Hybrid PERSIST model with ICF?
Curious about people’s thoughts on using the PERSIST model on ICF?
I know that ICF manufacturers will laud the benefits of ICF’s air tightness due to the concrete, but that doesn’t seem to account for instances where multiple building materials are being used, much less the several hundred feet of block joints.
For example, take an ICF wall where there is a stick frame on top…if we wanted to keep a single exterior plane (ie, a wall with no overhangs or lips), short of framing the stick wall out so so that the sheathing is flush with the ICF foam and then applying the PERSIST style WRB, this would mean adding about 6″ of rigid foam to reach something around R30 to the stick frame…but this now also means that the ICF wall (in keeping with a single plane) is now its R20ish + the rigid foam of R30…which is simply overkill and a waste of resources.
So what to do then? Do you frame out the stick wall so that it is a hybrid PERSIST in sort? With both external insulation on the exterior and then also some cavity insulation? This would be very similar to the ICF in respects to where the insulation sits if we put an airtight WRB on it.
Are we going to run into issues with a hybrid PERSIST model that incorporates some form of cavity insulation in addition to the external insulation?
Put another way, you might be able to ask the question like this: what issues are builders going to run into by applying Grace I&WS to an ICF structure, followed by additional exterior foam such as with the PERSIST model?
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Replies
I would use a product that is designed for ICF, such as Resisto / Colophene ICF. The traditional ice and weather shield could attack the ICF chemically.
Good point...so, presuming an ICF compatible product, the question still remains: how to handle the air barrier on the ICF and other structure elements?
My inclination is to think I would be OK with the WRB, external foam (R10ish), and cavity insulation (R20ish) for the stick framed portions...this would pretty well match the ICF, and from a composition perspective would be the same for ICF and stick framed portions (for ICF , it would be rigid foam > WRB > ICF wall), and overall, the structure would resemble the PERSIST model very closely.
I've not done ICF, but from what I've seen is mostly used in two ways. All ICF structure, so in that case no joints or plane changes or ICF basements with stick frame on top.
In the 2nd case, you can start with a platform frame for your first floor right above the ICF. You can set these so that the rim joist is set back from the exterior face of the ICF your wall sheathing + rigid insulation thickness. Your walls would go up from this point so everything would be in plane.
Most codes allow for a small amount of cantilever as long as less than joist depth.
There also tapered ICF forms that would give you a wider base to set your walls onto but these mean more thermal bridging as there is less exterior rigid insulation.