[GSBN] Perlite u/floor insulation

[Derek Roff]

Perlite is made up of small bits of expanded volcanic glass with a very 
rough surface texture and some quantity of gaseous bubbles trapped in 
the glass (see first link below).  While neither the glassy material 
nor the trapped bubbles will absorb water, the rough surface and the 
space between the perlite grains can hold water.  For this reason, 
perlite is sometimes called "absorbent".  Although I think the usage is 
imprecise, water can saturate a layer of loose perlite, and eliminate 
most of its insulation ability.  The second link below says that 
perlite has "the ability to hold 200 to 600 percent of its weight in 
water."  This is an obvious concern in any construction situation where 
water intrusion or significant moisture/water vapor is involved.

<http://www.schundler.com/filler.htm>
<http://www.ehow.com/facts_7844068_perlite-calcium-silicate-water-absorption.html>

Laura Bartels used bagged perlite as under-floor insulation in a recent 
project, so perhaps she will add her perspective.  Andy, from your 
description of the "water logged site", it sounds like perlite would 
not be a reliable choice for underfloor insulation, unless moisture can 
be excluded.

Best wishes,
Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu

--On Monday, April 4, 2011 11:57 AM +0200 Andy Horn 
<andy at ecodesignarchitects.co.za> wrote:

Hi

I am trying to get more info on the use of Perlite for underfloor 
insulation. (been to www.perlite.org)

I am involved as a consultant to a project in Johannesburg, where we 
are using under floor hot water heating for winter (pipes are being 
embedded inside a concrete slab) and so need to insulate under the 
slab. I have suggested we look at using a layer of 100mm of Perlite.

However, the Project manager has just come back to say we must use the 
closed cell polystyrene as he is worried about the Perlite absorbing 
water and is concerned that as a result it will loose its insulating 
properties. We have quite a water logged site with lots of clay and 
silt in the soil and the buildings in question are being dug into the 
back of the slope.

Various info says it is "water repellent" but I notice the details -- 
the 2 that I could find -- included some kind of a capillary break 
under and over the Perlite or a layer of plastic damp proofing 
underneath (as would be under a concrete slab anyway) and a layer of 
"oil paper" on top to separate it from the concrete. I also see it is 
used in insulating lightweight screeds and concrete work.....where it 
clearly retains its insulating properties despite being encapsulated in 
concrete with its relatively high moisture content......or does one 
treat the Perlite in some way to prevent it absorbing moisture???

Can anyone input here ....as while we can get the Perlite here, the 
suppliers have no experience with using it in the building 
industry......and I am loath to let the project use the closed cell 
polystyrene as it is a particularly environmentally damaging material 
which, uses benzene and ozone damaging HCFC's in its manufacture.


Any input would be really valued,

Thanks

Andy Horn