2×4 exterior side Mooney type wall with R-15 mineral wool
Hello all,
I’ve been lurking for a long time and have really learned a lot about building science from this website and others. I have a 1947 1 1/2 story house in northern Minnesota (zone 7 or 8). 2×4 walls with tongue and groove sheathing (3/4″ I think) followed by tar paper and painted redwood siding.
It seems like my best solution to add R-value is during residing but I don’t want to use foam since most walls have interior poly and want the wall to be vapor open. So here is my idea, remove the siding and tar paper, install cross hatched 2×4’s standing on end to help with thermal bridging using recessed structural fasteners. I would fill this space with R-15 mineral wool batts which I can get for .61 cents per sqft.
Now for my questions, is it okay to leave the tongue and groove sheathing in place and secure the 2×4’s over it (after removing tar paper of course)?
And on the outside of this new 2×4 wall, would I have to sheath it using plywood/OSB then housewrap or could I get away with just housewrap and a drainage plane with furring strips and then siding?
Any issues you could see with this wall assembly or suggestions on improving it?
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This isn't the question you asked, but if you're still interested in foam at all, you might want to look into unfaced EPS and see if it was vapor open enough for your application.
From a quick search I found an article detailing the vapor permeability of EPS foam. At 1" EPS foam is 1.1 perms and at 2" it is 0.55 perms. I think I would need at least 2" to stay away from the condensing issues I've read about regarding exterior foam sheathing being too thin. Increasing the thickness lowers the permeability to levels that I'm not comfortable with (no vapor barrier sandwich for this guy).
I've done something similar for an over-roof, works quite well, just have to watch for a couple of details.
For this setup to work well, you need a well defined air barrier. Probably the simplest is in the middle of the wall. This means either plywood+taped seams over the T&G or a layer of peel and stick over it.
To reduce the thermal bridging of the interior and exterior studs, you would want the 2x4 installed horizontally, insulation between the studs, WRB over the 2x4, vertical strapping over that then your siding. You can go with vertical 2x4 over the existing studs but you loose some R value (assuming 2x4 studs with batts on the inside) you get R21 vs R27.
You are also be in cold enough climate, depending on your energy costs, that going with 2x6 and R23 batts might be worth it (R35 whole assembly with horizontal 2x6). You can do a quick energy calc based on your HDD to figure it out.
I like your idea if I understand it correctly. How exactly are you securing the horizontal 2x4's on end? By cross-hatch, do you mean some vertical 2x4's to keep horizontal 2x4's from sagging toward the exterior, or ? Does the attached diagram depict your wall correctly (minus the cross-hatch) ??
I believe high performance homebuilding supplier FourSevenFive.com suggests (their own upgraded) housewrap with no sheathing, followed by vertical furring in a drainage plane.
That is pretty much the assembly. The sheathing in the middle should be detailed as the air barrier.
For the over roof, I went with long screws through the 2x straight into the rafters underneath (easiest is to pre-drill the holes). It was pretty solid so never put any blocking in between.
If you go with 2x6, blocking would be needed. For 2x4 and light weight siding (wood, cement board, vinyl or metal), I doubt you would need much, but it wouldn't hurt.
If you google Mooney wall and look at a picture you’ll see the basic concept. Usually they do horizontal 2x2’s on the interior of the house to help with thermal bridging issues and usually they do blow in insulation.
My idea was based on this concept but expanded to full sized 2x4’s and moved to the exterior of the house. So the wall from inside out is as follows
Drywall with vapor barrier (existing)
Vertical 2x4 with batt insulation (existing)
Air sealed tongue & groove sheathing (existing)
Horizontal 2x4 filled with mineral wool
Housewrap (WRB)
Vertical furring strips
Siding
Securing the horizontal 2x4’s would be long structural screws recessed slightly.
By cross hatch I just meant the new 2x4’s would be running perpendicular to the old ones.
I will have to consider the 2x6’s. The issue is limited supply of mineral wool in anything thicker than 2x4 in my area and/or the price. I would love dense mineral wool board style insulation that I could install like exterior foam but again it’s a supplier and/or cost issue. I would be DIYing the project so something That is simple to do and using the materials that are available to me was how I came up with the idea.
>"...Minnesota (zone 7 or 8)..."
There are no zone 8 locations in Minnesota. Even northern Minnesota is the "warm" half of zone 7. Zone 8 starts at 12,600 HDD/65F or 7000HDD/18C:
https://www.greenbuildingadvisor.com/article/all-about-climate-zones
Zone 8 in your region starts a bit north of the north shore of Lake Winnepeg, 200 crow-miles north of the US/Canada border. From a design point of view, assume zone 7.
Play around with this tool a bit, looking at the 1976- 2005 averages when clicking on a location to get a sense of just how far away you are from zone 8:
https://climateatlas.ca/map/canada/hdd_2060_85#z=7&lat=53.12&lng=-94.77&city=180
A Mooney wall approach makes more sense here than exterior foam. It would take ~3" of unfaced Type VIII EPS to hit the same clear-wall performance as 2x4/R15 spaced at 24", and even unfaced it would run about 1 perm @ 3", close to the Canadian NBC's definition for "vapour barrier". Unfaced Type I would be only slightly more vapor open, but way too easy to damage in handling. The minimum needed for dew point control would be 2.5", which isn't substantially more vapor open either.
A Mooney wall approach with moisture tolerant highly permeable asphalted fiberboard sheathing would offer the most resilience, but OSB/CDX sheathing should still be OK.
Thanks for the clarification on the zones Dana. I must’ve looked at a outdated or incorrect zone map. I’m 80 miles by road from the Canadian border.
How much performance would I lose having the horizontal 2x4’s at 16” OC since that is the size of the R15 batts?
Would vinyl siding be a bad choice here even with the drainage channel? I was hoping for something maintenance free.
>"I’m 80 miles by road from the Canadian border."
... making that probably 250 crow miles from Zone 8.
Vinyl siding would be fine in this application.
The performance hit from being 16" o.c. rather than 24" o.c. isn't enough to be concerned about. If one used 2x3s with 1" foam board (any type) edge strips the performance would be even higher that 2x4s 24" o.c.. (but probably not worth the trouble here.)
Is it still okay for me to leave the insulation exposed and just house wrap everything without adding a layer of plywood/OSB and then house wrap everything? No matter which way I choose to go both would then have a furring strip drainage layer then siding.