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Old 10-10-2010, 12:28 PM   #1
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Before the Blow: Assessing DIY siding job before insulating

QUESTION: Would you do anything to this siding job to protect from bulk exterior water, or interior condensation, before blowing insulation in the wall cavities?


100+ year old house in Alleghany mountains with empty wall cavities.

Prior owner laid 1/2" foil covered polyiso foam (low permeability) over the old lapsiding
Name:  PolyIso-over-Lapsiding.JPG
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He butted the foam up to the original trim work (no caulk). In some place the foam and trim makes a flat surface like in this pic:

Name:  PolyIso-Butts-To-Trim.JPG
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And in others the foam is proud of the trim like in this one:
Name:  Jchannel-Proud-Of-Trim.JPG
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The Jchannel around doors and windows runs along the edge of the foam, and does not overlap the trim at all. At best, the back flange on the jchannel is in the same plane as the face of the trim, leaving a small gap between the two. Like in the above pic, in other places the foam is in plain view.

There is no caulk, flashing, tape, or house wrap anywhere on the job.

We'd love to seal the cavities with closed cell foam but that's way out of budget. We're thinking of blowing with a loose fill, from the outside. Before we blow the cavities (and put an end or at least slow down drying via the cavities) should we tweak the siding job to control bulk water from outside, or condensation from inside, and what would you recommend?

Thanks for your comments.
Steve El


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Old 10-11-2010, 08:04 PM   #2
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BTW, none of the windows have storms. Some original windows were replaced with inserts and some were not. I plan to replace or repair the old ones, fur out the old trim, wrap the old trim with aluminum and then hang storms on all the windows. For the better windows the main reason to hang storms is aesthetic, because otherwise I'd end up having a sunken look to them and I don't want to reset them. That still leaves the gap between the new aluminum capped trim and the jchannel, and I don't really trust caulkall that much.


Last edited by steveel; 10-11-2010 at 10:07 PM.
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Old 10-11-2010, 11:15 PM   #3
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Foil faced polyiso needs the foil facing to waterproof it. If it gets any nail holes, rips, tears, or punctures and gets wet, it can absorb 7x more water than EXP.
You need to use metal flashing at windows and horizontal panel joints as well as butting trim underneath, and tape/seal any exposed edge. Water can reach behind it through spaces as thin as a dime. Think like a rain drop, as they say, one that can even run uphill. All vertical joints require taping along with tears, etc. Tape exposed board edges at metal flashings. Wind-driven rain cannot get to the foam board core. Vinyl siding has a perm rating of 70, because of the joints at individual pieces. It is your drainage plain, with the vinyl shedding most of the liquid. I wouldn’t caulk the bottom of the sheets, rather let any water vapor escape (from interior) after condensing on the backside foil face.(If it ever happens) This is wherein the problem lies. Your foil is vapor impermeable when it should be vapor semi-permeable.

“Some people worry that using impermeable sheathing in a cold climate will trap moisture within wall cavities. Building scientists have learned that as airborne moisture leaks into wall cavities, so does heat that warms walls to safe levels. While studies show it might be ok to use impermeable foil-faced sheathings in cold climates, you have to ask: why take the chance? It’s simply safer to use more permeable sheathings. Extruded polystyrene (XEPS) and molded polystyrene (MEPS) products give thermal protection with perms of 1+/inch and 2+/inch of thickness respectively. Polyisocyanurate products like Rmax Durasheath® (perm 1+/ inch) and Dow Sturdy-R (perm 3+/inch) are available too. Vapor permeable sheathings allow the wall cavities to dry faster if they get wet.” From:

Your job is cut-out for you, making that foam a drainage plane. Controlling the indoor humidity to an acceptable level.
In a search on “foil-faced foam under vinyl siding” one mentioned the siding detaching itself off the wall from the vinyl heating up because of the radiant foil. No problems yet, you’re probably good to go….

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Old 10-12-2010, 07:21 AM   #4
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Thank you very much Gary, I was hoping you'd chime in on this.
.....siding detaching itself off the wall.....
Phooey. I wish that would happen here, it would save a ton of work!

I didn't understand your suggested approach for horizontal panel joints. You mentioned metal flashing and but NOT caulking the lower edge of panels. Are you thinking of z flashing?


If things work out with job and school plans we plan to move in 8 years. The vinyl is in good condition. At a minimum I will furr out and wrap door and window trim and install storms. Beyond that our options seem to be:

(a) Caulk between jchannel and aluminum wrappin on trim but do nothing more to walls, with belief that cavities and foam will dry if they get wet. This option means keeping careful eye on conditions for condensation by doing even more than usual to keep winter indoor humidity under control and not cranking summer AC

(b) Strip the vinyl, turn the foil face into a proper drain plain, rehang the vinyl on existing nails

(c) Strip the vinyl and drive those nails flush, leave the foil facing as is so water vapor can escape, but add a new drain plain using Tyvek, then rehang the vinyl with new nails

Does anybody see any other big picture options?

Thanks for attention,
Steve El

Last edited by steveel; 10-12-2010 at 07:25 AM.
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