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Community and Q&A

Choosing an Energy-Efficient Cold-Climate Wall Assembly

scottwoodward | Posted in Energy Efficiency and Durability on

I’m entering the framing stage of a project to build a 24′ x 32′ garage with a roughly 10′ x 20′ mudroom on the garage level and a roughly 720 sq. ft. apartment above, with the stairs open below to the mudroom.

See attached floor plan drawings – very simple, open floor plan design. My goal is to achieve as high of energy efficient home as possible with a modest budget. I’d like to exceed code standards here in New Hampshire for R-value, but doubt I can afford the most energy efficient design. My main heating source will be a heat pump on the upper level with a back-up heating source of a pellet stove down in the mudroom. The structure will sit on a slab foundation and the slab will be insulated with rigid foam under the slab and I’m also planning to install radiant heat (exterior perimeter insulation already installed). Hot water will be a hybrid heat pump.

My carpenters are insisting on a 2’x6′ wall construction (foundation walls are 8 inches thick). At the moment, I’m leaning in the direction of the 2’x6′ wall construction with rigid foam insulation on the exterior to achieve an effective R-24 or higher with batt insulation in the wall cavities. I’ve read a lot of posts, but can’t quite make sense of it all, being new to all of this in general, both construction and energy efficiency. I’m grateful for the assistance I’ve received so far. It’s been helpful.

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Replies

  1. Expert Member
    DCContrarian | | #1

    Is the garage part of the heated part of the building? If so, you're biggest concert is going to be the doors. Insulation is often a matter of the weakest link and the doors are so large that if they're not good the rest of the building kind of doesn't matter. Really tight and well-insulated garage doors are rare and expensive.

    The other alternative is to have the garage be unheated and treat the partition wall and ceiling like exterior walls. The door between the garage and the living space has to be an exterior door too. This would probably give you better energy efficiency.

  2. tech1234 | | #2

    Scott, having built many similar buildings in NH I can tell you the garage doors will pretty much negate any effort for efficiency on the 1st floor. Also don't fall for the sales pitches on "energy efficient" garage doors.

    Although I love to see people excited about green building some designs are harder to make it work. If this was my build here's where my focus would be:
    large roof over hangs- think 16"-36"
    "2nd" floor:
    air seal
    decent windows (casements, not double hung, or even better fixed)
    lots of cellulose in the ceiling
    mini splits
    ERV
    PV panels if your site allows

  3. Robert Opaluch | | #3

    Agree with DCContrarian’s and tech1234’s points, very important. It is relatively cheap to do these high impact upgrades if you were not planning them already. Especially lots of insulation in your attic, the most cost-effective and easiest, if you plan some space in the attic, especially along the perimeter of the building. Raised heel trusses, a wide roof overhang, or lots of other ways to get R-25+ at the ceiling perimeter and R-60+ over most of the ceiling. Best bang for the buck.

    You can add to the wall insulation value by substituting denser, higher R-value fiberglass batts or fireproof Roxul/Rockwool mineral wool batts (R-23). No extra labor. You could frame the walls with 24” on center studs for fewer studs (and a bit more insulation and a bit less labor), probably this is planned already. Even if the garage is considered insulated and part of the conditioned space, I would install some insulation in the garage ceiling/apartment floor to keep the apartment heat losses lower.

    If the builder is hesitant about adding exterior foam, it would be easier to do and more effective on some of your walls, and not others. You might consider walls with fewer windows, or just the upstairs story, to be better candidates, and the garage door wall not worth bothering. Another option is for the upstairs apartment only: Add “Mooney wall” additional insulation on the interior; or use 2x8 stud framing, for deeper cavity insulation. A double stud wall is a good approach but you would lose even more interior space in your already compact design.

