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Hi Frazer,
I have some comments on this.
The argument against poly is perhaps one of
two. Some concrete guys don't like to put poly under the slab because it
slows down drying (excess water can't drain out below) and it can dry unevenly
(top dries faster than bottom) which may result in cracking.
It ain't necessarily so. First, they should
be using low slump concrete, with only as much water as necessary, so there
won't be excess moisture that needs to drain out. This requires a bit more
muscle, because the concrete isn't like soup. However, it also creates a
stronger slab. Regarding the uneven drying, it will be less of an issue
with lower water content, but on hot, dry days, the slab can be covered after
the pour to slow evaporative drying from the top. I think this is what the
concrete institute recommends. This slower drying also helps strengthen
the slab. So I don't buy the argument that poly is bad. In fact, in
my own addition, I insisted on the poly. And if it make's anyone feel
comfortable, they can punch holes in the poly. Holes? Vapor
retarders work as a function of their permeability and their area. Poly
has very low permeability, and a few dozen pipe sized holes won't effect how the
poly functions as a vapor retarder very much.
Regarding the insulation, this is definitely a must
if there will be radiant floor heating. Although the delta-T isn't huge,
it's raised because you raise the floor temperature. And it can get
expensive to heat the Earth with your boiler. A minimum of 2" blueboard
should be used. The seams don't need to be taped. The stuff is
tongue and groove, and stays together very well with a good friction fit and the
weight of the wire mesh and tubing.
Now the question about the poly becomes more
interesting when we add the insulation. The physics says moisture wants to
flow from hot to cold, so moisture in the slab will what to flow from the warm
slab through the semipermeable insulation to the cold ground. Poly would
prevent this flow. And Lstiburek recommends either poly or insulation, but
not both. (He also recommends controlling the outside water so it doesn't
get to the slab in the first place!) However, given that it's a radiant
slab, if you control the outside water, I don't think you'll have to worry about
the moisture. Again, anecdotes don't prove anything, but my radiant slab,
with the insulation and the poly underneath, is very dry.
I'm skeptical about the TekFoil. Radiant
barriers work best for radiant flow. We're primarily concerned about the
conduction losses from the slab to the ground. Plus, I don't understand
how the installer could specify this product when he just got just saying he
doesn't want to use poly. I suspect the bubble wrap and foil sandwich
probably has a perm rating as low of the poly. Why would it be ok if the
poly isn't (of course, the poly is ok. I'm just wondering about what
appears to be a logical inconsistency.
Hope this helps,
Mike
----- Original Message -----
Sent: Friday, October 26, 2001 12:09
PM
Subject: [GBlist] Re: Big Green: Radiant
Floor Heating
This from a client of ours:
"The radiant installer is very interested
and intrigued to know why we're not putting down a vapor barrier. I told him
that I thought it had to do with the proper drying of the concrete. He felt
that by putting the TekFoil or some radiant layer under the cement or 2" blue
board would be cost effective.
Should the blue board seams be
taped?"
Questions for the group are:
Why no poly under the insulated slab
??
Is TekFoil (foil faced bubble insulation)
cost-effective ??
Thanks
-------------------------------------
Frazer
Dougherty
North Fork Retrofit & Solar Options
Mail: Box 4016, East
Hampton, NY 11937
Shipping: 75 Industrial Rd, Wainscott, NY
11975
tel. 631.537.1654
fax. 631.537.1637
cel.
631-645-3534
www.home-performance.org
www.solaroptions.com
Insight '00
#1048, citrus yellow, 61.1 lmpg
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