Insulate room over porch

Question

I have looked at this How can I insulate the floor of a room above a porch?, but I think my situation is a bit different. We are insulating the bedroom floor over an open porch (verandah). In Portland OR, so mixed climate. We have complete access to the rafters/joists. There is no wiring or plumbing in this area. The ceiling of the porch has 8" rafters. Then then is a 4" space, then 10" joists for the bedroom floor. A bit like this:

floor (currently removed)====================

joists |............. |.................... |......

space

rafters |...................| ...................|

wood ceiling ===================

We have put house wrap (Tvek) across the tops of the rafters and lapped it up the sides a bit and taped it. This is part of the air barrier. This has created a paper floor for the insulation. We have kraft-paper-faced fibreglass batts that we plan onto this floor and squeeze into the joist bays (which are very irregular - between 14" and 16" OC). I bought faced batts and don't have the strength to take 'em back and get unfaced. We will cut channels in the batts to get them around the joists.

So two questions: 1. Should the batts go face down, so the paper is against the house wrap? I think they should, otherwise we will create a moisture trap. 2. Are there any major problems with this perhaps unorthodox approach?

Thanks Catherine

Answer 1

An air barrier (housewrap) is quite specifically NOT a vapor barrier.

Vapor barrier should go on the warm side (out in primarily cooling, in for primarily heating) - look up the heating degree days and cooling degree days for your area as one reference, or ask yourself if you use the A/C or the heat on more days of the year.

A loose fill product (cellulose, for example) will deal much better with odd shapes and framing irregularities and voids. Densely packed cellulose can also be used without a vapor barrier as its vapor transmission rate is quite low as a bulk material, thus eliminating the issue of where to put a vapor barrier in a mixed climate.