Installing rigid foam in rafter bays – Foamular 150?
ccampbell
| Posted in Energy, Heating & Insulation on
I’m thinking about installing layers of rigid foam (secured with spray foam) in the rafter bays to insulate a roof over a finished attic. The roof is unvented. My home depot sells 2″ Foamular 150 that looks like the right thing. Their product sheet says it’s closed cell. Is the main difference between that and their other products just the strength? Is the 150 a good choice for this installation? Also, do you normally glue the sheets in first with construction adhesive, then spray them in when cured?
Replies
Using construction adhesive is overkill. The sprayed foam, when cured, will hold it just fine. All you need is something to hold it in place until the foam cures.
If you glue it up besure to to a compatable glue as most construction adhesives will melt foam.
A screw at each corner to hold it till the foam sets up is fine.
ripping the sheets so they are a snug fit isn't as hard as it would seem if there aren't a lot of wires and whatnot. Spray foam the gaps and cracks and a little between each layer so normal expansion and contraction won't let the foam simply fall out if the framing moves a little.
When ripping the foam on a tablesaw be careful of hand placement since the foam can and will kickback although it seems like a hot knife through butter.....
You're going to drywall that
You're going to drywall that ceiling after insulating, correct? XPS needs thermal/flame protection.
Yes, it will be drywalled.
I think I'd tend to consider
I think I'd tend to consider e.g. Thermax as an alternative. Higher R per inch. Somewhat higher cost, but cost per R may be better and you'd get more R in the depth of the rafter (if you needed or wanted it).
You may need a fire rated install ... double check w/ the BO about installing it (or another person who knows such details).