Trouble deciding how to do foundation

Started by Alan Gage, March 31, 2011, 08:04:15 PM

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Alan Gage

The old house is almost torn down and soon it will be time to pour the foundation for the new place (1000 sq. ft. single story double stud wall in NW Iowa). I'm really struggling trying to decide how to do it though. I'm new to concrete so maybe I'm just psyching myself out.

I know it will be a slab on grade and will be heavily insulated. But what's the best way to get there?

A Frost Protected Shallow Footing monolithic slab would certainly seem to be the easiest way but since I'll be insulating under the slab as well (4" EPS) I'll need a lot of horizontal insulation extending away from the footing to pull it off, like 4-5' worth. I also have to worry about the detail between the exterior insulation on the foundation and the house wall. The easiest way around that would be to build the outside wall with 2x6 and let it hang over the edge of the foundation 1 1/2 inches so after the sheathing is installed it would be flush with the foam (assuming I used 2" there). I'd like to build it from 2x4 though since I saved enough from the old house to frame both walls (inside and outside).

So I'm looking for other options that would let me use 2x4 for the outside wall and limit the use of foam (for practical and environmental reasons). That got me thinking about a full depth frost wall (4') and only using foam on the inside. Under the slab and as a short vertical thermal break between the slab and frost wall (not insulating the full 4 vertical feet)

I'm thinking if I went with a full frost wall I could get by without a thickened footing. The soil is sand and gravel (good bearing capacity) so if I got someone in with an 8" footing trencher I'd think it would be pretty painless to do a 4' frost wall (+ 8" above grade). It would be a little over twice as much concrete as opposed to monolithic slab footings but there would be a lot less foam, which would be good for $$$ and less hassle in detailing.

When I tried to do some research on trenched footings it seemed they were all left level with (or slightly below) grade. Would there be a problem doing above ground forms to get the wall up another 8"?

Any thoughts?  If I did a 4' frost wall would it be a mistake to not use full vertical insulation?

Alan

bayview

    . . . said the focus was safety, not filling town coffers with permit money . . .


Alan Gage

Bayview:

Thanks for the response but ICFs don't interest me a whole lot for my specific purposes. If I did full depth frost footings and use ICFs I'd have to excavate a pretty wide hole to give enough room for me and the forms. If I trenched it in there would be no form work below grade and minimal excavation, that's what has me researching that direction. ICFs don't solve my exterior foam issue either.

Maybe I'm missing something. If I am please point it out.

If I was doing a full basement I'd take a good hard look at them though.

Alan

archimedes

I'm not sure I understand what you're asking.

Are you talking about excavating a 4' crawl space?

On my last cottage,   I formed a continuous footing using 2X material and poured it myself (16'X24').  And then I layed block above that.  I'm in FL so I didn't have to go down very deep.  But it was fairly easy to do.  Not nearly as bad as I had anticipated.

I would think block on a footer would be your cheapest and easiest diy foundation.  Then insulate the exposed stem wall and rim joist.
Give me a place to stand and a lever long enough,  and I will move the world.

Alan Gage

Quote from: archimedes on April 01, 2011, 05:08:48 PM
I'm not sure I understand what you're asking.

Are you talking about excavating a 4' crawl space?

No crawl space, it will be slab on grade. I'm trying to decide if I should do a monolithic slab with shallow frost protected footings or dig a full 4' frost wall (8") and pour the slab inside of it. No thickened footing so the 8" wall will bear on the soil. If I went with a 4' frost wall I'd want someone to come in with a trencher so there would be no forms below grade as the trench would be 8" wide. The insulation would cover under the entire slab and between the slab and frost wall for a thermal break. It seems like it would be a nice way to go but being unfamiliar with concrete work maybe there's a fatal flaw I'm unaware of.

The soil has good bearing capacity and since it's slab on grade the only loads on the footings will be the weight of the concrete, house itself (single story) and snow (40psf). Sounds like 8" should be plenty.

Alan


OlJarhead

I'm not entirely sure of what you are looking for either but if you are doing a slab on grade then I'd think you'd just put in the 2'x2' foundation trenches and tie those into the rest of the slab with rebar and steel grid.  Then pour it.

MountainDon

He still has to deal with the frost depth, unless he does the frost protected shallow method, which does use a lot of rigid foam.

Alan, sorry I keep forgetting, but you are or are not required to build to code?  Here in the NM desert, code requires R10 under the slab-on-grade to meet energy saving goals. The floor is warmer in winter too. That does need as much foam as the wide horizontal area around the perimeter is not needed.
Just because something has been done and has not failed, doesn't mean it is good design.

