[Greenbuilding] fastening a subfloor over foam over a slab

JOHN SALMEN terrain at shaw.ca
Tue May 31 16:52:56 PDT 2011


I would say only if you are using 3" or more of insulation. My concern here
is having some integrity in the foam/ply slab itself that keeps the sand
layer in plane. I think 3" is a little thin but had a contractor use it
instead of 6" for a kindergarten addition to a school. It was a large open
room with no interior partitions so my fear was 40 kids jumping up and down
and shifting the sand base. I visited last year and it was 6 years since it
was built and the floor is good - so 3" works. Just a lot easier to build
and level with more (not to mention the insul value).

I think if you have an existing slab there are a lot easier ways to level
it. Thick foam can span a large area (even without the ply). I would
probably use mortar to spot level before putting the foam down (making sure
you have sufficient bearing area). 

As for fastening the foam to the earth - I don't - think of it akin to a
floating concrete slab on grade but made out of 98% air. Unlike concrete it
will not settle under its own weight and bend and crack (most slabs do).
Also it can accommodate and compensate for earth movement. Buildings are
layered so by the time the flooring goes down, partition walls etc it is
pretty well locked in place. I looked at extending the ply over the fnd
sills but its not practical as part of the course of building.

-----Original Message-----
From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of
Benjamin Pratt
Sent: May-31-11 9:44 AM
To: Green Building
Subject: Re: [Greenbuilding] fastening a subfloor over foam over a slab

John,
Can you use sand over concrete to level the floor? With a sand base,
how are you fastening down the foam?
I was told to put plywood over foam, and screw them both down with
long tapcon screws. But that project has been put on hold
indefinitely.
Ben

On Tue, May 31, 2011 at 8:29 AM, JOHN SALMEN <terrain at shaw.ca> wrote:
> I have 4 projects done with ply over eps. The first was 3” eps foam over
> concrete slab with 5/8 ply w/ wood and tile flooring over (tile area had a
> 2nd layer of ½” ply over). I visited it after 10 years and it was fine.
> Subsequent floors have been eps over sand base with ply laminated. I’ve
not
> used anything less than 3” as there is some reliance on the foam to span
any
> voids or deflections. Also it is difficult to lay and set 1” sheets.
>
>
>
> For the concrete slab application the floor was levelled (high spots
ground
> out and a levelling mix like quickcrete).
>
> Polyurethane foam is the most reliable bonding adhesive for eps foam board
> (low expanding type rated for subfloor – enerbond or equiv.) laid in a
> squiggle pattern at 3” o.c.
>
> Sheets need to be weighted till the adhesive forms its initial set – flat
> weights (16x16 ply square with about 30lbs – conc block or bucket with
sand
> works nicely).  I think for 1” weight spacing would be about 3’ o.c.. If
you
> cut the sheets to 4’ you can leapfrog the weighting.
>
>
>
> Seams were foamed and then shaved.  Ply is laid with the same adhesive
> pattern (seams staggered over foam). With 5/8 or ¾ t&g ply the end seams
are
> foamed and splined (biscuit at 16 o.c. or something equivalent). Ply has
to
> be weighted as well and needs to be flat (not deformed). 2 layers of ½ “
are
> nice (no end joining) and for wood floor over - the second layer was foam
> glued and screw fastened a 6” on edge and 10” field or for tile-  at 3”
o.c.
>
>
>
> Makes a nice floor – no tile/grout movement (without ditra)
>
>
>
> With 1” foam I would be tempted to use a stucco basecoat adhesive
(designed
> for eps foam) with a notched trowel (3/8x3/8) for laying the foam sheets
to
> the concrete. Depends on how the concrete was finished.
>
>
>
> Since the floor is tile I would simplify the installation and materials
used
> by using 1.5” foam with ½” layer of cemboard laminated over (with seams
> taped). This would be as stiff as the 2x ply and the cemboard installs
nice
> and flat (i’ve used this over foam with tile). Both of these assemblies
are
> quite stiff in comparison to a normal joist floor so not sure what the
need
> for the ditra is.
>
>
>
>
>
> From: greenbuilding-bounces at lists.bioenergylists.org
> [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan
> Abrams
> Sent: May-31-11 5:05 AM
> To: listserv Green Building new
> Subject: [Greenbuilding] fastening a subfloor over foam over a slab
>
>
>
> does anyone have a reliable spec for fastening a subfloor over foam, over
a
> slab?  my recollection is that JLC or FHB had an article on this, but
cannot
> google it up.
>
> space is very tight in this case; allowing for 1" of xps, two cross plies
of
> 1/2" cdx, and finish flooring (tile on Ditra), to stay within code ceiling
> height...
>
> AA
>
> Alan Abrams
> Abrams Design Build LLC
> A sustainable approach to beautiful space
>
> 6411 Orchard Avenue Suite 102
> Takoma Park, MD 20912
> office  301-270-NET- ZERO (301-270-6380)
> fax      301-270-1466
> cell     202-437-8583
> alan at abramsdesignbuild.com
> www.abramsdesignbuild.com
>
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-- 


b e n j a m i n p r a t t

professor art+design
the university of wisconsin stout

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