Cheapest foundation/Frost protected shallow foundation cost?
Last Post 06 Apr 2018 05:07 PM by patonbike. 44 Replies.
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patonbikeUser is Offline
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29 Dec 2015 08:17 AM
What is the cheapest sound foundation option in Vermont  and can you give me an idea of what it costs per sq ft for a ~800-900 sq ft building?
For comparison,
I think our 8 ft cellar  was ~27 sq ft including labor, excavation, concrete, and R15 on 5 sides.

No basement needed on this one.

ICFconstructionUser is Offline
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03 Jan 2016 08:39 PM
There is always something cheaper. What is best? I much prefer a full depth frost wall of poured concrete, ICF it the building is going to have climate control.

What is the use of the building, climate controlled? What is frost depth there? Soil type?

I recommend you put together some costs, pros and cons so that you can make an informed decision.
Brad Kvanbek - ICFconstruction.net
jonrUser is Offline
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03 Jan 2016 09:23 PM
Since you want to insulate anyway, my guess is that a frost protected shallow foundation designed for a non-heated building will be the lowest cost with good performance.
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04 Jan 2016 05:55 PM
The building would be an accessory dwelling, i.e. a guest house. We would climate control it with a ductless mini split most likely. I guess what I meant is that I don't need a basement and hope to save money by not doing a basement - I love my basement in my house but I don't think I need it for the accessory dwelling

My thought was that by doing a FPSF , we can also use the concrete as the floor of the dwelling, then also save on flooring.

Frost depth - about 4-5 feet
Soil - clay


jonrUser is Offline
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04 Jan 2016 08:56 PM
An unheated FPSF design (and blowing out the water pipes) will allow you to safely turn the heat off with no danger to the slab. In some other foundation designs, there is some possibility that a vertical concrete wall could conduct cold to below the footing and cause that area to freeze.
patonbikeUser is Offline
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22 Mar 2016 01:55 PM
It makes more sense if I explain what I'm trying to do...

I want to build a ~26x28 cottage or apartment with cost being a factor.

I'm trying to figure out if it makes more sense from a cost perspective to put the finished living area on a frost protected slab or put it on top of a garage basically like a high posted cape (not 2 full stories... it would look pretty bad!)

If I put it on a slab, I'd want to add an attached unheated storage area , making the full building dimensions roughly 34x28 on 1 story rather than 26x28 on 2 stories.

If I put it on the slab I can have no stairs, no floor joists, use stained concrete for the finished flooring, etc. but I've got a slightly larger footprint, more roof etc..
If I put it on top of the garage, I assume the slab will be cheaper than the frost protected slab, insulating the floor joists from the garage below should be very easy, but I'll have stairs, finished flooring to deal with.

Slab seems just super simple and would be hard to beat for cost and ease of work?

Any thoughts?
Bob IUser is Offline
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22 Mar 2016 02:05 PM
build the larger single floor building; it's a PIA to insulate and air seal over a garage, plus the room the stairs take up, time & $$ to build. A second floor also means you have to frame and finish a wood floor.
Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant
patonbikeUser is Offline
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25 Mar 2016 02:00 PM
Hmm how are water pipes frost protected when the water line doesn't enter the building below the frost level?
jonrUser is Offline
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25 Mar 2016 09:01 PM
As long as the water pipe enters the house under the FPSF horizontal perimeter foam, it's not exposed to freezing temperatures. Think of it as "the frost level is moved up".

Of course any house that is left unheated needs an underground/behind foam valve.
patonbikeUser is Offline
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20 Apr 2016 07:49 PM
Any suggestions on where to find someone who can do a FPSF correctly in Vermont? It doesn't seem to be a very common thing here.

