Can kneewall polyiso boards be left exposed on the attic side?
ps24eva
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Can kneewall polyiso boards be left exposed on the attic side?
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In many/most areas if it has fire rated facer it's fine to leave it uncovered. Currently that narrows the product offerings down to Dow Thermax line of polyiso.
John,
It usually depends on whether the attic is accessible -- for example, via a door.
If the attic is inaccessible, most building departments will rule that a single layer of drywall (on the interior side of the kneewall) provides adequate fire protection. But code interpretations vary from jurisdiction to jurisdiction -- so it's always a good idea to call up your local building department to discover what they think.
Instead of polyiso, what specific housewrap can I use?
John,
Housewrap and polyiso are both air barriers, so in that sense they are comparable. However, unlike housewrap, polyiso has R-value (usually between R-6 and R-6.5 per inch). So if you switch to housewrap, you won't be getting the benefit of a continuous insulation layer that interrupts thermal bridging through the studs.
As with polyiso, it's important to contact your local building department to ask whether they will allow housewrap to be left exposed in this location.
What brand polyiso do you use?
John,
The brand of polyiso isn't too important (unless your building department tells you that Thermax is required for code reasons). Buy whatever brand is sold locally.
Many green builders prefer to buy used (recycled or reclaimed) polyiso. Look on Craigslist.
I tend to use whatever brand my local foam reclaimer and factory-seconds foam outlet vendors have on hand, which varies. YMMV.
Where it's cost sensitive and higher-R, a layer of ultra-cheap reclaimed foam with a layer of 1/2" Thermax facing the attic air works.
This is actually for a bonus room above a garage.
There will be no opening after completion.
I plan on affixing the polyiso to the outside stud edge.
There are already fiberglass batts.
What kind of nail to attach 2 inch thick polyiso board to studs?
Cap nails
I don't see cap nails longer than 2 inches
John,
You can find plastic washers in varies sizes from 1" diameter to 3" and possibly more. I was able to order a box of 1000 of the 2.5" diameter for pretty cheap. Do a quick search for plastic foamboard washers and you should get a ton of options to look through.
http://www.lowes.com/pd/Grip-Rite-1-lb-3-in-Round-Plastic-Cap-Nails/3295296
They are also available at all our local lumberyards here.
I bought those nails from Lowes.
Do you need to use adhesive too, or are they good enough?
John,
They have very good holding power. I wouldn't worry about adhesive.
Do you tape or foam spray in the gaps?
John,
To seal the seams between panels of foil-faced polyiso, you can use either a high-quality tape (housewrap tape or foil tape) or canned spray foam. Tape is probably the easier approach.
When I need to use foam, for example to block the floor joists, is this fireblock foam fine?
http://www.lowes.com/pd/Dow-GREAT-STUFF-Fireblock-Spray-Foam-Insulation/3052753
John,
That type of canned spray foam is fine. But if there is no code reason to use fireblock foam, you can use ordinary canned foam instead.
Which one of these 2 is the right stuff?:
http://www.lowes.com/pd/Dow-GREAT-STUFF-Gaps-and-Cracks-12-fl-oz-Spray-Foam-Insulation/3012216
http://www.lowes.com/pd/Dow-GREAT-STUFF-Big-Gap-Filler-12-fl-oz-Spray-Foam-Insulation/50214633
John,
You're overthinking this. Either type will work.