Alan Gage

Quote from: MountainDon on April 01, 2011, 07:07:21 PM
He still has to deal with the frost depth, unless he does the frost protected shallow method, which does use a lot of rigid foam.

Alan, sorry I keep forgetting, but you are or are not required to build to code?  Here in the NM desert, code requires R10 under the slab-on-grade to meet energy saving goals. The floor is warmer in winter too. That does need as much foam as the wide horizontal area around the perimeter is not needed.

You're correct, no inspections here other than electrical and septic.

I will be insulating under the slab (R15) but from what I understand I'll need more horizontal (wing) insulation extending away from the footing in that case. With no under slab insulation the FPSF relies on heat escaping from the home through the slab and keeping the soil under the slab and footings warmer. If you add insulation under the slab this heat source is gone so more wing insulation is required. So it needs to be designed the same as an unheated building, which in this climate means 4-5' of wing insulation. That's a lot of insulation and I'm worried it will be a hassle in the future when I want to do any work with a shovel around the house. With no under slab insulation about 2' would suffice but then, like you said, the slab would be colder.

I don't see any downsides from a comfort/energy loss standpoint of an uninsulated full depth frost wall with insulation isolating it from the slab. Seems like it shouldn't lose any more heat than if the foundation wall had full depth exterior insulation.

Can't wait until the build has started and these decisions are behind me!

Alan


Don_P

QuoteCan't wait until the build has started and these decisions are behind me!
I've had bad decisions run back around and stand there right in front of me again, usually with their hand out  d*

I've never seen what you are proposing done. I kind of doubt you'll get a nice clean vertical 8" trench four feet deep. If it is looking doubtful the day of the dig I'd want to have plan B in place so you can easily move on to something else without having to bring the excavator back in.


Alan Gage

Quote from: Don_P on April 01, 2011, 10:14:41 PM

I've never seen what you are proposing done. I kind of doubt you'll get a nice clean vertical 8" trench four feet deep. If it is looking doubtful the day of the dig I'd want to have plan B in place so you can easily move on to something else without having to bring the excavator back in.

I'd never heard of it before either but was talking to my dad about foundations the other day and he told me that's what they did when they built the last building he had put up. He was skeptical when that's what they proposed but apparently it's fairly common around here and one of the large construction companies does it regularly. I'll have to do some more checking around locally. It's got me curious.

Some photos I found online:







Alan



archimedes

I've never seen what your describing done either.  But every region is a little bit different.  Especially if there are no codes.

I can't help but thinking that if you're going to dig a 4' hole than why not remove all the soil between the stem walls and at least have a cellar.  It can't cost that much more to remove that soil.  Then you'll also have a place to put your waterheater,  furnace,  access for plumbing (and don't under estimate how important that one is)  and room for storage.  If you're going down 4' (or so) with the frost wall, then go 2' (or so) above ground and you've got a usable space under the house which you can insulate.  You can even drystack the blocks yourself.  You don't necessarily need to make it living space.

You're paying someone to come out and move dirt,  for the trench,  is it that much more to remove the rest of it - esp. if you have a place on your property to put the dirt?  It maybe a little more up front,  but your getting additional usable space in return.

Sorry,  don't mean to complicate your decision making even more,  but it's something to think about.  There's a reason so many northern homes have basements.  If your gonna dig a hole anyway..........

Looking forward to watching your build.   d*
Give me a place to stand and a lever long enough,  and I will move the world.

jr1318

#11
check out my thread here for spff  picture on page 3, for vetrical insulation on slab build forms to cabinsize, on the  inside of the form put 1 /2  treated plywood scewed to inside of form with 7 in galv lanscape spikes through the plywood and 1 inch foam on insie of plywood, what this did is it let me attach plywood into the slab so I didn't have to worry about glueing plywood to foam. Look at pics on bottom of page 3. It worked well, also 2 feet past perimeter of forms is plenty, just make sure to use hi density foam that is made to pour concrete on. If you'd like to talk e-mail phone number maybe I can explain better.
https://s103.photobucket.com/albums/m159/jr1318/cabin/

JRR

#12
Alan, I see nothing wrong with your plan.  On the wall detail, I might want to add a little vertical steel ... at the expense of removing some from that lower level ... maybe have a second tied-up layer of steel ready to drop in the ditch as the wet concrete pour nears ground level.

