Exterior foam and rain screen bottom wall detail
rantipoler72
| Posted in General Questions on
I am wanting to add exterior foam to a garage I am converting into an apartment. I have already wrapped with a WRB and flashed windows in plane with the WRB. I am ready to add 1” polyiso foam and 3/8 rain screen and believe I know how to install around my windows and doors. But I am struggling with how to terminate at bottom of the wall. I know I need a combination of bug screen and flashing but protecting foam as well as directing bulk water away and any mitigated water in rain screen cavity out has me a little stumped with how to proceed with best practice. Thanks for any help!
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Replies
You want a perforated J trim like:
https://www.maibec.com/en/resistech/products
This should go under the foam and extend out all the way out over your rain screen strapping. This way it protects the foam but allows air flow.
A local sheetmetal shop should be able to bend you some to fit. I've also bent perforated soffit covers in a pinch.
Hammer and Hand's Best Practices Manual has a complete chapter on rainscreen details.
Looking through the chapter may help you visualize how you want all the details of your walls to come together with insect screen and flashings:
https://hammerandhand.com/best-practices/manual/4-rain-screens/4-1-top-wall/
I second the advice on the hammer and hand practice. I just finished the wall cladding on my ADU using this technique.
There is a slight modification needed though if you also have exterior insulation. The H&H system is assuming no insulation. With no insulation layer then the strapping and the Cor-a-Vent completely block bug access to the rainscreen.
However if you have an insulation layer behind the strapping then you'll also need to use an additional insect barrier to block access to the insulation. (I used rockwool which ants would love.) I used insect screen fabric tacked to the sheathing before putting on the insulation layer. Just let the screen material dangle down. Then after you have the strapping and Cor-a-Vent installed then wrap the screen back up tight and staple to the strapping.
If that's not clear then I could post a photo.
Bill,
I think insect screen is too susceptible to mechanical damage to be a good solution over time. Builders here have all switched to the flashing Akos posted. It would need to be custom bent to cover both the insulation and strapping.
Another solution for exterior foam is to move the WRB to the outside of the insulation. Then the bottom of the foam can be covered by any solid material.
Malcolm, do you think this Maibec ventilated flashing could replace the need for Cor-a-Vent?
Cor-a-Vent is still needed with insect screen fabric as the fabric is not rigid enough to make a good seal against the siding starter strip without the bulk of the Cor-a-Vent strip behind it.
However I'd think with this more rigid ventilated metal flashing that you could make a seal against the siding - perhaps by even running a bead of caulking along the flashing before tacking on the starter strip. On the other hand any dents or deformation in the metal flashing would provide a path for insects. Perhaps too risky?
Bill,
Here in BC where every house is required to have a rain-screen gap, perforated J flashing replaced insect-screen probably close to a decade ago, and most lumberyards carry it.
One advantage is you can tack it onto the sheathing and it acts as a base to set your foam and furring strips in. I haven't seen or heard of issues with using it.
Here is a picture of a rains-screen from one of my projects (without exterior foam):
I installed a pressure treated board at the base of my wall that the foam sits on. I then stapled 8” bug screen on, installed my rain screen and then folded it up and stapled to the rain screen.
Brian,
My reservations about relying on insect screen aside, your method works as long as the WRB is located in front of the foam, or your foam is detailed as the WRB. If, as in Shawn's case, the WRB is back at the sheathing, I'm not sure a solid block under the foam is great idea.
I agree with you, I was apprehensive about just having mesh but I’m never going to be trimming right next to the siding so I didn’t see much reason to spend a couple hundred more dollars. I also have my foil faced polyiso detailed as a secondary water barrier. If any water gets past that, I didn’t caulk the treated base so it should migrate through.
Is there a concern about the drainage plane being behind the polyiso? I was under the impression that polyiso doesn't handle water well, so unless it's protected via foil facing and taped edges it might degrade over time. My instincts tell me that the WRB should have been over the polyiso.
You would generally want the WRB behind the polyiso, but it’s fairly common to tape foil faced polyiso as a WRB too, sometimes as a secondary WRB if you have redundant layers.
Bill
Thanks Bill, I'm still curious as to why you want it behind. Doesn't this increase the risk of wetting the polyiso?
Provided the polyiso is foil faced, there is no issues with the WRB behind the foam.
Polyiso is only problematic when in ground contact or exposed to a lot water. Behind your siding, it should never see enough liquid water to matter.
Even if some liquid water does make it in, it will just reduce the effective R value of the foam until it dries up, the foam won't be damaged by the water.
The facing on fiber faced polyiso (standard roofing foam) can get damaged from liquid water, if you are using that, the WRB needs to be above the foam.