Exterior CI over GP ForceField Sheathing
jb_429
| Posted in General Questions on
I am planning to use GP ForceField sheathing along with exterior insulation (EPS or XPS) and faced fiberglass batts in the wall. Sheathing will be taped, but the plan would be to not tape the exterior insulation. Siding will be vinyl. I have some questions about this process.
1a) Is a drainage plane needed between the foam and the sheathing?
1b) If a drainage plane is needed, I really don’t want to install a housewrap. What are thoughts on something like this that I came across in a Matt Risinger video. Starts at 1:00 and involves adding small amounts of silicone to create a gap under the panels: https://www.youtube.com/watch?v=kj2QZmDpiIA
2a) I am in Zone 5, so 2021 IRC says 20+R5, but I know it is more commonly recommended to have 30% on the exterior. If I use R-19 cavity insulation, which when compressed will be R-18, then I should be fine using 1 1/2in R-6 EPS. Does that sound correct?
2b) If that plan seems to be fine, then my next question is if it is best to use a laminated EPS or unfaced EPS for durability. I for sure can’t use a foil faced with the vinyl. I think the permabilty is <1 for the laminated and ~3 for 1 1/2 EPS.
With this plan, I would be able to use 4″ roofing nails and penetrate 1 1/4in into the studs which the vinyl company says would be fine and the “warranty” would be valid. I don’t expect an actual warranty, but their stating it would still be warranty compliant gives me some satisfaction that the install should be ok.
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Replies
1A. Nothing is needed. Install the rigid right over the force shield. The sheathing will be your main air and water barrier and should be detailed as such. You don't have to tape the foam but it doesn't hurt if you do.
2B. It is always better to have a permeable assembly, so I would choose rigid with higher perm rating.
The 30% is for no warm side vapor retarder. With faced batts, you can use less without issues. 2x6+R5 with vapor barrier is a very common assembly around me (zone 5) and works great. I would go for the thickest insulation that still lets you do a simple siding install with reasonable fasteners.
I would also add some extra blocking around the corners so you have a bit more surface to work with there.
Agree with Akos that you don’t need any gap between the exterior insulation and sheathing. In fact, an air gap might lead to convective currents which would negate a lot of the thermal resistance.
I would consider fiberglass faced polyiso. You might be able to get some reclaimed roofing polyiso from a roofing contractor in your area. You say no to the foil-faced version, but I’m not sure why? Is it because of possible heat and the thermal expansion of the vinyl siding? If permeability is your reason that is not a good reason. There is nothing between the exterior insulation and the vinyl siding that would need permeable insulation. Also, most vinyl siding has built in vent channels.
Matthew,
Joe Lstiburek recommends a gap of 1/32" to 3/16" between the exterior insulation and the sheathing as a drainage plane for incidental water intrusion, and hygric redistribution. At 3/16" you lose 5% of the R-value.
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-111-double-play
"If permeability is your reason that is not a good reason. There is nothing between the exterior insulation and the vinyl siding that would need permeable insulation."
The advantage of permeable foam isn't at the cladding, it's from the sheathing inward, allowing some drying for the framed wall to the outside.
Good to hear that overall my plan should be fine.
I would love to go thicker than the 1 1/2in, I may reach out to the vinyl manufacturer and see what they say about 2" foam and if a longer fastener would still be approved.
@matthew25 Yes, the no foil-faced is a requirement of the vinyl siding manufacturer. I would assume you are correct that it is due to it causing an increase in expansion of the vinyl.
I do see a difference in suggestion between @akos and @matthew25 between the permeability of the foam. My thought is along the lines of the more permeable the better, so unfaced would be preferred. Though faced seems like it would be better from a durability standpoint. I would love to hear the reasoning behind each view on it though.