Is see no reason to design in a thermal bridge. Since you evidently don't want a monolithic slab, raise and extend the 2" EPS FOAM to the left, all the way to the wall. Add more stone to fill the gap. Keep the slab at 4" all the way.

Posted By jonr on 18 Jun 2011 07:44 AM
I would pour the footer and the slab as one piece - and put insulation outside of both. Ie, a thickened edge monolithic slab. Consider moving insulation to the basement ceiling and having an unheated basement.

Garage - frost protected monolithic slab. I probably wouldn't seal it up tight because of moisture issues. Warmth plus 100% humidity and maybe a little road salt and you have great conditions for rust.

Maybe I'm misunderstanding Jonr's post, but here goes...

In our area, 99% of the homes have basements. And, it seems to me that for the majority of those (not pre-cast) the placement of the footer, walls and slab are done in three separate steps (i.e. no monolithic slabs, no FPSF). This is just my impression, as a laymen outside the industry so I could be wrong. But it seems that the contractors I spoke with and the designer all seemed to follow this trend too. At this point I don’t think I want to go outside the normal workflow to ask for a footing-slab pour as one item. As a one-time newbie GC for our own home, I feel I need to stay somewhat in the realm of typical process for the contractors.

Re: Basement in/out (?) of conditioned space
I’ve read many times how in new construction you want to make the basement, if you have one, part of the conditioned space. Therefore, insulate under the slab, the walls and yada yada. So I just accepted that early on in researching about house construction and planning for our new house. But just within the past week or two, I’ve been wondering … WHY does the basement have to be part of the conditioned space? And then when I read Jonr’s post, I really started to think about it…

In our new house, we’ll have 1600 sq ft on the 1st floor for our use - more than enough space! And we’ll have a much smaller space upstairs for extended family visits. The basement will be storage. With planned mini split heat/AC and a ventilator (ERV or HRV) in a sound isolated enclosure in the 1st floor laundry room, we’ll have no ducting or HVAC below the bottom of the 1st floor floor trusses (~14” tall). However, the water heater and future solar heated water tank (water heater pre-heat aka BuildItSolar website) will be located in the basement. But I could fully insulate each of those tanks individually.

OK, so what all does this rambling get me. Well, if I do NOT insulate the basement, and instead insulated in the first floor trusses, is that the better way to go for our house?
- less material cost than insulating basement walls and under slab
- less volume to condition, esp. since basement is just storage
- avoid possible insect problems with exterior (underslab) foam
- less DIY slab prep, better for critical time in construction schedule prior to backfill and getting framer started

It is way different than I was thinking - obviously - since I've been working on the details of underslab insulation. Uninsulated basement - It's not sexy or new, but is it a better fit for this house and its described use for the next ~30 yrs. Thoughts?

Posted By jonr on 19 Jun 2011 02:38 PM
Is see no reason to design in a thermal bridge. Since you evidently don't want a monolithic slab, raise and extend the 2" EPS FOAM to the left, all the way to the wall. Add more stone to fill the gap. Keep the slab at 4" all the way.

I'm not trying to be contrary or anything, it's not that I don't want a monolithic slab ... its more about working within the scope of resources available to me. I agree, the thermal bridge between the 6" of footer and underside of the slab is not a good thing. About 4" of the 6" would eventually be under polyiso wall insulation - in a fully insulated basement about two years down the line.


My first choice would have been to do what you suggest above. The problem is that with 1st-floor top bearing trusses and the 9' poured basement walls, if I add more stone to flush with the top of the footer, then lay a bed of 2" EPS across footer and stone, then slab, I'll lose code required head room on egress stairs. I don't have two inches to spare. I asked for floor trusses to help us with our planned DIY plumbing and electrical. And we went with top bearing floor trusses to limit the number of stairs needed to get into the house. So one decision leads to another, and then oops, no more head room. With dwindling design funds and pre-construction schedule time, I'm very hesitant to go back to the drawing board on it.

Posted By BabyBldr on 19 Jun 2011 02:19 PM
I’m glad it was pointed out that I’m either in violation of the code, or dancing around the hairy edge it. I don’t want to do that, or to accidentally mess with anything structural. 

The concrete police will not come and arrest you if you only have 3" over the footer 

Maybe I'm starting to understand - you want to retain the full poured wall height. Could put the wall on top of the slab vs beside it.

Jonr,

That would work well for above grade.  For below grade, I would avoid a cold joint level with the top of the floor.  Monolithic footer and slab would eliminate the cold joint. 

Posted By cmkavala on 19 Jun 2011 03:55 PM

Posted By BabyBldr on 19 Jun 2011 02:19 PM
I’m glad it was pointed out that I’m either in violation of the code, or dancing around the hairy edge it. I don’t want to do that, or to accidentally mess with anything structural. 

