I am just about finished digging out my barn floor.
I intend to lay gravel, sand, damp proof sheet, insulation (polystyrene),
about 4 inches of ready mixed concrete with steel work.
My question is at what level/order do I place the polystyrene insulation and
what are the benefits of its location.
Thanks for any info.
The polystyrene should be placed on top of the vapor barrier and the
concrete cast upon the insulation. The reason is that the purpose of the
vapor barrier is to prevent moisture from being forced into the insulation
under the pressure that is created by the temp. diff. Bring the vapor
barrier up the sides if possible and lap the joints by about 12-18". Your
reference to the damp proof sheet, I asssumed to be the vapor barrier. You
should try to use 10 mil Visqueen but at least 6 mil if you can't find the
Sand should be put down under a vapor barior and on top of it
under to ensure the plastic is not torn on the jagged base and on top
so the concrete doesnt go off like a rocket while finishing it.
By "go off like a rocket", I assume you mean set up really quickly. I
thought that pouring conrete directly on a plastic vapor barrier
typically delays the setting up of the concrete, as there is nothing
to absorb the excess water used for placement. The sand layer on top
of the vapor barrier would provide a more normal setting up time, as
it can absorb some of the excess water. Have I got this backward?
No. You got it right. However, the tip about putting 2 layers of sand
with the vapor barrier between is a pretty good one.
BTW, ACI has issued new guidelines on using a plastic vapor barrier.
Basically, they are saying "DON'T USE ONE", unless you have a high ground
water problem and you have a flooring material that will have problems
with moisture (carpet, vinyl, wood).
In other words, try to take care of any moisture BEFORE it can migrate
through the slab. This will be the case in most installations, so in most
cases a vapor barrier is not needed.
Bob Morrison, PE, SE
R L Morrison Engineering Co
Sorry was out of town for a while..
No you are correct the sand layer on top does allow a more normal set
up time. The plastic accelerates the process greatly and on a hot day
concrete poured directly on plastic can damage the whole slab because
of the heat generation. To answer the question below (even though I was
not asked) yes you should spray down the sand or dirt or baserock or
whatever you are pouring over not wet wet but lightly moist.
Wayne Whitney wrote:
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