Depends on what you mean by "efficiency".
Unfortunately, foam is, generally, about the most efficient insulator on a straight-up "per inch" basis.

There's nothing stopping you from building non-load-bearing interior walls and filling the cavity with whatever insulation you want.

Rockwool
Fiberglass (batts or blown-in)
Cellulose
Spray foam

Whatever.

That's probably the ideal method, as it allows you to run wiring chases ahead of time...

In barns the siding is usually the sheathing. To keep it all from rotting out there needs to be an air gap between the siding & insulation.

Best bang/buck is often installing girts spanning between posts to maintain the air gap, onto which you can mount reclaimed roofing foam boards, which can be cheaper than batts per square foot at a given R value, and easier to make air tight. A layer of housewrap on the interior side of the foam detailed as an air barrier can also help the long-term air tightness prospects.

A continuous layer of 3" roofing polyiso (R16-R17) would beat current code minimums for SE PA if it also insulated the posts. Installing 2" foam (any type) onto the girts and a 2x4/R13 framed wall tight to the foam would also beat code min with margin.

There are several vendors of used foam in your area. Run these searches every few days and you may find a few more:

https://philadelphia.craigslist.org/search/sss?query=rigid+insulation

https://lancaster.craigslist.org/search/sss?query=rigid+insulation

https://allentown.craigslist.org/search/sss?query=rigid+insulation

In barns the siding is usually the sheathing. To keep it all from rotting out there needs to be an air gap between the siding & insulation.

In this case the siding is actually vertical 1x12 board and batten and its in good shape so I want to leave these intact and tackle this from the inside.

Best bang/buck is often installing girts spanning between posts to maintain the air gap

So what is a good air gap to shoot for between the foam board and the inside of the siding? I plan to run the girts from post to post so siding boards could be removed if ever needed without disturbing the insulation. In other words, should I use 1x or 2x, or even 3x material for the girts, and can they be flush against the backside of the siding or should the girts have an space as well?

Should the foam board seams be foamed or is tape sufficient?

Installing 2" foam (any type) onto the girts and a 2x4/R13 framed wall tight to the foam would also beat code min with margin.

Are you suggesting to use regular batts for the 2x4 wall with this approach?

How necessary is adding a layer of foam inside of the 8x8 posts? I had envisioned making the framing flush with the inside of the posts and then covering the framing, insulation and house wrap with T&G boards. Adding foam inside of the 8x8 posts will make for a much thicker wall, which is ok I guess but then there would be a larger air gap between the backside of the siding boards and where the foam board insulation starts, maybe 5-6". My gut tells me that that seems like a large air space to have and could lead to moist air getting trapped behind the siding. Maybe I can keep foam boards, girts and framing flush with inside of posts, then cover with house wrap to a super thin layer of foam stapled to posts as a thermal break? Thoughts?

Finally, as far as the house wrap goes, I had envisioned tacking it up to the inside of the girts against siding before applying the foam board, but you recommend applying inside the foam board between the insulation and interior wall cladding. Can you explain which is better? My thought was that wrap on backside (between siding and girts), would keep moisture from entering the seams on the back side of the foam board which cannot be sealed, but maybe the moisture is more likley to come from inside the house and that is why you recommend applying wrap inside the foam boards?

I'm also considering using foam board as suggested and then a DIY spray foam to seal the seams. What is the recommended air gap between the foam insulation and the interior wall cladding?

Thank you!

Nuclear.....yep

Posted By newbostonconst on 12 Dec 2018 03:55 PM
Nuclear.....yep

LOL.  Just looking for sound advice and opinions as this isn't a typical approach to construction.

A 1/4" air gap is a sufficient thermal break for tighter siding but the amount of wind-driven bulk water potential getting by well-aged board & batten the 1.5" depth of a 2x4 girt isn't a terrible idea and gives you something firmer to hang the foam board on, but 3/4" is fine. Contact between the girts & siding is not a problem.

Moist air simply can't get "trapped" by board & batten siding unless you sprayed the exterior with a heavy coat of sprayed roofing material or something. The bigger the air gap, the better the capillary break, but the benefits diminish after 3/4". In rainy western British Columbia they only demand 10mm (~3/8"), but it's easier to go with twice that using 1x lumber to establish the gap.

The posts themselves are a thermal bridge through the insulation layer, which means you'd need somewhat thicker insulation to achieve the average performance needed to hit code minimum.

