NH Garage Build - Questions/Advice

[audipwr1]

Am in the final discussions on specs with my GC and have a few questions for the pros. The garage is a 40x50 with 14 ceiling. Stick build with trusses (and without finished attic space).

Use - mostly storage of sports cars, and occasional working in there (both summer and winter) with long periods of no use. I plan to keep the inside between 45-80 degrees year round (so heating and very occasional cooling) and when there keep it 65-70.

I have specc'ed the following

12 inch footings, 5 ft frost walls with 2 foot exposed (then 6x12 lumber to get my 14 foot ceiling)
4 inch (fiber or steel) reinforce slab, semi polished 5k PSI. with 6 inch spots to mount 2 post lift
For cost efficiency I am not going to do radiant floor heating - requires just too much up front cost
r30 in the ceiling, r20 in the walls, the rigid foam along perimeter r10 down 3 feet. Industrial garage doors r17

Here are my questions
1. GC wants to do 2 inch rigid foam insulation covering the frost walls, but NOT insulate under slab (he says super hard with this big a space to not crack the insulation and later causes potential cracks and issues with slab) - is this sufficient? The gravel under the slab will be surrounded on the sides by the perimeter insulation. What about moisture etc?

2. I am going to do a power troweled finish to begin with - after curing I am considering an epoxy coating in a low gloss grey. Any other considerations important here?

3. For HVAC I am contemplating MRcool DIY splits (2 units)with 2 propane backup units. I think I will need something around 50k BTU of capacity (heating) and somewhat less for cooling. Any suggestions on cost efficient solutions here I should be thinking of

4. Electrical - 200amp service. 2 minisplits, 2 post lift, 2 four post lifts, 1 air compressor, then lots of lights.

 

[jack stand]

Looks very nice.
I've not heard of foam being a cause for cracking (being polite to your GC) but I understand the wild expectations that his customers may have has him on guard.
4-6' (flat) around the perimeter would be a compromise. Fighting a cold slab becomes harder as you get deeper into winter.

 

[ConCretin]

1. GC wants to do 2 inch rigid foam insulation covering the frost walls, but NOT insulate under slab (he says super hard with this big a space to not crack the insulation and later causes potential cracks and issues with slab) - is this sufficient? The gravel under the slab will be surrounded on the sides by the perimeter insulation. What about moisture etc?

Placing insulation on the frost walls keeps the frost from penetrating into the soils under your slab and is good practice. Placing insulation under a slab is also a very common practice and if done properly doesn't contribute to cracking. With that said, I don't think it's worth the cost in terms of comfort or heating bills especially if you aren't going with radiant heat.

2. I am going to do a power troweled finish to begin with - after curing I am considering an epoxy coating in a low gloss grey. Any other considerations important here?

If you are considering an adhered floor system such as epoxy, you should install a good quality vapor barrier under the slab. This will block any potential water vapor from coming through the floor and wreaking havoc with the bond of your expensive new epoxy. You could use 6 or 10 mil poly but this degrades over tine and is subject to damage during construction. A more expensive but effective choice would be a 15 mil product like StegoWrap.

One thought about the plan you posted. Stepping the footings up and down at the doors is a little funky and will cost more than just running the footings level.

Take a look at my Guide to Floor Slabs in the link below for some additional thoughts on placing a slab. Congrats and good luck on your project.

 

[Metal-Marc]

I would put down insulation underneath the slab for sure. Weather you want radiant heat or not, I would put pex in the floor. You might change your mind in the future, or it can be a great selling point when you want to sell the property.

200A is a lot of electricity if you don't work full time in the garage with a crew.

 

[audipwr1]

thanks for the replies
It also sounds like I need to be thoughtful about frost heave here - (its very sandy soil)
I read the floor slab guide - is there something similar but as a "cold climate addendum" as it relates to how to endure i dont come to find a frost heave in my slab one weekend!

