vapor barrier, additional 12" blown in fiberglass on top.
    Well ventilated with generous soffit and attic ridge venting.

    Room Layout:

    • Living area is a rectangle of about 500 square feet includes living area (with Murphy bed for guests), kitchen and eating area.
    • One separately enclosed (master) bedroom.
    • Full master bath, that passes through
    • Small laundry - utility room for well pump, softener,water heater
    Current Situation:

    The studs are exposed, and time to plan heat. Cottage located in Pellston, Michigan on Douglas Lake, the "coldest spot in the nation" on average in the winter. Many below zero days.

    Usage:

    Will plan on living there 8-12 months a year.
    Already have Cadet baseboard heaters that I purchased for next to nothing when Builder's Square went out of business. Not concerned if they are not utilized. All ceilings are 8', no cathedral ceilings.

    Although I cannot offer LP or electric rates at present, am wondering about cost effectiveness, the "feel" when comparing the two. The ability to add A/C not an issue. Understand that a high-efficiency LP furnace is not practical if cottage is left unheated as residual water from 2nd heat exchanger would freeze?! Would not choose to leave LP heat on when not there and uncertain travel plans during winter.

    Since the cottage bedroom is small, I realize that bedding (blankets, bedspread/comforter) near baseboards is a fire hazard. Have about 2' clearance.

    Not concerned about using a humidifier attached to a central LP furnace as that would present additional winterizing blowout issues. Could use portable humidifiers in either option.

    Would expect insulated duct runs to go "under cottage" so that the floors are warmer if I go with LP.
    Although cost is an issue for a new LP furnace installation, I am mostly concerned about the cost of heating in a bitterly cold environment for 4 months, and the feel of the heat. Many of the year round residents on the lake use wood fired boilers with LP backup.

    Any thoughts? Thanks, Chuck
     
    Last edited: Oct 20, 2014
  2. Mandingo

    Mandingo New Member

    Joined:
    Oct 21, 2014
    Occupation:
    Civil engineer/construction PM
    Location:
    NY
    I haven't looked into the costs of pellets lately, but I believe it's down there with natural gas per BTU, if not lower. Since you already have baseboard heat, quite honestly off the top of my head I think I'd put in a nice pellet stove and use the baseboard to supplement the bedrooms. You could skip all your duct work. Are they allowed by code out there?
     
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  4. DonL

    DonL Jack of all trades Master of one

    Joined:
    Mar 30, 2011
    Occupation:
    Rocket Scientist
    Location:
    Houston, TX
    Properly Wired Electric is safer. And cheaper to install and operate.

    Electric Blankets are great too. Instead of heating a Room when not needed, to keep a pipe from freezing.

    Why would you need to add humidity if you live on the lake ? Does the Lake Freeze over ?


    Good Luck on your project.
     
  5. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    A 1.25- 1.5 -ton cold-climate ductless mini-split heat pumps can probably cover your existing load down to about -15F or so, and if you get a good one it'll only use about 35-40% of the power (a 60-65% savings) that electric baseboards would use. You'll still likely want to have some baseboards in any doored-off rooms for temperature balance (since like a pellet or wood stove, mini-splits are point-source heating), and to cover you for when it drops in to the -20s.

    The installed cost in my neighborhood would be about $4-4.5K. YMMV.

    Models worth considering are Mitsubishi MSZ-FH15NA or MSZ FE18NA, Fujitsu AOU-15RLS2 and AOU-15RLS2-H.

    If you don't intend to spend much time there in the winter, the 1-ton versions of these series will probably cut it down to 0F or so without much baseboard backup use. That'll typically save $500-700 in up front cost over a 1.5 tonner.

    The Mitsubishi units are designed to turn off at -18F (though the grapevine has it they don't actually turn off until about -25F), then automatically re-start once it warms back up to -13F or so. This is partly a self-protection feature, due to the innaccuracy of their temperature sensing on the outdoor unit once it gets too cold causing some error. The Fujitsu units are somewhat lower efficiency, but they keep on truckin' at -30F or lower (according to folks in Quebec heating with them.) At -10F the efficiency is only about 2x that of baseboards, but at +15 it's 3x, and at 35F it's getting on to 4x as efficient. In your climate you'll probably average about 2.5x as efficient over a whole season- but that's still well under half the total power used.

    They all come with high-efficiency air conditioning built in.

    With your wall stack-up the last thing you want to use is a humidifier. The fan-fold blueboard has facers that bring it's vapor permeance down to between 0.6-0.8 perms, and you have no air-barrier or vapor retarder between the interior air-leaky t & g wall. If the humidity on the interior doesn't track the outdoor humidity you're at risk of moisture accumulation in the exterior siding/sheathing. (It would have been a good idea to use an interior side air-barrier, or even a polyethylene vapor barrier detailed as an air barrier between the finish t & g paneling and the studs.)

    Kraft facers on batts don't cut it as an air-barrier, and with the low-perm underlayment on the exteiror side, are inadequate as a vapor retarder. They run about 0.4-0.5 perms when bone dry, but become more vapor open as the humidity levels rise. That's not terrible, but it's not great. It would have been better to use either 2-3" foam on the exterior (which raises the temp at the wood sheathing layer to above the dew point of the interior air.), or to use a higher-permeance (but still air-tight) underlayment, given the high air-permeability of the interior finish wall. If you are not there much for 3-4 months of winter and aren't intentionally adding humidity, it won't much matter.

    No floor insulation other than the R1-ish rugs? If not, it's worth it (both financially, and for comfort!)