Also I am assuming I don't need to heat the slab itself?  I was planning on just putting a mini split in this ~728 sq ft building.  it shouldn't be a big issue leaving it ~60F over winter.
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24 Apr 2016 11:28 AM
In basement country, FPSFs turn up most often under additions to existing houses. Remodelers may be able to point you in the right direction. Or these people: http://www.vtgreenbuildingnetwork.org/ I'm thinking contractors pour waffle slabs right along for outbuildings in VT, and could easily pivot to a FPFS if you spot them the design. The designer doesn't have to be local.

FPSFs can be designed for unheated buildings so, yes, a mini split set at 60 is all you need.
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26 Apr 2016 04:36 PM
I thought 60F was considered "fully heated" by Green Mountain standards! :-)

IIRC the Fujitsu mini-splits have "minimum heat" setting that keeps it about 50F, which would reduce the heating power used during low occupancy times considerably.
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27 Apr 2016 10:34 AM
A freeze-proof design + no heat will save energy and be safe when the power fails. When its 0F outside (agreed, not the average temp), 60F might save only 14% over 70F.
patonbikeUser is Offline
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22 May 2016 08:59 AM
Posted By Dana1 on 26 Apr 2016 04:36 PM
I thought 60F was considered "fully heated" by Green Mountain standards! :-)

IIRC the Fujitsu mini-splits have "minimum heat" setting that keeps it about 50F, which would reduce the heating power used during low occupancy times considerably.

ha ha... the first year we had our condo up on the mountain, we kept it right around 60 because I was so worried about what the heating bills would be. 
patonbikeUser is Offline
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22 May 2016 09:03 AM
i don't see many buildings on piers around here, but would this be a viable solution?
jonrUser is Offline
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22 May 2016 09:14 AM
Sure. Thermally, it is similar to a well vented crawl space.
patonbikeUser is Offline
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14 Oct 2016 01:26 PM
Seems like we've gone a different direction and now the plan is a garage with an apartment on top.
My thought would be to heat the apartment, not the garage.  I don't need heat in the garage and it's an added expense (both to insulate, finish and then to keep heated), but my concern is then how do you get the water from the ground, presumably through the garage, and  into the apartment without worrying about it freezing?
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17 Oct 2016 11:10 PM

If you have gone to the garage option due to difficulties with the design of the foundation,
I may be wrong but is seems that you have skipped one option. Instead of a FPSF. You can do a Normal foundation with insulated slab on grade.
This is what I did for a workshop almost the same size last year in central Oregon.

Our Code required foundation depth here is 24 inches to bottom of footer. We used an 8 inch thick footer, and 23 inch foundation walls. I used 1 inch XPS inside and outside foundation walls and on top of the footer on the outside.
On the inside the XPS is tapered to 1/2 inch at the surface so trim will cover it. We used 2" of XPS under the 4 inch slab. I would have preferred to use eps but my concrete man was afraid of it and would not guarantee the slab on it.
The rest of the building is wrapped in 1 inch of Polyiso. and heated with a Mitsubishi Hyperheat minisplit. R27 walls R55 ceiling

The highest heating bill I had last winter was $23.00

Your mileage may vary, we enjoy an electricity rate of .0663/KWH

patonbikeUser is Offline
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26 Oct 2016 08:09 PM
It ended up being due to the design we thought would meet our needs and our space the best.

The local concrete guys recommend a normal slab, not FPSF, 48" deep footer, I think a 4" slab, not positive on that.   They said it should cost $10k and I'm not sure if that includes excavation for 24x30.  I haven't gotten additional quotes yet.

I am concerned about how you get water upstairs if I don't heat the garage.

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28 Oct 2016 10:18 AM
Water should not be too difficult to protect from freezing. I assume the main supply will run from the meter to a point in the garage (under the slab) at same depth as now required. Assuming the stairs to the second floor apartment is in the garage structure, the line would run up the interior stair wall. Thermostatically controlled heat tape would be used for the short distance the line is in unconditioned wall space to then be distributed within conditioned floor/wall space.

If exterior stairs then you will have to carve out some garage space for a mechanical chase.  A chase may be a better option because you may have the same freeze concerns with the sewer run.
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