    For ideas, see:
    https://www.greenbuildingadvisor.com/article/how-to-design-a-wall

    I designed a similar 1BR apartment over a garage. I had the stairway and a garage level room enclosed, separated both from the garage and the apartment above. I note that you are using the windows over the stairwell for daylighting. Note that this open stairwell will introduce greater heat losses and air leakage, being open to the lower level, which is okay if you have reasons for it, but you have to account for that. Personally, I’d isolate the apartment from the ground floor garage and entry room areas, but that means losing your stairwell window daylighting, or enclosing that stairwell downstairs, so probably a non-starter.

    Main question: Which direction does the kitchen windows face (the bottom of the floor plan drawing). Hopefully this faces south, and you have no trees or other buildings shading these windows. That will bring a substantial amount of daylighting and solar gains during winter, which would reach your living area in the AM and midday. You would get more light in the morning and afternoon, and less daylighting in the late afternoon during your long, dark, cold winters. Less heat gain in the (sometimes hot) summer afternoons. Are the apartment windows predominantly on those two sides for views, avoiding noise from a highway, or why?

    There are no windows on the apartment walls on the wall at the top of the floor plan drawing. Hopefully this faces north. If it faces south, you would get little light in the AM except the bedroom; and you would get not much daylighting until afternoon. The living room would not be well lit from daylight, especially in winter.

    The dimensions of your building shell, bedroom, bath and kitchen are all compact and efficient, good. Nice layout planning. Might be a little tight getting furniture onto the stairway though. I would not have the “bump in” entry door area. You lose interior space, and create extra wall space and exterior wall corners that are neither energy efficient nor space efficient (unless you are planning shelves or storage in the two pockets on the sides of the door). And the slab is exposed to the cold and weather? Why is the door not flush with the longer wall at the bottom of the floor plan diagram? (You could add something like a separate small exterior porch, entry area, or roof over the door for weather protection). That would reduce five wall corners in that area to one. Or why not at least eliminate the bump out at the bottom right corner, for a larger covered door (5 wall corners reduced to 3)? All these opportunities for heat losses are open to your apartment above, not to mention construction complexity.

    How to you plan to open and close windows, or adjust any window shades, on the windows on the stairway?

    1. scottwoodward | | #4

      Bob,

      Thanks for the many pieces of input. See attached photos to help with the visual. There's a few goofy things with my software design that I need to fix, like the eaves and the walls around the entry door (see Southwest view photo). Here are answers to your questions:

      1.) I thought about enclosing the stairwell, but that created a much tighter mudroom and created less space for the hot water heat pump to work, so I decided to keep the stairwell open. But at least the mudroom is closed off from the garage bays.
      2.) There are no windows in the mudroom, only two exterior doors on each end (I know one looks like a window, but it's actually an exterior door with a dog door)
      3.) I can't get rid of the bump out now because the foundation guy poured the foundation that way. The idea of it was to have a covered porch on the main door without having to create that porch extended from the exterior wall outward. More of aesthetic thing trying to keep clean lines.
      4.) The house is oriented at about 210 degrees. There are no double-hung windows in the apartment. Only three casement windows and the remainder being fixed. The orientation of almost all of the windows are on the south and west walls.
      5.) The window choices were mainly because of the view and the southern exposure (modestly nice view of Prospect Mountain in Alton, NH). The windows you mention -- the kitchen bank of windows face west as do the corner fixed windows. The windows along the stairwell side face almost due south. Those are fixed.
      6.) No opening/closing issues, but I haven't thought yet about shades. I'm sure I can adjust the living room side windows from the stairwell and the kitchen side fixed window by reaching over what will be a half wall by the stove.
      7.) There's a bathroom window that faces sort of north west (fixed) and one egress window in the bedroom facing north (large casement) which in the event of a fire would provide egress right out onto the driveway where there would be emergency responders.
      8.) I'm not intending the heat the garage bays other than the previously mentioned radiant heat. There are four double-hung windows in the garage bays. They are not serving any purpose other than letting in daylight and aesthetics.

      1. Expert Member
        DCContrarian | | #5

        "8.) I'm not intending the heat the garage bays other than the previously mentioned radiant heat."

