[Greenbuilding] fastening a subfloor over foam over a slab

[JOHN SALMEN]

I don't know... this is one of those applications that really is designed
for an adhesive both from strength and application. We tend to like the
notion of mechanical fastening but does it offer any benefit in this type of
application and are the materials suited to mechanical fastening?

 

If you are looking at running a termite shield or vb then mechanical
fastening will puncture that?.  Is the slab suitable for tapcon - or will it
spall or crumble or grab causing screws to spinout or shear. Will the 1"
foam and 1" ply combination float over low spots and/ or will the screw hold
the floor in suspension over the voids (over time...).  will there still be
flex between fastening points and will the fastening points be too stiff in
comparison to overall flex (too stiff under tile compared to non screwed
areas).  Even 1" foam is subject to some deformation over time and will this
cause screw heads to pop or loosen.  Those are some of the thoughts and
concerns I would have with mechanically fastening.

 

A gluedown would arguably be stronger in this type of assembly and has a
little more redundancy built into it than a fastener @24oc. Most effective
physical termite barriers are adhered now or peel and stick. So an adhered
membrane with foam glued over with cemboard glued down strikes me as an
ideal use of appropriate material that takes advantage of the benefits of
each material and has fewer compromises. The foam/cemboard is essentially
providing the same function as the tile membrane so that becomes redundant -
so the overall assembly is probably at less cost.

 

I think if a contractor can they be safely relied upon to properly set
tapcons (not easy to do well) gluing down some foam should be a little
simpler. 

 

 

From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan
Abrams
Sent: June-01-11 11:39 AM
To: Green Building
Cc: satjiwan at alumni.brandeis.edu
Subject: Re: [Greenbuilding] fastening a subfloor over foam over a slab

 

so an important question is mechanical versus adhesive fastening.  Screw vs
Goo, if you will.  Without being responsible for the installation
personally, I am more comfortable spec'ing mechanical fastening.  I'm
thinking to fasten the lams of plywood at 8" centers EW, and secure it to
the slab with flathead tapcons at 24" OCEW, with 3/4" embedment. 

A remaining question is resistance of the foam to insect infestation.  This
suggests a rugged vapor barrier over the slab--as for insect supression as
for vapor migration.  (in this case the basement is well drained, with
little risk of significant bulk water anywhere.)

AA

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