sand bed vapor barrier?

[Drake]

Most of the examples I have found of sand bed construction show vapor barrier placed under/surrounding sand bed. If wet sand has better thermal properties wouldn't vapor barrier be better located between sand and concrete slab? Am planning a shallow 12" sand/slab/hydronic basement floor as a thermal shunt/short term storage as it is relative low cost as part of new construction.

[mrd]

This article from buildingscience.com explicitly states:

Never install a sand layer between a polyethylene groundcover and a slab. The sand layer becomes wet and holds water indefinitely, it can only dry upwards, not downwards, due to the polyethylene.

[Ryland]

every sand bed I worked on had a vapor barrier and 2" of foam under the sand (12" of sand) then a slab on top, the sand does not hold moisture very well and the concrete is more likely to hold moisture then the sand, so it might take a month or two to dry out the sand but it will dry out and at that point it will stay dry and insulated.

[Drake]

Always a conundrum - two polar recommendations from knowledgeable sources. Wouldn't want to trap moisture under non vapor barrier slab and won't wet sand bed be more efficient rather than dry? Double vapor barrier?

[philb]

This is interesting. There is also a discussion on Build-it- solar.com Sand bed storage for solar homes

A master plumber in my area told me I would be making a major mistake if I put the insulation under the sand bed and the PEX in the sand.

Now I'm totally confused.

[Drake]

Getting accurate advise is often not an easy thing. Have learned that just being accepted practice or even CODE by building Professionals is not always right.

[Ryland]

Originally Posted by philb

A master plumber in my area told me I would be making a major mistake if I put the insulation under the sand bed and the PEX in the sand.

Now I'm totally confused.

Why is that? The sand protects the PEX and adds to the thermal mass, having had talked to people who have done solar hot water and infloor heat for as much as 25 years they all seem to be in agreement on PEX in the sand bed, the toss up seems to be do you insulate under the sand bed or not, if you insulate it then it stays warmer year round, if you don't insulate it then you get some cooling in the summer, you for sure need to insulate around the ouside of the foundation and I think that a well designed house with good over hangs doesn't need much cooling either way so the heat loss in the winter should be the focal point.