        OK, now you've lost me. One of the biggest mistakes builders make is to have parts of the house that are kinda-sorta insulated and kinda-sorta conditioned and kinda-sorta not. You see this all the time in old houses (and some new ones) where the builder was never really sure whether the attic and crawl space were inside the building envelope or outside. These houses will have ludicrous insulation and air sealing attempts, and often no real way to fix it after the fact.

        It sounds to me like you're heading down that path. There is nothing magical about radiant heat that allows you to pump heat into a poorly insulated space and not have it be an energy black hole. If you are going to heat a space, you have to insulate the space, and air seal the space, and you have to do it well. Insulating only part of a space is like waterproofing only part of the bottom of a boat, it's almost indistinguishable from not at all.

        If you have those big garage doors I think it's a fool's errand to try and heat the garage. Make the floor of the apartment and the partition wall your building envelope. Detail those as if the garage doors were open all winter.

        1. scottwoodward | | #6

          Understood, thanks. I was actually only planning to put radiant heat in the mudroom area and treat that as part of the building envelope and not do anything with the garage bays. My builder has the pipe to put radiant heat in the garage bays too, so I said yes, but to your point, that would be a fool's errand.

          Are you also saying there's no point in putting any kind of insulation in the garage, except for the ceiling, underneath the flooring above?

          1. Expert Member
            Zephyr7 | | #7

            Insulating the garage will help to lessen the temperature swings it will see, which will mean warmer nightime lows and lower daytime highs inside the garage. If you do every decide to heat the garage, that insulation would help you too. What I would not do is try to heat the garage continously -- you'd be much better off putting in something like a Modine Hot Dawg heater (made for things like garages), and only heat the garage when you need it, such as while you're working on your car. Leave the garage unconditioned the rest of the time. This way all the air leaks only hurt your energy efficiency during the relatively infrequent times you're actually heating the space.

            Bill

    2. Bas Sommer | | #12

      Bob,
      just two points:
      - would a porch as entrance (air lock) in front of the mud room make sense to you? No door between garage and mud room but garage to porch. Porch has three doors: simple weatherproof door from outside in, uninsulated (fireproof) door to garage, insulated (not weather-resistant) door to the mud room. Maybe aligne doors to better pass through. That way you safe that (insulated? not so fireproof?) door in the inner garage wall. One door more but simpler wall build and a mud-porch..
      question 2: did you ever look at pellet stoves which heat only air and not water? Maybe that was what you want anyway. With the stove in the mud room you can easliy heat the whole assembly even if power is out.

      regards - Bas

  4. scottwoodward | | #8

    Thanks Bill. That's good advice. I recently changed direction on HVAC to use heat pumps instead of propane. My original HVAC plan had a Modine Hot Dog heater for the garage for the very purpose you describe. I'm trying to avoid having two kinds of heating sources (actually three with the pellet stove in the mud room). I wonder if putting a mini split in the garage is feasible for the same purpose of a Modine? If that's not feasible, maybe the thing to do is to have propane as the sole back-up heating source, which I would do before next fall, only because I have a limited budget at this point.

    1. Robert Opaluch | | #9

      The pictures are helpful. I guess I would build a full height east wall, and have a small window in the living area and maybe the bedroom, but this works too. Complicates the construction some.

      Definitely insulate and air seal the garage walls! Fiberglass batts are almost free, installation is quick, and they provide a zero maintenance heat loss service. If nothing else, your car is warmer for your comfort when you take it out in the winter.

      The garage might be predicted to stay about half way between the apartment temp and the outdoor temp. After all, the ground temp is warmer than the outdoors, and the heat lost from the apartment and mudroom warms the garage some, and the east and west facing windows will provide some solar heat gains in sunny weather.

