[Greenbuilding] building floor on top of rigid foam... (basement/garage retrofit)

[JOHN SALMEN]

The foam at 5” thickness acts as a v.b. as does the ply (a factor of
thickness with eps). The granular base (gravel or sand) is the capillary
break from groundwater but also is what forms the level base so it either
needs to be fine enough to compact and screed (level) and/or the foam needs
to be of sufficient thickness to span any discrepancies in the base
(laziness in levelling). I’ve found that coarse sand (cement sand) compacts
well and screeds well to form a decent level. 4” foam will span
irregularities on roughly 16” centers and even more once bonded to ply. 6”
of foam is great in what it will span. (levelling the sand can be improved
with a water/latex spray mix).

 

The basic technique is that sand is compacted to within an inch or so of
level and then you place screeds in the sand at final level and fill in and
screed so the sand is level. Lay the foam down and spray adhesive in an
overlapped 4”  squiggle pattern on the foam, lay down the t&g ply
overlapping foam seams and weight it down, stagger the ply seams, etc. 

 

Depending on the area the sand can be screeded in sections followed by foam
and ply – glue can set in a relatively short time depending on temp/humidity
so the weight can be shifted to the next area (polyurethane foam adhesion is
helped in moistening the ply backs)

 

1-1/8” ply is major overkill as the foam is providing the span and the ply
is only acting as a skin and a nailbase. The extra thickness could  work
against you. The concern with ply is that it is flat. If it is overly thick
and is warped it will take a lot of weight to flatten to the foam and any
warpage can delaminate and form pockets.  I’ve found 5/8 t&g to be the best
working minimum that works but my personal preference is 2 layers of ½” ply
laminated for ease of installation and stiffness as it tends to flatten and
compensate for any poor bond areas and you can eliminate having to set the
t&g by using the overlap. It is hard to describe in words but this assembly
requires good workpersonship in the layup so it is best to work with an
assembly that can compensate for any problems. I’ve found the best assembly
is thick foam that can span (and insulate) and thin layers above layered to
form a stiff skin that spans any pockets and provides sufficient nailbase
for wood flooring and sufficient stiffness for tile. Think of it like a
fibreglass layup.

 

The end product even with a single 5/8” layer tends to be flatter and
stiffer than a floor on joists and if a solid wood floor is being nailed to
that the final assembly is quite stiff and is basically a floating
structural floor. Anticipated deflection can be as much as a ¼ “ depending
on loading but I’ve gone back to look at floors that had a combination of
wood and tile and have not seen any movement that has popped a grout line.

 

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Reuben
Deumling
Sent: April 20, 2011 10:38 PM
To: Green Building
Subject: Re: [Greenbuilding] building floor on top of rigid foam...
(basement/garage retrofit)

 

Any particular reason not to vary the relative thicknesses of the two
materials? I've planned to do as you say, but have secured at a good price
enough 2.5" foam and 1-1/8" T&G plywood. I was going to put these atop
compacted 1/4" minus gravel, with a vapor barrier (or not? I see you didn't
mention one). 

Thanks for any thoughts.

Reuben Deumling

On Wed, Apr 20, 2011 at 9:18 PM, JOHN SALMEN <terrain at shaw.ca> wrote:

The basic spec. is rigid eps board (minimum 4”)  on a level surface w/ t&g
ply sheathing laminated directly to the foam with a suitable adhesive. That
is it.  The level surface does not have to be concrete – it can be a
granular base (sand). 

 

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