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Last Updated: , Created: Thursday, December 13th, 2001

Is my crawl space insulation right?

Joe and Joan from St. Thomas Ontario wrote: "Our kitchen has a 2 foot crawl space underneath. We hired a contractor to insulate the floor with pink insulation. He used plastic to hold it up. The kitchen was still not warm enough so we nailed up Styrofoam over the plastic. The following year we added another layer of Styrofoam giving us R30. Am I in trouble with the plastic between the insulation and do I need plastic on the floor of the crawl space?

It is always a good idea to have a plastic on a dirt floor of a crawl space to keep the humidity from evaporating out of the soil.

For the insulation, in theory you should have a vapour barrier on the warm in winter side of the insulation, but in practice, a plywood floor will stop enough moisture to avoid problems from moisture migrating from the house to the crawlspace.

The problem is that plastic sheet under the fibreglass. If that sheet is a "house wrap" like Tyvek or Typar, which are not vapour barriers, no problem. If it is a polyethylene transparent sheet, then you have a vapour barrier on the wrong side of the insulation and this could trap moisture in the joist space. However, the addition of the Styrofoam may have solved, or almost solved the problem.

You can break the "warm in winter" rule for the vapour barrier as long as you have twice as much insulation on the cold side of the plastic sheet as on the warm side. In theory with R-20 of fibreglass in the floor you would have to have R-40 of insulation below the plastic to avoid problems. In a floor however, there is less tendency for moisture to move downward (through air flow), so if you got the Styrofoam up to 4 inches (R-20) you will probably stay out of trouble with this floor. On walls and ceilings I would insist on the 1/3rd - 2/3rds rule.

Keywords: Crawl Space, Vapour Barrier, Insulation, Techniques

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