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BigJimmy 04-07-2013 10:31 PM

Vapor barrier or not?
OK, here is a question for y'all:

Our house was built in 1907. Apparently, there were two exterior, open porches, one above the other, that were at some point in time, enclosed to create additional interior space.

I am remodeling the first floor of this home and this "porch" is being converted into a mudroom. The original floor was sloped for rain run-off and because this space is now enclosed, I wanted to make the floor level and plumb. Based on what I found after the tear-off of the finish floor, I decided to abate and replace all of the existing subfloor and framing.

As this was originally a porch, there is no foundation below it. Strangely, in the past, someone installed a vent pipe all the way up to and through the roof.

Once I installed the new floor joists and framing, I wanted to install some rigid foam insulation boards so that the flexible mini-ducts (Unico system) could be above the cold-space. Therein, I built a lattice of 2x4's below the ledgers to carry/support these insulation boards. At this point, the 3-1/2" foam boards and floor joists are in place and I have sealed around the edges and seams with closed-cell insulation.

I want to add additional insulation between the rigid foam boards and the tops of the new floor joists. The insulation that I have has an integral vapor barrier with nailing wings and my initial instinct would be to lay it down with the vapor barrier facing up, toward the conditioned space. Given that this insulation is fiberglass however, it can be assumed that some heat would be lost and might condense against the rigid insulation below (which represents the barrier between non-conditioned and conditioned spaces).

In the end, I am trying to decide if this insulation should be installed with the vapor barrier up, above and stapled to the tops of the floor joists (in the conditioned space) or if it should be removed.

Any insight is appreciated!

AllanJ 04-07-2013 11:01 PM

The foam boards with sealing around the edges are a vapor barrier and it is not a good idea to have a second vapor barrier in the form of fiberglass insulation paper backing.

Cut rectangular holes in the fiberglass backing paper amounting to about 25% of the surface area. This eliminates the vapor barrier attribute. You can still use the edges of the backing to staple the batts in place with, if the holes are not too large each and not too close to the edges.

Windows on Wash 04-07-2013 11:08 PM

What type of foam boards did you use on the underside of the framing?

If the foam has a low permeance rating, the insulation should be unfaced ideally.

BigJimmy 04-07-2013 11:30 PM


Originally Posted by Windows on Wash (Post 1154533)
What type of foam boards did you use on the underside of the framing?

If the foam has a low permeance rating, the insulation should be unfaced ideally.

Owens Corning Foamular 250

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