    To keep the floors comfortable a -10F outdoor you don't want to fill the joist bays completely up- leave 3-4" of air between the fiber insulation and the subfloor, which evens out the temperature difference between the floor over the cavity vs. the floor directly over the joists. If you have 2x10 joists, R23 rock wool or R21 fiberglass batts are going to be worth it. If deeper you can go deeper. If you then added 3-4" of rigid EPS or rigid-rock wool below the joists (held in place with 1x4 furring through-screwed to the joists 24" o.c. it would reduce the cold-striping at the joists even further. It's probably worth installing 1/2" plywood or OSB to the bottom side of the joists, detailing it as an air barrier, and air-sealing the band-joist/sill-beam to keep outdoor air from bypassing your floor insulation layer.
     
  6. chuck b

    chuck b sea-bee

    Joined:
    Jan 19, 2011
    Location:
    levering, michigan

    * Thank you for your lengthy and authoritative reply. You know your stuff and have given me pause to think about a few things.
    First, the t & g knotty pine is not up yet. Would a layer of say Tyvek over the kraft faced insulation stapled to the the studs speak to your concern about an air barrier and vapor permeance? How does Typar differ from Tyvek?
    Was planning on filling the floor joists with closed cell sprayed foam. Figured it would do a better job of discouraging critters from making a home there in the winter. It also has a higher "R" value and of course, $$$ intensive but not so labor wise. Would that layer of plywood or OSB not cause vapor issues between the bottom of the floor and the bottom of the joists if sealed? But also the DWV plumbing system protrudes at various points above and below the floor joists making the sheathing difficult to attach,

    Looked into a Mitsubishi mini split and was told that where my cottage is, Pellston, Michigan, the coldest town in the USA on average in the winter, it would be too cold and would need to be supplemented with baseboards as you suggested. The lake does freeze over of course. Guessing your comment about the lake and the ubiquitous humidity was spot on. Would not likely need humidification except in the dead of winter.

    Dana you know your stuff. Thank you and look forward to your reply as time permits. Chuck
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Tyvek runs about 30 perms, Typar about 10 perms- neither of which qualifies as even a class-III vapor retarder though Typar comes close. Either can be detailed as an air barrier, and air-tightness is more important than vapor retardency, and critical in your stack up.

    A layer of 5/8" thick t & g pine with a shellac finish runs about 1.5 perms- it's class-III vapor retarder, but is impossible to make air tight. In your stackup & climate you would ideally have the vapor permeance toward the interior be no higher than the XPS you have on the exterior. That could be a layer of sheet rock with "vapor barrier latex" primer, which runs about 0.5 perms under the pine, detailing it to be air tight, or you could use the same fan-fold XPS under the pine that you used on the exterior, taping the seams with house-wrap tape (sealed over with paintbrush-applied duct mastic to guarantee long term adhesion), sealing the edges with caulk or can-foam.

    A layer of 6-mil polyethylene is an order of magnitude more vapor tight, at about 0.05 perms, and while that would block moisture from the interior from ending up in the sheathing (if perfectly air tight) it would also block all drying toward the interior. It's cheap & effective when done right, but it's something of a double-edged sword.

    Filling R1.2/inch joist framing with R6/inch foam is a waste of expensive foam, since at almost any framing fraction the framing will be conducting the lions share of the heat, undercutting the performance of the foam (by a LOT!) It's better to save the high-R foam budget for where it can be applied in continuous layers, thermally breaking the conductive framing timbers. A full fill of closed cell foam is more labor intensive than you might think, since it has to be installed in 2" lifts, with a cooling period between lifts, otherwise there is a signficant fire-risk during the curing period, and even if it doesn't catch on fire, installing it in thicker lifts causes adhesion & shrinkage issues. If you're going for a foam solution, a full fill of open cell foam can be applied safely in 5.5" lifts (with substantial cooling period between lifts), and it will air-seal somewhat more tightly than closed cell.

    A layer of plywood or OSB on the bottom side of the joists will make it more air-tight, even more so if you take the time to put a bead of caulk between the joists & sheet-goods edges, and tape & duct-mastic all of the seams. If you have to put up some 2x furring (or if it deeper, a set of non-structural joists perpendicular to the existing joists on framing ties) to accommodate the plumbing & electrical excursions. Furring should be screwed in with pancake head timber screws (3"-3.5" FastenMaster Headloks or similar), penetrating the joist by 1.5" which would be able to support a substantial amount of insulation or other weight over time without splitting the furring.

    Unlike wall sheathing, there is no exterior wetting issues to deal with on the under side of floor joists, and it can be left open to the air (which maximizes it's drying rate) rather than closed up with #15 felt, which runs about 1-2 perms when dry, 5 perms when soaked, or ~0.7 perm XPS undelayment. Plywood is preferable to OSB, since it becomes more vapor permeable more quickly with rising adsorbed moisture. If you put rigid foam on the underside of the joists, unfaced EPS allows the most drying. Type-II EPS is still only ~1 perm @ 3"/R12.6), a thickness that would allow you to put as much as R15 rock wool or R15 fiberglass (3.5" thick) between the joists directly above the EPS without condensation concerns in your climate. Between thermally breaking the joist edges with R12+ foam and the side-exposure of the joists to the cavity between the fiber and the floor above, the temperature striping that you would otherwise feel on that floor during cold weather goes away. It's a more comfortable floor,( even at lower R-values) when you do it that way. If the joists are full dimension 2 x 10s, you could just drop 8" of fiber between the batts, leaving 2" of cavity to mitigate the temperature striping and still have a decent R-value (if below code-min for climate zone 7 for a year-round residence, but it's a cabin that you're not keeping at 70F when it's -25F out.)

    Any plumbing that needs to be freeze protected would have to be fully inside the insulation & air-pressure boundary, however you decide to insulate the floors & walls, even if it means you have to skimp on R in places to make that happen.
     
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