 

[racecougar]

In addition to the comments above, I would seriously recommend upping the attic insulation.

As far as mini-splits go, if you're comfortable flaring lines and pulling a vacuum, that will open you up to a ton of other brands at potentially less cost / higher efficiency. It also allows you to make the line sets whatever length you need, rather than coiling up excess length.

 

[audipwr1]

pulling vacuum a pain without right tools IMO
you think r49 has incremental returns for the cost RE building insulation?

 

[ConCretin]

I read the floor slab guide - is there something similar but as a "cold climate addendum" as it relates to how to endure i dont come to find a frost heave in my slab one weekend!

Edit; Got my threads mixed. re-writing answer

 

[racecougar]

Pulling a vacuum is pretty simple with the proper tools, which cost less than paying up for Mr. Cool units. You also end up with new tools.

Yes, increasing to at least R49 in the ceiling is absolutely worth the minimal increase in cost, assuming you're not building this garage just to sell it.

 

[CraigStu]

Why is the floor below ground level?

 

[ConCretin]

I read the floor slab guide - is there something similar but as a "cold climate addendum" as it relates to how to endure i dont come to find a frost heave in my slab one weekend!

The insulation on the frost walls will prevent that. The under-slab insulation relates only to heating efficiency and comfort, which is debatable.

 

[racecougar]

Why is the floor below ground level?

Where are you seeing that?

 

[CraigStu]

Maybe I am misinterpreting but the front elevation looks like the grass out on both sides is 1/2 way up the lowest garage door segment.

 

[racecougar]

Maybe I am misinterpreting but the front elevation looks like the grass out on both sides is 1/2 way up the lowest garage door segment.

Ah, I see what you mean. Yes, I'd question the grading here, too. That looks like an opportunity for water to run the wrong way (into the building).

 

[D.Gribble]

A compromise on underslab insulation that I see on commercial projects is running the insulation 4' wide at the perimeter. If your GC is worried about the insulation breaking you can always lay down 40 psi or 60 psi instead of the typical 25 psi.

I agree with @ConCretin on putting down 15 mil stego. It is cheap and pretty much standard practice in the commercial world.

 

[billconner]

I am a little surprised by insulation planned as the NH code requires 49 in the ceiling and 20 (2 southern counties) or 25 (rest) for walls. And the next edition will require R60 in ceilings and at least R25 in all walls with at least R5 of that in Continuous Insulation. I'd certainly do the R60 and R25 and I'm old, so not long for payback. I did R60/R40+ for a 460 sf addition and stayed warm when below zero outside with a 1200 watt electric heater until I had hot water hooked up. It could payback just in less heating and cooling capacity.

Id suggest raise heel (energy) trusses as well.

Just curious if you considered a first protected shallow foundation? Just foam under thickened edge slab and several feet out, no deep footings or stem walks.

PS Meant to say I'm in NY on Northern border in zone 6.

 

[dcg9381]

3. For HVAC I am contemplating MRcool DIY splits (2 units)with 2 propane backup units. I think I will need something around 50k BTU of capacity (heating) and somewhat less for cooling. Any suggestions on cost efficient solutions here I should be thinking of

I think you've hit is spot on. I helped a buddy with Mr. Cool recently and they've made the copper line connections very consumer friendly.

In terms of cost effective, often you can get 2 x <size> units for less than the price of <large unit>. Remember they are somewhat directional cooling. It's OK to "over cool" with ductless units that are inverter driven and you can always kick down to just using one.

I prefer to put the lines in the walls versus kicking them straight out. The lines (and control wires) will go through electrical PVC. So when I frame, design a drop inside the walls for the lines as well as use 3/4" PVC for the drains.

We use propane as aux heat in the winter too. Non-vented units do increase indoor condensation, so there is that to consider.

 

[audipwr1]

Why is the floor below ground level?

