New house construction – Minneapolis, MN
I am getting close to finalizing the plans for our new single-family house construction on a typical urban lot in Minneapolis, MN (climate zone 6A) and would appreciate some input from the GBA community. For context, it is an East-West lot, and the house is set back from the street and neighboring houses to get good solar exposure. Passive house inspired, but not looking to get certified. We are not planning on running a gas line to the house, so everything will be electric. We would like to build an office/studio above the garage that could also be an occasional guest house. My wife and I are planning on doing a fair amount of the construction to keep the costs financially feasible, so simple is better.
I’m not an architect and have never built a house, so any general comments about the plans would be appreciated. In particular, I’m wondering about the following:
1) Ductless mini-split recommendations. We are proposing a single ductless mini-split in the downstairs living room of the house. I have seen a lot of suggestions for installing a second unit upstairs for cooling in the summer, but I’m not concerned about cooling. I’d prefer to rely on natural ventilation, and only expect to use the A/C for a week or two out of the year. I’m leaning towards a ceiling recessed model for aesthetics and freeing up the wall space for other things. Is there a significant efficiency difference between a wall mounted or ceiling recessed model? Is one more appropriate for the space?
2) Heat load calculations. I ran some quick heat loss calculations and came up with a design load of 20,600 BTUs for the house and 4,800 for the upstairs of the garage. Does this seem reasonable and should I install a 21,000 BTU head unit or go up to 24,000 to be safe? We will need to have electric baseboard backups regardless.
3) Applied roof supports. I like the idea of cutting off the rafters to reduce thermal bridging and simplify air sealing, but I am a little confused about how to support the roof overhangs. Currently, I am proposing vertical 2x’s as an applied rafter over the Zip sheathing. Is this reasonable of is there a better solution? The only other option I have seen is adding the overhang support with an applied soffit which I think would look kind of funny with the modern design of the house.
4) Are the roof and wall assemblies appropriate for the zone and climate? I know the R-value of the wall is a bit under the Pretty Good House target, but hanging more than 4″ of insulation plus the rainscreen off the side of the house seemed like a lot. Also, the minimum exterior insulation ratio requirements for foam would not apply to rock wool correct?
5) Are the HDP bearing block thermal breaks on the glulam post footings necessary? I imagine the subslab condition away from the perimeter is fairly temperate, so maybe I shouldn’t be too concerned about maintaining thermal breaks there.
Current house plans and the heat load calcs are attached. Thanks for any and all input!
-Kellen
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Replies
Hi Kellen.
I'm not going to try to answer too much here, because there are other GBA users who know a lot more than me, particularly about the mechanicals. I'll simply say that your first floor plan looks good for one minisplit head, at least from the feedback I have gotten from architects and builders who have tried to minimize equipment. Second floors, with lots of separate spaces, like yours, tend to be the problemed areas. But if you are not concerned about cooling and will be installing back up electric baseboard heaters in the bedrooms, maybe it will be fine.
I've seen applied roof overhangs done a few different ways. The best method I have seen from a thermal bridging perspective is when the overhangs are simply fastened through the continuous exterior insulation with structural screws to blocking installed as part of the wall or above the top plate. The roof sheathing then ties the overhang assembly back to the rafters. Your drawings make it look as though you'll need to install your overhang above the rafters and first layer of decking. I've seen that done too. It takes up some of your insulation space, but not much.
The minimum exterior continuous insulation R-values are about controlling dew point temperature at the sheathing, thus condensation potential, and have implications about interior vapor control. From the perspective of keeping your sheathing warm to minimize condensation potential, they do still apply with mineral wool. However, because it is more common for builders to use rigid foam for continuous exterior insulation, which limits outward drying, those walls need to be able to dry inward and therefore builders often use class III vapor retarders (just the painted drywall) or smart vapor retarders. With mineral wool, however, your wall will still be able to dry to the exterior. You are building a more vapor open assembly than most.
Have you worked through all of your air barrier details, not just the materials for the slab, walls, and roof, but what you are going to do at all of the transitions? Now's the time to do that.
Thanks for the reply Brian. Yes, I am currently thinking that the overhangs would be installed above the rafters and decking. If I'm understanding the alternative that you mentioned correctly, the overhang would be bearing on the exterior insulation, in which case I would need to use EPS or XPS instead of mineral wool, right?