      Could plug in a cheap resistance electric heater on occasions when you need heat to work more comfortably in the garage. You can move it nearer to where you are working. Its unlikely that you would save that much with a heat pump to recover the costs, unless you are planning to work in the garage in the winter on a regular basis. Heat pumps work most efficiently when used continuously at lower power, not full blast to quickly warm a room, then off most of the time.

    2. Expert Member
      Zephyr7 | | #15

      A minisplit probably isn't a good fit to ocassionally heat a garage. The reason is that minisplits work best when they're maintaining a steady temperature. Minisplits aren't very good at bringing a very cold room (like a garage) up to a comfy temperature in a short period of time.

      I agree with Robert here, electric resistance might be your way to go here if you plan to only rarely heat your garage. What I would do is get a 5-6kw electric resistance heater, and put in a 240v outlet for it in the garage. When you need heat, run that. You can get pretty big ones if you want, and keep in mind that 1kw is about 3,412 BTUs, so a 5kw heater is only about 17,000 BTU -- around half the size of the smaller Hot Dawgs. This just means it will take longer to heat your garage compared to a larger heater, but it's easier to run a single 5-6kw heater than it is to run 3-4 1.5kw space heaters, which would also need a bunch of electric circuits.

      BTW, a single large electric resistance heater is what I use to warm up my garage if I need to work out there. I have an outlet tapped off of the circuit that I use to run the charger for my car. Note that you need a circuit breaker to do this, so I have a 20A breaker in a small breaker box that is tapped off of the 40A 240V circuit that feeds the charger.

      Bill

      1. scottwoodward | | #16

        Thanks Bill! Good advice.

  5. Expert Member
    Malcolm Taylor | | #10

    Scott,

    Sort of unrelated, but I'd deep-six that small window with the sloped head, and separate the corner ones more. Both situations are more trouble than they are worth.

    1. scottwoodward | | #11

      Thanks for the recommendation Malcom. The sloped head window was more of an aesthetic thing, so no real loss in dumping it. I'm not gaining anything from it. What's the problem with the separation on the corner windows? More potential for thermal bridging?

      1. Expert Member
        Malcolm Taylor | | #13

        Scott,

        No, just a bear to frame and trim. The centre post has to be treated differently than any other jamb because it can't just be cased and trimmed.

        I don't want to intrude on your design, as that's not what you were asking about, but my own preference for the wall over the stairs would be two smaller windows symmetrically arranged at each end, and a much deeper window in the middle which could would allow views from the stairs and introduce more light.

        1. scottwoodward | | #14

          Thanks Malcom. I appreciate your advice. I don't know anything about framing and I don't think my carpenters have really thought all that deeply about the challenges the windows might pose (at least they haven't said anything). Attached is a photo, which was the inspiration for the corner windows. There's a really nice sunset view from that corner, which will be even better from the second level. That said, I don't want to create headaches for the carpenters or create potential problems.

          What did you mean by a deeper window in the middle? The window that's there in the middle is 70" wide. Standing in the right place, the sunset view will be visible through that window as well, in addition to the corner windows.

          1. Expert Member
            Malcolm Taylor | | #17

            Scott,

            By deep I meant a lower sill height, while keeping the head height consistent with the rest of the windows. So something like 70" x 70".

            These are only suggestions made without any idea of the site conditions, or other considerations you may have.

            One other question just out of curiosity: How are you planning to frame the asymmetrical roof? Trusses or ?

  6. scottwoodward | | #18

    Hi Malcom,

    Taking earlier advice, I would use the raised heel truss for the roof framing. I'm using a Chief Architect product (Home Designer) and it automatically renders the roof framing (but not the raised heel truss). I'm still in the midst of figure out how the raised heel truss would work for this design. It may be as simple as raising both sides the requisite amount for the heel.

    1. Bas Sommer | | #19

      Good to have the visual!
      So raised heel to the left and cathedral to the right?

      1. scottwoodward | | #20

        Yes, I think so, unless there's a way to get the raised heel on both sides, but if not, at least I've got the raised heel on one side. Will also extend the overhand all around to be 18" or 24".

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