With foil facers on the foam board a temperature rated aluminum tape would be sufficient. If asphalted paper or fiberglass, housewrap tape overlaid with troweled-on duct mastic works. Spray foam doesn't have sufficient adhesion to form a reliable air seal on the seams unless applied fairly thickly. Tape & seal the foam to the posts too, and put a bead of caulk between the end stud and post when installing the studwall.

DIY foam kits use high environmental impact blowing agents and should be used only sparingly. Caulking the non-structural framed wall to the foam facers with polyurethane caulk the full perimeter of each stud bay and using rock wool batts is both more fireproof and more environmentally friendly, and surprisingly comparable in thermal performance despite the lower center-cavity R. (The thermal bridging of the studs robs higher R/inch goods of their potential performance.)

A 2" layer of foam board is sufficient dew point control for a 2x6 /R23 rock wool wall, which would be 7.5" actual depth, 8" if including wallboard. At 2" rigid polyiso has has a huge amount of margin on a 2x4/R15 wall (6" actual depth with half-inch wallboard), which what you'd probably be building. If the 2x8 posts are full dimension 8.0" nominal that would be plenty of room for 1.5" thick girts. If they're milled 2x8s with 7.25" nominal depth, drop back to 1x4s for the girts. At center cavity with a 2x4/R15 + 2" reclaimed polyiso wall you'd be at R26-ish at center-cavity, and would beat code minimum performance by quite a bit if the framing fraction of the posts & beams are less than 10% of the total face area of the wall, which it probably is. (It might be about 5%.)

The location of the housewrap depends on where you set the window depth & flashing. With "outie" windows the glass is roughly co-planar with the siding the housewrap would go between the girts & foam. If the glass is set just slightly proud of the exterior stud edges (which is a bit more common) it's an "innie" mount and the window flashing and housewrap is on the interior facing side of the foam board. When there is no sheathing layer between the foam board and studwall it's probably better to go outie on the windows, and drain into the gap established by the girts.

Posted By barnrestoration on 22 Dec 2018 11:54 PM

Very helpful information Dana thank you.  I picked up 30 sheets of 1.5" XPS and another 30 sheets of 1.5" paper backed polyiso insulation today for next to nothing to get started.  Based on your advice so far I'm thinking I can use 2 layers of rigid foam for the walls with no additional insulation and perform reasonably well in SE PA. Correct?  Would you recommend a layer of XPS attached directly to the girts, then a layer of polyiso inside of that or vice versa?  Should each layer be sealed at the seams or only the innermost layer's joints?  Vapor barrier required or no?  Interior cladding will be either drywall or tng 1x.

For the ceiling/roof, part sloped and part flat, I'm thinking of a similar approach but then adding batts to acheive required r value.  It sounds like the air sealing at the roof level will be most important, especially since the existing roofing is metal over purlins with no tar paper or ice & water shield and condensation inside the metal roofing could be likely if any air leaks through.  So what would be the best approach at the ceiling/roof given the foam board I now have?

Thanks! 

A nearly continuous 3" of foam (any type) will pretty much meet IRC code-minimum in your stackup.

If alternating layers of polystyrene and polyisocyanurate, the polystyrene goes on the cold in winter side of the assembly. As the average temperature through the layer drops, the performance of polystyrene increases monotonically. Most paper faced roofing polyiso increases in performance until the mean temp through the layer hits the high 40s or low 50s F, the falls. With 1.5"/ R6.3-R7.5 polystyrene on the exterior, the R8.5-ish polyiso will always be warm enough to meet or exceed it's rated R value, but would have to be derated a bit if on the exterior.

Per the FTC the labeled nominal R-value is required to be it's performance at a 75F mean temperature, at a 50F temperature difference. (50F on the cold side, 100F on the hot side.) That tends to exaggerate the cold weather performance of most polyiso. The only vendor who has beaten that is Dow's Thermax wall sheathing polyiso, which like polystrene increases monotonically with falling temperature (at temperatures that matter for building insulation purposes, at lease.)

Tape the seams of both layers. There is no such thing as "too tight".

At 1.5" XPS is a Class II vapor retarder, all paper faced polyiso sold in the US is also a Class II vapor retarder. With an air gap between the board & batten and the foam there is no way that vapor diffusion through the foam would cause a moisture problem for the sheathing- even air leaks would not be a moisture problem in this stackup. In short, don't install a vapor barrier- it's not necessary, and you already have one (the foam), from a practical standpoint. Make it air tight, but don't worry about water vapor diffusion here.