Instead of a direct floor slab atop the foam, would it (?) be better to:  (#1) first, pour a few inches of gravel only.  This can be delivered by concete ready-mix trucks.  Install gravel drains, if needed ... install subfloor services ... level and tamp the gravel.  Then (#2), pour a thin (2"?) welded-wire reinforced concrete cap.  Screed very flat.  (#3), Add layers of foam insulation.  Finish with (#4) plywood flooring, then tile, etc. (you might skip the plyboard if a thick reinforced grout bed and floor tile were used.)  This might be a cheaper scheme and would keep the foam from being so weight loaded ... may be able to use a less expensive foam board.  Might be easier to provide a better, flatter surface for the foam to be on.  I have not done the numbers ... just thinking aloud.

I know the foam is supposed to be OK against the ground, but I think it would "Live" better in the above plan.  The insulation would be closer to the floor material, ... I would like this better.  Quicker warm-ups.  Of course, if you were planning on using the slab's concrete surface as your final floor, this above plan may add expense.

OlJarhead

Quote from: Alan Gage on April 01, 2011, 10:51:55 PM
I'd never heard of it before either but was talking to my dad about foundations the other day and he told me that's what they did when they built the last building he had put up. He was skeptical when that's what they proposed but apparently it's fairly common around here and one of the large construction companies does it regularly. I'll have to do some more checking around locally. It's got me curious.

Some photos I found online:







Alan




These look more like the shallow ones we had done around our slab.  Ours were two feet deep and two feet wide and the slab was poured on top of all but 6" of the footing (stepped to allow the slab to be poured onto it) and tied together with rebar etc...wish I had pics of it but that was almost 10 years ago!


Don_P

I'm never sure when to nit pick when pics get posted but there are a few problems in the pics that might be helpful to point out. The first two are shallow 2' wide trenches. Remove organic matter from inside the future structure. Rebar needs to have a minimum of 3" of concrete cover, it is not thrown down in the mud, it is supended on chairs or hung from above. The mud really shouldn't be there.  The third pic is I think what you are envisioning, a trencher laying a narrow deep trench. Alot of loose dirt is in that trench. I'm doing good to fit in an 18" trench with a shovel and make any headway towards cleaning it out. What you propose can work, I just havent seen it done and I suspect that is for a reason. Where I'm going with plan B is saying that as you follow the excavator with a shovel... if you cannot clean the trench or if the trench is caving, i'd have him widen it to standard footing width right then while he's there and count on a formed pour or block wall. There is nothing wrong with experimenting and trying it. If it works, great, you've saved some money, if it doesn't I'm trying to save two full trips around.

JRR

#15
I agree with Don_P, a footing trench needs to be very clean, dry, and firm.  I usually follow the trenching back hoe with a shovel, removing at least an couple inches of earth all around... making sure all organic material is removed.  If the rain surprises, its back in the ditch ... mucking and digging further down.  I often use a compacting hammer to firm-up the full length of the trench bottom ... even if its "virgin" earth.  If re-bar is not "covered" by concrete ...above/below... it can be counted on to soon rust away.  Removed earth should be located a couple of feet away from the trench if possible ... or it will find its way back in the trench, just where its not needed.  Many locales have a inspection requirement of trenches, with rebar in place ... before concrete is ordered.

As the ready-mix trucks arrive, the last task is to get me out of the trench.

Alan Gage

I talked to a couple people around town about the 8" trench footings and while neither of them had a problem with the process itself they couldn't think of anyone off hand who actually had a trencher specifically for doing footings. They said most people had been using a couple of the local electricians to do the footings since they already had trenchers. Didn't seem to be any concerns with too much loose soil falling into the trench. When I was researching it online I found mention that it was recommended to dig the footing trench in 2 stages. The first to within a foot or so of final depth and then finishing it off to final depth on the second pass. This was to keep the bottom of the trench as clean as possible.

I'm leaning away from going that route though. I'd feel better if there was someone around with a trencher that used it primarily for foundations rather than hiring an electrician to do it. Maybe I'll talk to one of them though and see what they say.

I am leaning back towards a FPSF. After reading through everything again I realized that for a building with horizontal insulation running under the entire slab vertical insulation on the outside of the wall isn't required. This would solve my dilemma with how to deal with that exposed foam. Instead I'd poor 24" stem walls (16" below grade and 8" above) and insulate the vertical interior wall for energy efficiency, then poor the slab inside the walls.

Supposed to be bad weather this weekend so it will be a good one for staying inside and nailing all this down. Guy will start tearing out the old foundation on Monday.

Thanks for all the feedback guys!

Alan