The reasoning behind proposing mineral wool on the roof deck is that I'm using 12" rafters and filling the cavity with dense pack cellulose along with the walls seems logical. But, that is already about R-40, which means I would need at least 10" of EPS on the roof to reach the minimum 51% exterior foam ratio recommended for my climate zone. If my target for the roof is R-60, I'm overshooting that by quite a ways and installing 10 inches of exterior foam sounds challenging. Even though mineral wool is more expensive per inch, it may be more economical to reach my goal of R-60 if I can get by with only 5 inches of exterior insulation because I don't need to be concerned about condensation in the roof if it can dry to both the interior and exterior. Does that make sense?
For air barriers, since the overhangs are applied above the roof decking, creating a continuous air seal between the roof and wall planes seems pretty simple with Zip sheathing. I haven't decided on tape vs. liquid flashing for seams and penetrations. The underslab Stego barrier would wrap above the ICF stem walls to tie into the Zip wall sheathing and create a continuous air barrier. That's the plan at least.
>" would need at least 10" of EPS on the roof to reach the minimum 51% exterior foam ratio recommended for my climate zone."
With NO cavity insulation it could still meet code (on a U-factor basis) with just 6" of polyiso on the exterior, leaving the rafter bays empty. Or, drop back to 4" polyiso up top with 5.5" of fiber insulation (any type) tight to the roof deck.
Dense-packing the rafters with 12" of cellulose isn't recommended with only 5" (R20) of rigid rock wool above the roof you'd be limited to 5.5" of cellulose. That would still meet code (on a U-factor basis), at about U0.025 (=R40 "whole assembly") with credit for the air films in the empty cavity.
The Pretty Good House standard is about "whole-assembly -R" with all thermally bridging framing and the R values of non-insulation layers factored in, not center-cavity R. For an unvented roof the costs can be high enough that it's not necessarily rational to go better than code there, even though BA-1005 is calling for an R60 "whole-assembly" for a "compact roof" in zone 6, in Table 2 :
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Dana,
I have reread some of the GBA articles on cathedral ceilings and figured out what I was misunderstanding about the condensation issue. I like the simplicity and security of keeping the primary air barrier above the roof deck and wrapping it continuously to the wall sheathing/air barrier with both protected by continuous exterior insulation, which seems incompatible with a vented cathedral ceiling assembly.
So, there are two options I'm considering:
1) Apply 3" of closed cell foam to the bottom of the roof decking to prevent interior moisture from reaching the decking and fill the rest of the cavity with dense pack cellulose. That would be about an R-49 assembly, so I could add 3" of mineral wool to the exterior to reach my R-60 target.
2) Put 8" of mineral wool on the exterior to reach R-32 (I realize that polyiso or XPS would need fewer inches, but I'd prefer to use mineral wool for a variety of reasons) and then only partially fill the rafter bays with R-30 batts.
You seem to be pretty knowledgeable about the mechanicals, do you have any input on the mini split locations, types, or models shown in the plans?
Thanks!
Are the rafters 2x12s (11.25") or are they 12" I-joist type rafters? (it makes a difference).
Option #1 delivers about R30 to the exterior of the foam/fiber boundary (R32-R33 if HFO blown closed cell foam) and with I-joists, 9" of cellulose another R33. That's still more fiber-R on the interior, but the hygric buffering capacity of the cellulose will likely prove adequate if an interior side smart vapor retarder is used.
If 2x12s you'd be looking at R30-R31 of cellulose, which is closer to making it without the interior vapor retarder, but it's still a good idea.
Option #2 is pretty expensive, but works. That too would need an interior side smart vapor retarder.
For the above-the-garage zone, a design load of 4800 BTU/hr @ -12F a separate Mitsubishi FH06NA is something of a slam-dunk, since it can modulate just fine at the range of the average winter load, and still musters 6430 BTU/hr @ -13F (a 1.34 x oversize factor, which is enough to cover the load down into the -20s or until the sensor-error feature temporarily turns it off.)
https://neep-ashp-prod.herokuapp.com/#!/product/25907
http://meus1.mylinkdrive.com/files/MSZ-FH06NA_MUZ-FH06NAH_ProductDataSheet.pdf
For the 20,600 load of the house it's probably better to put a pair of KA09 or KA12 four-way ceiling cassettes, each about 10' from the ends of the house. A single wall-coil or ceiling cassette in one corner of the house isn't going adequately heat the spaces next to windows 40 feet away. With a big glass slider on the opposite side and other end of the house from where the wall-coil is drawn It's important to get this figured out ahead of time, since it requires custom framing to accommodate that type of cassette.