The drawing contemplates just building up the 3 non driveway sides with gravel to hide the large (2 foot above grade) footing walls. To get the 14 foot ceiling its 2 foot of concrete and 12 feet of lumber. We are now debating just using a stucco covered insulation instead of this - cheaper than the gravel

I am a little surprised by insulation planned as the NH code requires 49 in the ceiling and 20 (2 southern counties) or 25 (rest) for walls. And the next edition will require R60 in ceilings and at least R25 in all walls with at least R5 of that in Continuous Insulation. I'd certainly do the R60 and R25 and I'm old, so not long for payback. I did R60/R40+ for a 460 sf addition and stayed warm when below zero outside with a 1200 watt electric heater until I had hot water hooked up. It could payback just in less heating and cooling capacity.

Id suggest raise heel (energy) trusses as well.

Just curious if you considered a first protected shallow foundation? Just foam under thickened edge slab and several feet out, no deep footings or stem walks.

PS Meant to say I'm in NY on Northern border in zone 6.

That is the code yes agree. I will level up the insulation

 

[audipwr1]

The insulation on the frost walls will prevent that. The under-slab insulation relates only to heating efficiency and comfort, which is debatable.

Ok thank you. I am trying to work out the level of "cost" that not insulating the slab will have in terms of heating in the winter to make the building itself 65-70 to work in. I dont work on the floor (2 post lift) so less about the feel of the cement more about the big heat sink into the soil?

 

[audipwr1]

Additionally - is 3 feet of frost wall on a 12inch footing deep enough? Wont that heave? I think probably needs to be 4 foot under grade + 12 inch footing?

 

[egdede]

As asked above: Is that floor below grade? I can't think of a good reason to design that feature....

 

[audipwr1]

No its not - that drawing has backfill in it to hide the 2 foot exposed concrete walls on 3 sides. We are instead going to stucco the insulation that is exposed 2 feet of vertical around the building with a metal flashing on top

It was the architect's idea that I didnt like and would be insanely expensive to build up 1 foot of gravel grading around the 3 sides

 

[billconner]

Additionally - is 3 feet of frost wall on a 12inch footing deep enough? Wont that heave? I think probably needs to be 4 foot under grade + 12 inch footing?

Bottom of footing at or below frost line. When I look at maps, looks like no deeper than where I am, so 48".

I'm not crazy about exterior foam with coatings. Masonry veneer products may be too pricey, but there are cement board products made for this. Just about durability for me - lawnmowers, string trimmers, etc.

 

[ConCretin]

Additionally - is 3 feet of frost wall on a 12inch footing deep enough? Wont that heave? I think probably needs to be 4 foot under grade + 12 inch footing?

That would be adequate up here in Maine but it depends on your local frost depth.

 

[audipwr1]

Bottom of footing at or below frost line. When I look at maps, looks like no deeper than where I am, so 48".

I'm not crazy about exterior foam with coatings. Masonry veneer products may be too pricey, but there are cement board products made for this. Just about durability for me - lawnmowers, string trimmers, etc.

This is why we originally wanted to slope up the grade along the edges

any national brands you like for cement board? Then you need to finish it with something - whats affordable / low labor intensity to install?

The veneers I have seen dont have corner pieces would imagine that makes the building corners look weird?

 

[billconner]

No help on brands of cement board.

My neighbor had a lot of stone veneer applied last summer. I'll try to take a picture of a corner. I googled "stone veneer corners" and bound a lot. My neighbor has a kind of matching bullnose piece between foundation and siding even.

 

[dcg9381]

any national brands you like for cement board? Then you need to finish it with something - whats affordable / low labor intensity to install?

James Hardie. You paint it. Board and batten is a common finish (at the edges) for their version that looks like stucco, but there are all sorts of lap and other exterior appearance options.

 

[billconner]

Here's one that is made for this purpose: https://duraskirt.com/duraskirt-insulation-covering/

In looking around found a couple of posts using grey FRP for this. Interesting idea. Dull side out, spray foam adhesive. Also interesting.