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basement exterior insulation

mwalter | Posted in Energy, Heating & Insulation on March 23, 2009 07:32am

It looks like I am falling short (budget and local expertise in Columbus Ohio) in my attempt to get an ICF system for a basement addition. Right now I am planning to go to the other side of the spectrum with concrete block. I have mostly 12″ block for architectural brick support. I will have exterior and interior drains connected to a sump pump. I am looking at radiant floor heating in the slab. There is a fair bit of water in the ground around our house.

I obviously have the opportunity to do whatever I want on the exterior and under the slab. I am looking for suggestions for the foundation wall system:
a) what damp proofing system
b) how much rigid foam should I put on the exterior (what make sense)
c) best way to apply the rigid foam?
d) backfilling?
e) any other products (e.g., below grade drainage materials?)

What about under the slab?
a) which vapor barrier and where should it go
c) how much rigid foam
d) best way to avoid thermal bridges at the sides?

In talking to contractors, most say a block foundation will be ok. It is about $4k less than poured wals. Am I making a mistake to save $4k? I feel like since I cannot get an ICF system, I might as well go cheaper and try to put on a good exerior insulation system. I’d put an interior insulation system (foam-based) at a later date. Is all this just as good as an ICF system?

Sorry for all the questions. Any help would be greatly appreciated.

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  1. DaveRicheson | Mar 24, 2009 02:28pm | #1

    http://www.buildingscience.com/index_html

    Look around the above site for your specific area .

    I personaly like poured foundation walls over block walls. Our cuurent code in KY calls for a minimum of 1" rigid foam on the outside of foundation walls.

    There are some people here with radiant in slab experience and will be along to adise on that. I planned my last slab for radiant, but didn't go that way because of budget constraints. I insulated under the slab wit 2" rigid foam, taped the joints, covered with 6 mil poly and the my 4" slab. The slab was alos insulated at the edge with 2" foam agianst the foundation wall. Even without the radiant heat installed the slab was noticeably more comfortable than my uninsulated slab on grade in my shop next door.



    Edited 3/24/2009 7:40 am ET by DaveRicheson

  2. Clewless1 | Mar 24, 2009 03:21pm | #2

    2 inch foam on the outside of the wall is pretty 'standard' (even though there is no such thing). I used 3 inches on mine. You need to run it up to the top of the wall ... which leaves it exposed. You have to protect the exposed part from weather and damage ... flash w/ metal or do a stucco type thing on it. I waterproofed the wall w/ that black tar stuff ... don't recall what it's called ... someone else can help you w/ that. If you have water problems and drains installed ... consider Enka drain (sp?).

    Under the heated slab. I did 2 inch foam under the slab. Can't remember where the VB went ... not sure it matters ... although concrete guy might say under the foam. Around the edge ... you could bring the 2 inches up to the top of the slab ... then you have a finish detail to deal with. Options include using 1/2 inch Celotex fiberboard that soft, black fiberous sheet material used for concrete joints). This thermally separates the slab w/out a big finish issue (but it is only 1/2 inch ... you could use a couple of layers. Another option is to use the 2 inch w/ a 45 taper on it while leaving 1/2 inch of foam at the tip instead of a sharp point. Some think this leaves a weak slap edge subject to cracking, etc.

    Definately as good as ICF ... IMO. ICF has the beauty of insulation, forms, and concrete pour all in one. If you are doing it yourself, I'd think it would be inexpensive (relatively speaking.

    Food for thought

    1. DaveRicheson | Mar 24, 2009 03:55pm | #3

      I did the 2" rigid foam on my slab perimeter. I then used a small surform plane to knock it down to about 1/4" below the finished level of the slab. Then filled the 1/4" with a self leveling caulk. With drywall and base installed I ended up with 1" of the caulk joint exposed, but that was no biggie in the studio/kennel building. I think a 1" rigid foam perimeter insulation would work just fine in a more finished area.

      1. Clewless1 | Mar 25, 2009 03:24pm | #5

        Yeah, there are a number of ways to do it. Pay your money take your choice. It tends to be a somewhat awkward detail ... unless you simply get used to the 'new' mindset and pick a method that seems to work for the application. Like everything, the success lies in the details and how well you execute those details. I've heard people use e.g. a 2x2 for the top edge ... place for carpet nail strips to attach.

        On your application, if you only had 1/4" of insulation exposed, how is it you ended up with a full one inch caulk joint AFTER the drywall and the baseboard??

        Drywall and baseboard will cover up to say 1 1/4" of construction ... so really you should be able to leave half of a 2" insulation board at the top of the slab.

        1. DaveRicheson | Mar 25, 2009 06:09pm | #6

          Let me try to attach a paint sketch of what I ended up with

          As you can see I ended up with 3/4 to 1" of the exposed caulk line. Not bad for my application with no finished floor. It wouldn't work for carpet, but a 1" rigid foam probably would.

          Edited 3/25/2009 11:09 am ET by DaveRicheson

          Edited 3/25/2009 11:13 am ET by DaveRicheson

          1. Clewless1 | Mar 26, 2009 05:27am | #7

            I see ... I thought you had said you trimmed/tapered the foam back. I see ... this was a garage/shop application? It works as long as you don't have a finish to apply. Although maybe tile could 'float' over it? Could cover it w/ a double layer of 1x baseboard and had nothing left. On a finished application, I would bevel the foam 45 deg and left say 1" expose to be covered by baseboard and wallboard. That provides a good thermal break.

          2. DaveRicheson | Mar 26, 2009 02:36pm | #8

            It is just a kennel building and studio for DW's hobbies. Only about 900sf and we were more concerned about energy efficiency than aesthetics.

            Besides, it going to be so wild looking inside (purple, lime green, hot pink windows) that knowone will every look down at that 1" caulk joint <G>

  3. Scott | Mar 25, 2009 03:39am | #4

    Same thing happened to me... budget wasn't so much a problem (ICF doesn't cost much more), but there were no contractors with ICF experience and I didn't want to be version 1.0.

    So I went with a poured foundation with 2" SM for 24" depth around the perimeter.

    I went with conventional sprayed bituminous damp proofing. If I were to do it again I'd consider covering the whole wall with something like peel & stick ice & water shield.

    Good luck,

    Scott.

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