Where to “true” the exterior rigid foam?
YardHammer
| Posted in General Questions on
Putting 2″ polyiso over existing vinyl siding on a 1920’s wood frame with the usual wiggles and wobbles on exterior. I’ve been reading GBA how-to’s on this until I have to go sit in the swing for a bit, and still can’t cipher out what I need to do.
Should the foam be snugged against existing siding/sheathing and then the new siding be shimmed along the wall leaving a gap between the foam and new siding?
Or,
should I draw down the foam to make it true for the new siding, but leave a gap between foam and existing siding/sheathing?
Thank-you,
Bill G
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Replies
What's underneath the vinyl siding? Do you have a particular reason for not removing it?
The vinyl siding and all its edge moldings has to come off--this takes mere minutes. Get that foamboard snug against whatever sheathing is behind it. If you don't, you'll have all sorts of unknown air movement throughout the vinyl channels, which would reduce the effectiveness of the foam. Not to mention, the toughest part of exterior foam retrofitting is trimming out all those newly-inset windows and doors, and I can't imagine how complicated it would be to work around old vinyl moldings. Tape the new foam seams for airtightness, then shim your new siding with or without firring strips.
Steve,
Steve,
Existing sheathing is patchwork of 3/8 plywood and OSB.
The reason for not removing it was I saw a foam manufacturer website where the external foam was placed on existing siding, then covered with new siding.
Thank-you
Andy,
There's always folks who ask questions or tell you stuff in workshops you don't want to answer or hear.So, thank you.
But I think you are right.
What kind of siding are you planning to install? 3/8 inch sheathing is pretty thin. Did you mean 5/8 inch?
I’m assuming you are not in zone 7 or 8. But where are you located? See this article to better understand why I’m asking these questions: https://www.greenbuildingadvisor.com/articles/dept/musings/combining-exterior-rigid-foam-fluffy-insulation
Bill,
Here's one more vote in favor of removing the siding. After you have removed the siding, inspect the sheathing. If the sheathing is inadequate, you may need to add a layer of plywood or OSB.
Then install your WRB, and one or more layers of rigid foam, according to the advice in these two articles:
Calculating the Minimum Thickness of Rigid Foam Sheathing
How to Install Rigid Foam Sheathing
Steve, Martin, Andy:
I'm on the cusp of zone 3 and 4 in southern TN. The walls are pretty stout- old hard as brick oak studs. I've done a bunch of tear out, interior is gutted; new sills, joists, sub floor. Got the LVL beam under the loft. Roof is real wide 5v and dry.
My orig plan was to tear off existing siding but I broke my back a little over a year ago and now kinda have a "hitch in my get along" (Dizzy Dean), plus I'm almost 70. So I was trying to limit how much I was up a ladder and found this video. Could this work if the existing siding was sealed on the edges?
This link from Dow is what I was going on:
http://www.revitalizehome.com/photo_video.htm , the "Insulate While Residing" video.
thanks
Bill G
Well, now I know where the first reply went. Sorry
BG
Hey Bill, that video makes it look super easy to nail foamboard over existing siding, which it is, but it totally minimizes the serious amount of ladder time that you'll spend on details of air- and weather-sealing and trim if you want the job done right. Without airsealing you won't achieve the huge insulating upgrade advertised in the video ($8,000-$11,000 payback over 15 years! Really, DOW?) and without VERY careful weathersealing you may get damaging water intrusion around windows and doors. In my work remodeling I frequently spend hours repairing the results of badly-installed vinyl siding.
For what it's worth, the guys in the video were covering up what looked like T1-11 plywood and old 12" hardboard siding on a relatively new house. They were NOT nailing through wobbly vinyl siding into a patchwork of sheathing on a century old house. Hard to know without seeing it, but I'd guess you'd not only get a better job if you yank off the existing siding first, it would also go quicker.
That's tough to be hobbled by a back injury--hope you can be back on the horse soon.
Andy,
Ok,
I give. Existing siding must be removed. Gaah!
I have an Appaloosa mule (Spock), but can't really ride because I broke my back bush hogging last year which added to damage from bad landings jumping in the Army in '69, so I sit sideways a lot.
Anyway, you said what I thought but didn't want to hear.
Know any siding folks in So TN?
bg