# Adding foam board insulation & plywood to underside of roof



## Gary in WA (Mar 11, 2009)

I'll move you to "Insulation" for a better response.


Any plywoo0d or OSB product is a vapor retarder with 0.75 perm value: http://performancepanels.com/index.cfm?content=app_pp_atr_perm

If the supply inlet vents are lower than the plywood install, I would be concerned with ventilation. Without it the attic space can get full area venting or with correct conditions, cross venting, all depending on wind direction, vent sizes (NFVA), location, attic pressure, etc. The plywood may block some venting- collecting condensation to grow mold, may not, hard to see without doing a smoke test. It seems to me the air would be forced against the roof rake, not picking up as much of the inside attic air, acting as a giant baffle. If you have a leaky (air) attic, air seal it first as this is more important than fiberglass insulation against ice dams. Air travels right through f.g., though it is a good filter. If in a cold climate, warm moist attic air could condense on the cold backside of the plywood at outdoor temp. from the rafters thermal bridging and the cold outside air in the cavity. This would be a stretch, depending on the attic and outdoor temperatures, amount of moisture in the air, and plywood coverage, along with airflow, but a possibility.

Radiant barriers are effective if you live in FL or other hot climate without minimum Code value of insulation. Don’t substitute radiant for insulation. If in a heating climate, the values of radiant are minimal or even a hit against you. You want the solar gain in the winter to help with the heating loads below. Only a 2-10% reduction in cooling bill. Need an air space next to radiant barrier. 



http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920704.html

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/00/000915.html#fig5

http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem

Gary


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