http://meus1.mylinkdrive.com/files/SLZ-KA09NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf
http://meus1.mylinkdrive.com/files/SLZ-KA12NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf
You might be able to do OK with a pair of 1-way KP09s (or KP12s) which fit in 16" o.c. joist bays, if they can blow in an optimal direction:
http://meus1.mylinkdrive.com/files/MLZ-KP09NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal-en.pdf
The 3c24NAHZ doesn't have enough capacity to cover a 20K load with any margin @ -12F, but the 3c30NAHZ does:
https://neep-ashp-prod.herokuapp.com/#!/product/26170
https://neep-ashp-prod.herokuapp.com/#!/product/26173
If going with separate single zone wall coil units, a pair of FH15NAH , one on each end of the house gets you there (but a pair of FH12s fall short)
http://meus1.mylinkdrive.com/files/MSZ-FH15NA-MUZ-FH15NAH_ProductDataSheet.pdf
Thanks for the thorough reply Dana! Your suggestions make a lot of sense. If I go with the pair of KP9s or KP12s, would the "optimal direction" be to point them both towards the center of the house?
To answer your question, I will be using 11 7/8" TGI rafters. I think I was a little too generous with my rounding, it seems like 4" of exterior insulation would be needed to keep over 50% of the whole roof R-value above the cellulose.
Kellen,
It's beyond the remit of your questions, and probably too late to change, but I would reconsider relying on a small circular stair as your only access to the second floor. It effectively means you will have to hoist beds, furniture, and the washer + dryer over the second floor railing to get them upstairs, and using these stairs is seen as so unsafe that our code has recently banned them altogether.
Echoing Malcom Taylor's comment, I completely agree that you will come to regret that stair when carrying a sick child or a breakfast tray or one of the thousand other everyday activities that will be made more awkward and dangerous by the stair. I don't accept though that it's too late to change. You don't give a dimension for the width of that space where the stair fits, I looks to be about 66" and you'll need about 10" more for a standard out-and-back stair. You should be able to steal this from the 1/2 bath, and with a little adjustment to the structural post layout you should be good to go. A second layout point, you have the cooktop separated from the sink by a main traffic path. This is also a potential hazard - consider carrying a pan of boiling pasta water to the sink to empty it and tripping over the dog in the process. I recommend moving the refrigerator to the wall where the sink is and move the sink in front of the big kitchen window.
Otherwise a pretty tidy plan. Dimensioning note, you might want to run dimensions to the centerline of the windows and doors instead of to the rough opening widths so that a small size variation as specs are firmed up doesn't lead to problems, also you might want to run exterior dimensions from outside of framing rather than the sheathing so the sheathing can dress down over the face of the stem wall.
All the best, have fun!
Most people seem to have a pretty negative opinion of spiral staircases. My wife grew up in a house with one and her parents and grandparents use it regularly without issue. I've never had an issue with it. Moving large furniture between floors is obviously an issue, but with the balcony I think I can design a pulley system that may be preferable to hoisting stuff up a typical staircase anyway.
The kitchen design comments are interesting. All of the plumbing in the house is in the wall between the kitchen and bathroom which is the primary reason for the sink being where it is. I'll have to think about it.
The window and wall dimensioning comments make sense. I'm sure this is a super amateur question, but if I dimension the walls to an even 40' and the zip sheathing comes in 4' increments won't there be a gap in the air barrier at the corners? Or is this how it is supposed to be done and wrapping tape around the corner is sufficient?
Thanks!
Hey Kellen, yes there are advantages to a compact plumbing layout but it's often possible to have your cake and eat it too. Here's a very rough sketch of a simple rearrangement that would give a safer and more convenient kitchen layout while retaining the compact plumbing advantages, of course you could keep the sink directly against the plumbing wall in this layout if you preferred it. Myself I like a lot of daylight at the sink location.
Looks like a bright and cheery interior for winter especially, with all that south-facing glazing. Have you computed your (potentially substantial) south-facing solar gains during January? (Or Oct thru March?) Your space heating load could be substantially less on sunny and partly sunny days. Perhaps you may not need so many windows on the upper story, as it appears the corridor is only connecting rooms, not for lingering. Could get pretty warm up there, so assume those are for partly for ventilation.
I wish I could see a plot plan of the lot and neighboring homes and other potential shading. Do or have someone do a solar shading analysis? You mentioned setting your home farther back in your lot to avoid shading from neighboring homes on EW lots. Unless you are on very good terms with your neighbor, you may wish to consider paying your south-facing neighbor to sign a legal solar access agreement to avoid someone in the future planting a screen or building a structure that reduces your wintertime solar gains from your south-facing glazing. I would consider beefing up that R-4 foundation edge insulation, and/or add buried wing insulation to protect your foundation wall/slab edge from heat losses. That's your solar-heated thermal mass. Shouldn't the French drains be positioned down lower at the footer?
I also like the centralized plumbing on the two-story interior partition plumbing walls, very efficient, and minimal wait time for hot water.
