many foam boards specify an air gap to achieve their alleged R factor…is this gap to the inside or outside? what’s the R without it?
the blue foam is often mentioned here as having about an R-5 per inch – is that with or without gappage?
and that foil backed bubble wrap – like Reflectix, but I get it from a metal building company much cheaper of course – claims to have an amazing R-10 in a roof application, so I think they’re taking some anti- radiant value into account, since they claim less for side and floor use…with gapposis of course…
anybody know more than the advertizing…?
Replies
toledo,
The air gap is for reflectorized foam boards. The shiny side has to face air to boost the R-value. I recall 1/2" as the standard distance. Others here will know more.
Bill
>> The shiny side has to face air to boost the R-value.And, if I'm not mistaken, it has to stay shiny to stay effective. If it gets a thick coat of dust on it, it no longer reflects very well.
thanks, Bill - I hope I'm not being obtuse here, but, again, does this air gap want to be between the foam and the siding, or between the foam and the sheetrock, or do it matter?
Edited 2/10/2005 7:53 am ET by toledo
Edited 2/10/2005 7:55 am ET by toledo
actually.. the shiny part has nothing to do with the gap R-value..
the shiny is some kind of value they give their product for Reflectance
the R-value is for a gap of 3/4" to 1" which is assumed to be DEAD air.. if so, dead air of 1" has an R-value of 1
if teh gap gets larger, then convective currents will start to form and the R-value will reduce
a lot of the shiny reflective values get degraded over time because they get dull or dust covered
most of the shiny bubble wrap claims are unsupported by rigorous testing...
BTW... the outside of a heated surface also has an r-value of about 1, and the inside surface too.. both assume a turbulent layer
the Total R-value for any section will include all of these components.. including wood studs, insulation values, dead air spaces, inner & outer layers, etcMike Smith Rhode Island : Design / Build / Repair / Restore
thanks, Mike - you answered my now deleted edit about the dead air - that was my assumption about the air gap....
too bad about the bubble wrap...it's very handy for some awkward situations...like bad crawl spaces...