The concrete police will not come and arrest you if you only have 3" over the footer 

Cmkavala - well I'm glad to hear I can avoid jail time. ;-)

Jonr, yes you understand and state it better than I did - I'm tyring to retain the full poured wall height.

Regarding my rambling post above on WHY is it recommended to put the basement in the conditioned space - even if it is just for storage. I'm not sure, but maybe it has to do with how darn difficult it is to properly insulate the floor trusses. I googled around a little and found that people working on that issue as a reno (i.e. basement already in and not part of conditioned space) were all lamenting the difficulty of getting the floor trusses well insulated. By comparison, insulating under a slab is pretty easy - just lay the stuff down, cover and pour. So, that's what I'll do - not to answer my own post or anything...  ...but the schedule is getting short. Got Mom, Sis and her kids coming this week so have to juggle hosting with GCing - and we all get to share the one upstairs bathroom. Boy it sure will be nice when we get this new house built and have a 2nd bath for guests! Family reunion end of the week, ground breaking on new house early July ... So I want to settle the basement issue. I'll order 2" treated EPS for underslab. I'll ask the supplier if it comes in thicknesses less than 1" for the ~6" strip over the footer. That will help reduce the thermal bridging there, and keep the slab as close to 4" as possible. Not an ideal solution, but one I'm willing to live with - and light years ahead of my current home.

OK - thanks for your time and help everyone.

my detail is similar..
the FAD I guess is some kind of footing drain ????
the way we do it is leave the FAD rght where it is shown.... but the 2" eps is continuous under the 4" slab ON TOP of the FOOTING
and it is also a full 2" at the perimeter edge


this reduces the head room by 2" , but the insulation and the continuous bearing with no steps is superior

a lot of the areas we build have high water tables
so we usually set our footings ( 10x20) on 12" of 3/4" crushed stone, then we fill between the footings with 10" of 3/4" crushed stone

our "FAD" goes in this area leading to a gravity drain if possible... or to a sump if no drain to daylight is possible

OP checking back in with some progress, yippee.

I ended up purchasing the foam I need from a distributor in my area - at a price premium. By comparison, I could have purchased direct from a different manufacturer in RI some distance away, paid shipping and still have come out at slightly cheaper. After the rather poor customer service, IMHO, from the local distributor and manufacturer, I was sorely tempted to go this rout. But in the end, I figured the local buy, even at a premium price and no-so-hot C.S. was the more logical way to go. I don't mind distribution systems - IF they add value. That is, if the distributor is familiar with, or at least aware of (!) the product and offers some value for their cut of the transaction. In this case, the distributor felt like a hindrance, IMO. But, in the end, I'll have my foam in the later this week.

Thanks everyone for you help and especially to Kermit who suggested the alternate source that helped me figure out how to go about getting this product. We broke ground today ... fun, fun.

Posted By cmkavala on 18 Jun 2011 10:56 AM

Posted By acwizard on 18 Jun 2011 10:33 AM
Uniform Building Code section 1904.4 states minimum must be 3.5". There are no exceptions to the rule

I agree and a 3" slab poured directly on an 8" footing is an 11" slab with a cold joint. 

1. From a practical standpoint, there are no guarantees that a 3-1/2" formed slab is a consistent thickness thruout. What I can guarantee is it will be less than 3-1/2" in spots

2. It is perfectly acceptable to top off an existing slab with a 2" overpour cold joint , the slab in essence is still  over 4" in thickness (with a cold joint)

I would like to agree with your point of view.... Well said....

Posted By BabyBldr on 17 Jun 2011 03:01 PM
REFERENCE ONLY, SEE POST BELOW...

Hi,
I've got my EPS foam. I plan to use a thinner foam over the footer so I get some thermal break there, but maintain as close as possible to 4" slab too. I need to prep the foam, cutting strips for the footer and for the 'isolation' strip between the wall and slab. A few detail questions if you've got experience with this type of stuff, THANKS!

I plan to put foam UNDER a 15 mil vapor barrier. I bought the VB with a special 'aggressive' double sided tape to attach it to the basement wall. But note, in diagram above, I'm not sure how to handle detail of isolation foam strip (between wall and slab) and VB attachment. Should the VB attach to the wall above the finished slab by several inches? I plan to eventually finish the inside of the basement walls with rigid foam and drywall (for fire protection), so I guess the VB showing isn't the worst thing in the world. But I'm not sure if the VP showing above slab will mess up what the concrete finish guys want to do.

Should I allow slack in the VB at the slab/wall/footer corner so it doesn't prevent concrete from flowing into the corner - any tips on that.

Do you cut the isolation foam strip (between wall and slab) at an angle as shown in diagram?

