Vinyl siding over 2″ foam, no strapping?
I’m retrofitting a 100 year old, balloon framed, two story farmhouse in Wisconsin (zone 6). All new windows, removing old siding to board sheathing. Will be blowing cellulose into wall cavities where possible (some have urea formeldahyde from a 1970’s attempt). Planning to apply ARB over boards, then 2″ polyiso, then vinyl siding. Window/door openings will be picture-framed with boards inset into the foam layer (3/4″ board over 1 1/4″ foam). Other penetrations will be treated like windows. I would like to use thicker foam but roof overhang is only 12″ and adding 2″ to 3″ is about max. Foam to extend downward over ring and foundation to 2′ below grade. Will also be cleaning out old insulation in vented attic, updating wiring then applying 2″ closed-cell spray-foam over plates and entire lathe/plaster ceiling to achieve an effective ARB prior to adding loose fill cellulose for a total of R60. Mechanical ventilation to be added to the hopefully tight house via two pair of Lunos (upstairs and down) plus spot exhaust in bathrooms and kitchen range hood.
Here’s the issue: I need a minimum of 2″ exterior foam for condensation protection and to come close to R value goals. Since I can’t go much thicker than 2″ and because vinyl siding won’t benefit from a rain-screen, I had hoped to have the siding applied directly over the foam (no strapping or OSB and lower cost) with 3 1/2″ nails. I have read from various sources that this is the maximum foam thickness for vinyl but workable ( BSC article: http://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh). Problem is that the vinyl installers I’ve talked to are not so sure and haven’t done installations over foam this thick. So my question is: Should I plan to apply vinyl siding over 2″ foam with the only solid nailing at windows/doors and penetrations???
My alternate strategy is to use 2 1/2″ of glass-faced, roofing polyiso covered by 7/16″ OSB, like a site built SIP. This will provide a smooth bearing surface and nailing, but much more expensive!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Edward,
The GBA detail library includes details for installing vinyl siding over 1.5 inch of rigid foam (see image below).
GBA has also quoted Joe Lstiburek as saying that he doesn't mind nailing window flanges through 1.5 inch of rigid foam, but he wouldn't use that attachment technique for windows with rigid foam that was thicker than 1.5 inch. (Here is a link to the article I'm referring to: Nailing Window Flanges Through Foam.)
So if you want to try this technique with 2 inches of rigid foam, you will be pushing the envelope a little. That's not to say that people haven't done it; see this link:
http://forums.finehomebuilding.com/breaktime/archives/vinyl-siding-over-rigid-foam-sheathing
You wrote, "The vinyl installers I've talked to are not so sure and haven't done installations over foam this thick."
So it sounds like your next task is to try to convince your installers.
One more point: you can always install furring strips over the rigid foam, but that raises a whole other set of issues. More info here: Can Vinyl Siding be Applied Over Furring Strips?
.
A full layer of 2" closed cell foam over the attic side of the plaster & lath ceiling is extreme overkill, less than green, and completely unnecessary in a zone 6 vented attic.
If the attic ventilation isn't up to snuff and you feel the a lower vapor retardency is necessary, that can be had much more cheaply ( and green-ly) with a coat of vapor barrier latex primer on the ceiling. If the attic ventilation IS up to snuff, the amount of moisture getting into the attic via vapor diffusion through the ceiling paint and over a foot of cellulose and being retained inside the attic is truly miniscule.
A couple of inches of OPEN cell foam would also be overkill for air sealing the lath & plaster, but it would be only 1/4 the amount of polymer of 2lb foam, and blown with water, not HFC245fa (with a 100 year global warming potential of ~1000x CO2). It would be roughly a third the cost of closed cell foam. If you feel a full shot of foam is the only reliable way to air seal it, making it open cell would be both cheaper and kinder to the planet.
Thanks so much for the quick and thoughtful responses. GBA is truly a great resource!
Martin, thanks for the links. That's one I hadn't discovered. Its nice to see that at least one person tried this before. To be clear, my plan does provide for solid nailing for the windows/doors (picture-framed boards flush with field of 2" foam, layered over 1 1/4"foam and screwed into frame) just the field of the siding is over the 2" foam. The window flanges, trim, and J channel will be directly on a wood surface. I think you are right that my final answer will be the siding installer. Hopefully a couple of more folks might chime in to provide a bit more anecdotal data to help us decide to "push the envelope" or not.
Dana, good point on the spray foam. I think I will look into the open-cell option. The reason that I was considering spraying the entire surface rather than just the plates and penetration is that the original plaster ceiling is heavily cracked and now covered with strapping and a layer of drywall. I am still concerned that the 3/4" void between the newer drywall and original plaster is an avenue for air leakage to find its way into the attic via the cracked plaster. A continuous layer of foam on top, in the attic, could be an effective seal to cap off the thermal envelope. As you point out though, open-cell can accomplish that with less cost to me and the ozone layer. Does this now make more sense?
Again, many thanks to both of you;
Ed
Edward,
Did you move forward with your original plan?
Im going to be installing windows and vinyl siding over 2” foam. Using a similar site built insulated window buck.
None of these details scare me, but im also the installer. I have never filed or heard of a manufacturer honoring a warrenty years after install so that doesnt concern me.
Edward,
Did you install vinyl siding over 2” foam? I would like to install insulated vinyl siding over 2” foam and can’t find info/experience on this assembly.
Thanks
Andy
Deleted