"I also like the centralized plumbing on the two-story interior partition plumbing walls"
But it is done at the expense of having the only bathroom on the second floor as far as possible from all the bedrooms. It would be much better to have it next to the master bedroom. Long after you have forgotten about the efficiencies of the rough in plumbing, you are still walking the full length of the hall in the middle of the night
Thanks Robert! I have only done a rudimentary solar gain analysis myself in Sketchup. The awnings are designed to totally block solar gain on the summer solstice and maximize solar gain on the winter solstice (that is what the gray dashed line coming through the window on the house section represents). I know that most of the overheating issues happen in the shoulder seasons, so I just tried to keep the glazing at a max 12% of the floor area given the thermal mass of the concrete floor. The heating design loads don't factor in solar gain (since the coldest part of the day typically happens at night), so I don't think hiring someone to do a more precise calculation would really change anything. I certainly value the solar gain and natural ventilation and think it will substantially lower heating/cooling costs throughout the year, I'm just considering the benefits mostly as a bonus.
The french drains are something I'm still trying to figure out. I don't think they're required by code, it's more of a better safe than sorry approach to include them and relieve hydrostatic pressure. But the lot is basically flat, so there is no means to daylight them, and I don't have a basement to put a sump basket in. So. I was toying with the idea of bringing the french drain closer to the surface and draining it to a dry well somewhere in the yard. I'm probably being overly cautious and they will just be omitted.
You are building a home that you will have to live in the rest of your life. I hope it is everything is exactly how you want it.
Couple questions/comments.
-Why bother with the icf stemwall versus field applied insulation?
-French drain is peculiar unless your soil is poor, or the lot has water issues.
-Kitchen triangle is non existent.
-A 14" web truss for what looks to be a 10' span with no need to run ducts is overkill. I know MN loves their web trusses but you could do that satisfactorily with a 9 1/2" I-joist and still drill for your Zehnder.
-A web truss typically does not need an LVL rim, especially with a small house like yours.
-Why use ZIP and cover it with other materials? That's a question for this entire forum as it seems to have become a standard even though you are using it for tape. Can't you tape commodity plywood or OSB?
-Why don't you monopour the post footing and use a cast in place anchor? That post detail seems overcomplicated.
.02
Zoning will let you build a garage almost as big as the house?
PS You don't need to explain any of the above to me, the ? marks are just, IDK, proper or improper grammar I guess. Just bites to think about, hope it maybe helps out.
-our general contractor expressed a preference for working with ICFs, and I think it will be cheaper overall to work with him than to hire a sub for a block foundation with field-applied insulation
-The web truss vs. I joists is a good question. The web trusses seem a lot easier to work with even for just running the HRV ducts and wiring, and if the ductless minisplit plan doesn't work out we always have the option of running ducts. I think the material cost savings of using I-joists would be under $1000, when accounting for the added labor for the electrician/plumber/HVAC I'm not sure it's worth it.
-The Zip sheathing is primarily an air barrier. I don't have a lot of faith in it as a true WRB, but with the exterior water resistant insulation and a rainscreen, the Zip sheathing shouldn't have to deal with much/any water. The other options for creating a continuous air barrier would be using plywood sheathing and field applying a liquid membrane or a Solitex product which I think would both be more expensive (although higher quality) options.
-Technically the garage is an ADU (accessory dwelling unit), and the size falls within the zoning ordinance size requirements as far as I know.
Thanks for your thoughts!
Kellen K,
What a great, efficient floor plan!! I generally like the wall details as well. Cathedral ceilings in cold climates are always tricky.
One very minor area for potential improvement: Rim Joist Insulation.
Your plan shows spray foam at the rim joist. Obviously, this is a common detail but I think it is unnecessary given your exterior mineral wool. I would just net the cavities and dense pack them 5.5" inches. Spray foam carries some risk, is expensive, and can be bad for the environment. Luckily you don't need it with your design.
Thanks Rick! The main reason for using spray foam at the rim joists was just ease of installation. I didn't think netting the cavities between every joist was really an option, but I'll look into alternatives.
I like the design.
I recommend http://www.bmomn.com for dirt cheap windows. Lots of Pella and Anderson.
Thanks for the info Steve! We will probably buy all of our windows from a single manufacturer to keep the install, style, and quality consistent. It seems like a great resource for a lot of other things though (interior doors, flooring, tile, appliances, etc.).
It's exciting to hear about your new house construction project in Minneapolis! Your focus on good solar exposure and going all-electric sounds like a sustainable approach. Building an office/studio above the garage is a fantastic idea for versatility.
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