There is a load bearning footer in the basement for a stair well wall. It was already poured with the other footers and is level with the top of the grave bed in the basement area. I'm not sure how to handl the load bearing footer with regard to covering (or not) with foam and vapor barrier. I'm thinking no foam on that area (so the concrete slab will actually be 6 inches there, not four). But I'll still put a VP over the load baring footer. I'll special cut/tape a section of the VB so that the slab concrete flows into the 2" hold since there is no foam there. Does that make sense?

Thanks!

Note that, as drawn, your footer provides a thermal and moisture bridge into the wall. You might get cracks where the slab goes from 3" to 4".

Posted By jonr on 12 Jul 2011 08:06 AM
Note that your footer provides a thermal and moisture bridge into the wall. You might get cracks where the slab goes from 3" to 4".

The slab will go from ~4" to ~3.5", since I'll use 1/2 foam on the top of the footer (2" in the main basement area - except over the load bearing footer). I can't do the 2" foam all the way to the wall because I need to maintain head room in the basement for egress stairs. The stack up for the footer area (not the main basement) from the footer up is: footer, drylock paint, 1/2 EPS, 15 mil VB, rebar on chairs.
I agree, there will be a thermal difference since the foam thickness changes, but this is about 7' down with pretty steady ground temps I would think.
I'm not sure why you think there will be a moisture issue - the VB is there and taped to the basement wall - so maybe I'm missing something. (?)

So, did this all come about because the designer didn't provide for the underslab insulation in addition to an already tight overhead clearance?

Why are you putting the vapor barrier above your insulation?  I put my basement slab vapor barrier under the insulation.  My stackup is earth (clay), ~6" clean gravel, 10 mil vapor barrier, 2# density EPS foam, 4" concrete slab.  Similar to the following attachment:

From a performance point of view it doesn't much matter which side of the foam the poly lives in a well-drained slab. In fact, 2" XPS is by-itself sufficiently vapor retardent that with taped-seams one might dispense with the poly altogether.

With EPS you'd still need the poly, but again, whether it's above or below the foam doesn't much matter.

To answer BabyBlder: If there is a capillary break material installed between the footer & the poured wall and the exterior is waterproofed there's no water vapor coming off the wall to speak of any way. But if there is no capillary break between the footing and the wall, matter how high you go with the interior vapor barrier, the moisture will just wick higher. Bottom line,, either way, trim it flush at the finish plane of the concrete and fuggedaboudit.

I've never seen the vertical XPS tapered, and it would only serve to create a weak point for the slab concrete to crumble making a rough edge. Come straight up with it- if you like, come 8' up with it, insulating the foundation wall (but that commits you to finishing the interior sooner than later.)

Posted By arkie6 on 12 Jul 2011 02:03 PM
Why are you putting the vapor barrier above your insulation?  I put my basement slab vapor barrier under the insulation.  My stackup is earth (clay), ~6" clean gravel, 10 mil vapor barrier, 2# density EPS foam, 4" concrete slab.  Similar to the following attachment:

arkei6, I'm a newbie and non-pro so I'm trying to figure it as I go. I could be wrong in placing the VB directly under the concrete, but here is a GBA Q&A that is very similar to the Q I asked here. In the Answer section, the 3rd answer explains why (this placement of the VB).

http://www.greenbuildingadvisor.com...lation

Posted By BabyBldr on 12 Jul 2011 02:36 PM

Posted By arkie6 on 12 Jul 2011 02:03 PM
Why are you putting the vapor barrier above your insulation?  I put my basement slab vapor barrier under the insulation.  My stackup is earth (clay), ~6" clean gravel, 10 mil vapor barrier, 2# density EPS foam, 4" concrete slab.  Similar to the following attachment:

arkei6, I'm a newbie and non-pro so I'm trying to figure it as I go. I could be wrong in placing the VB directly under the concrete, but here is a GBA Q&A that is very similar to the Q I asked here. In the Answer section, the 3rd answer explains why (this placement of the VB).

http://www.greenbuildingadvisor.com...lation

As with most things in life, you can get opininons on both sides of the fence, but I believe that the concern of "so that you don't trap water in the cracks between the foam" in the 3rd answer you referenced above is without merit.

While I respect Martin Holladay's opinions in general, I'm with arkie6 on this one unless this is a building where you'll be hosing down the slab on a daily basis, with a HUGE liquid-moisture drive from above. For a residential app where the slab stays mostly dry except for the occasional spilled mop bucket, the odds of there being long-term moisture accumulation in the XPS is nil.

But there's nothing wrong with putting the poly between the slab and the foam.

Just would like to know is the water beneath the soil responsible for the liquid moisture you are talking off.... If its so it would be impossible to construct what you are planning to construct....