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Insulated Metal Panels in a PERSIST-type system?

jchwang | Posted in Green Products and Materials on

3300 sqft house in Climate Zone 6 (Toronto). Considering using insulated metal panels (IMP) as external insulation layer in a PERSIST/REMOTE approach for wall and possibly roof.

These panels are polyiso foamed in place between 2 thin 26ga steel sheets, usually used in commercial applications On each panel, the 2 sheets form a complex tongue & groove profile at the ends, but do not actually touch one another (thermal break between interior face and exterior of about 2″). Fasteners go through ~3″ of the tongue and groove profile to the structure, and are hidden when the next panel slides over the fastener head. Panels are caulked with butyl to complete moisture and air seal.

I believe advantages relate to its ‘all-in-one’ functionality as a cladding/insulation/air barrier: Low cost (considering all functions it could replace), speed of installation, polyiso is sealed so loss of R due to gas escaping minimized, one continuous insulation piece (IMPs come in 40″ widths and can be 8′ to 40′ long). I plan on orienting them vertically. These advantages should allow some wall layers to be redundant, but I am unsure so the wall section currently still contains:

2×6 framing (5.5″)
5″ Roxul (mineral wool) for R16 (yes, am considering leaving 1/2″ for wiring / plumbing chase)
1/2″ Plywood structural sheathing
Tyvek Drainwrap
3″ Roxul Comfort CIS for R12 (supports fasteners, drainage, fire barrier)
4″ IMP for R28 with screws fastened through to framing

Total wall: 12″, R60 (R40 on cold side).

After reading many articles on this site, my concerns include:
1) Thermal bridging – even though bridge is only through screws into framing wood (I think), the metal facing on the IMPs still worries me.
2) Caulking failure, though manufacturer assures me the butyl is sheltered from UV in the tongue and grooves, so will remain pliable. Is a rainscreen needed to prevent moisture from being driven into the polyiso (degrades with water)?
3) Not sure I totally understand solar vapour drive, but thse metal panels would presumably heat up a lot in summer, though they theoretically wouldn’t hold any water and are vapour impermeable.
4) Because of 2) and possibly 3), I thought Roxul layers would be needed in case moisture distribution / drainage required, though not sure if I should have drainage holes top / bottom / both and how large? Will Tyvek ‘flatten’? Is drainage plane inside face of IMP, or outer face of Tyvek?
5) Do I need the Roxul Comfort IS board or should IMP directly touch DrainWrap (IMP inner surface is corrugated so there is a drainage challe)? Do I need the Tyvek (probably cheap insurance, so worth keeping?)
6) How many external insulation layers can continue down below grade? Don’t think IMPs can, but Roxul can?
7) Where to place windows? For energy performance, it should be in the middle (thermally) so at R30 plane, where 1″ Roxul and IMPs can be used to overinsulate the frame.
8) Installation issues –
a) Unsure how the panels are secured to structure if only one T&G edge can take a fastener. The other side relies on the next panel to interlock with it and be fastened down. Better to ‘hang’ the IMPs horizontally?
b) I think a 7″ fastener should be able to secure the panel (relatively light at 2.6pcf) but which brand/rproduct? There would be no furring, as it goes through the IMP T&G. Based on REMOTE manual, I think I can place screws 12″ OC vertically x 40″ panel widths. Should I also consider using a ledger? Building Science Corp did a Deep Energy Retrofit using these panels as insulated cladding oriented horizontally, though they seemed to be attached using z-girts and not direct attachment using screws into studs.
c) Framing is 16″ OC, so for the 40″ width, extra framing studs will be needed.

Haven’t thought through roof issues (another post….)

Not directly related to the IMPs, but also unsure whether:
a) I need MemBrain or vapour retarder (drywall / paint good enough?)
b) Tyvek Drainwrap or Grace Ice & Water Shield makes more sense.

Thank you all for any advice / insights.

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  1. GBA Editor
    Martin Holladay | | #1

    The biggest question (it seems to me) is whether these panels can be installed over 3 inches of Roxul. A secondary question -- assuming the answer to the first question is "yes" -- is how the panels will be fastened to the wall.

    Both of these questions need to be directed to the panel manufacturer. If your plan does not comply with the manufacturer's installation instructions, I don't think you should proceed with your plan.

  2. jchwang | | #2

    Thanks for the timely response Martin. I was contemplating those installation issues yesterday, and going by the articles in GBA on Roxul and external cladding and the BSC testing that has been done.

    At 2.6 lbs per ft3, this would be considered light cladding similar to fibre cement or Hardie Board so I think the Roxul and screwing into the framing shouldn't be an issue. I think the main differences and potential cause for concern is the area of the screw near the head. Instead of going through 1" wood furring, the screw secures through 2 pieces of 26ga steel. From the detail drawings on the manufacturer's website, it seems that self-tapping metal screws are usually used, not bolts, so hopefully this isn't a problem. Will check with manufacturer, but I guess I was inspired by BSC, which did a deep energy retrofit where they had to convince manufacturer that panels could be used as insulated cladding, not to original intent (I've attached presentation).

    Other than installation (#8), what do you think of the other risks? Thermally and moisture-wise, any red flags for you?

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