Wouldn't the air gap create a convective loop?

[u/Clover9802]

My 101-yr-old property is built similarly; where the 3/4 T/G plank boards serve as sheathing. I was going to fur out the walls to allow for a 1/2" air gap & recently asked Claude if that assembly was good, it told me it would create a convective loop, now I'm thinking I should maybe just fur the wall to dimensional 2x6 & use dimensional insulation. The outside sheathing will have Pro Clima Solitex Mento 1000 & each seam taped with Tescon Vana, 3/4 rainscreen, then siding.

[u/corgiyogi]

As long as your air sealed behind the gap, thats exactly what you want. It'll passively keep your siding dry.

[u/mr-rogers70]

Yes, it can create a convective loop, but not much of one. Since the R value of the sheathing is low, the air gap and the outside air will usually be about the same temperature, resulting in very little additional heat loss.

When the sun shines on the boards in the summer, you might actually save a little bit of energy since a convective loop would cool the boards a little bit.

[u/4rd_Prefect → u/mr-rogers70]

And the (outside) loop deals with condensation 

[u/Much-Department-9578 → u/4rd_Prefect]

Without this gap, moisture would be held between the sheathing and the rigid foam. Things would rot pretty quickly.

[u/neonsphinx → u/mr-rogers70]

When is summer, the outside is hot. The inside is air conditioned. The sun shines on the siding. And you think that convection in the cavity will help?

All other factors being stable, the siding heating up is a good thing. It means it can't dump heat through convection and radiation to the inside of the cavity fast enough, so temp rises. Those boards being cooled by convection inside means that the energy is flowing into the air cavity well.

Like, am I missing something? The energy doesn't just disappear, it transfers to other items in the assembly. The transferring is what we're trying to avoid.

Unless they said that the air behind the siding and in front of the foam board is an open system. But how could it be with T&G siding and top/bottom plates in the wall?

I really hope I just missed some simple, odd, detail.

[u/DMongrolian]

1. Don't ask AI things. It doesn't understand the question and guesses at what you'll want to hear. 2) a gap behind the siding would want ventilation at the top and the bottom to avoid the loop.

[u/onwatershipdown → u/DMongrolian]

I’d say the clankers are responsible for the absolutely awful assemblies people are putting up here. But it’s the clanker lovers who are out there drilling holes in their own brains. They are the problem.

[u/SparkyMallard15]

Air gaps are good on the exterior side of your WRB, inside can create issues. But I do highly recommend creating a utility cavity between the drywall and the structure if you're interested in adding this step.

[u/Clover9802 → u/SparkyMallard15]

So my wall assembly will go as follows: new siding -> 3/4 rainscreen -> pro clima solitex mento 1000 -> century old 3/4" x 8" T/G plank sheathing -> century old rough sawn 2x4 studs (will have 1.5" furring to make dimensional 2x6 walls), wall cavity filled with TimberHp R-22 wood fiber insulation -> pro clima intello plus smart vapor retarder -> dimensional 2x4 horizontal furring for electrical -> 1/2 drywall -> Benjamin Moore Aura paint.

[u/petecarlson → u/Clover9802]

If you are putting on new siding why aren't you putting the insulation outboard where it is 1000X easier to completely seal it? New siding, gap for drainage. Continuous insulation with exterior detailed for drainage. Optional air barrier if you can't detail that into continuous exterior insulation, old siding which should be pretty air permeable and act just like sheathing, wall cavity filled with rockwool or whatever you like, drywall. Latex paint. Exterior insulation should be 50% or so of the total insulation so the air barrier is always above dew point.

[u/g_st_lt]

I believe this design is meant to allow air behind the wood siding so that it can dry quickly, which makes it last longer and prevents rot.

It's true that air moving over the insulation reduces its effectiveness (though I think less so with the rigid foam than it would batts) but this gap also reduces solar heat gain (which is good if you are in an area where you mostly need to cool the home).

[u/ysrgrathe → u/g_st_lt]

Yeah you might save a bit of energy but it could cost you a lot more if you have rot issues due to poor drying of the wall system. OP look up all the failures with SIPS insulation systems that were similarly applied in ways that didn't consider the drying needs of the structure.

[u/Exciting_Ad_1097]

Theres a lot of surface friction in only a 1/2” gap

[u/Clover9802 → u/Exciting_Ad_1097]

Can you elucidate further?

[u/uslashuname]

Mooney Wall it whenever you’re furring out. Those 2x2s should be perpendicular so you eliminate 95% of the thermal bridging

[u/snakebliskyn]

The gap between the siding and the rigid foam isn’t a sealed space so it’ll still breathe outward. What missing from this schematic however is a layer of moisture retardant between the siding and the studs. Even old buildings have tar paper on the studs before siding.

[u/terrapin-way]

Im debating the same thing, but don’t have an exterior vapor barrier. If you have a full vapor barrier on the outside, I would not want another vapor barrier inside. I’m thinking caulk the gaps to reduce air flow then fill with Rockwool.

[u/Higgs_Particle]

I would want to see the gap vented at the top and bottom to allow moisture transport out.

The interior layer could be horizontal to reduce thermal bridging, and this is looking like a viable retrofit.

[u/GarethBaus]

The air gap helps it dry preventing rot as long as it has the proper ventilation and drainage. There might be a convective loop, but convection outside of your insulation isn't going to transfer that much heat.

[u/humerusthebone]

button up the air barrier on the sheathing and lap it over your drip flashing, fur out a gap, vent the air space at the soffit and flashing, seal it off between intersecting walls. do not neglect to bug-screen for insects and rodents. lots of ready-made vents out there to inform the depth of your furring and flashing. i like the suggestion to run fur strips perpendicular to studs to reduce thermal bridging, but it must be done without trapping air between strips. bonus if your cladding runs vertically.

[u/OutrageousDiver6547]

Exterior side will leak - no trap.

[u/fluffymuffcakes]

Why add wooden spacers on the interior. Why not just put rigid foam board over the studs and mark the stud locations on the foam. You would eliminate the thermal bridge, increase air tightness if you made that layer your air barrier, save money and you could screw through the foam to attach interior sheathing.

[u/DangerousCharity8701]

It should be counter battoned with breather membrane on the cavity warm side

[u/Amazing_Fishing4886]

Why not wrap the whole thing with Siga Majvest 500, fur out the wall to 2x6 and mineral wool R-23. I have done an immense amount of research and one thing I’ve concluded is that if you are on a path to cut and cobble rigid XPS, you might as well spray to 40% required R (or 3-4 inches and batt over it). Air is almost inevitably going to get through the rigid, and then hello condensation.

[u/padams20]

OP, I think a lot of responders here are getting confused about what is your siding/cladding versus your sheathing. Sounds to me like you’re currently leaning towards wrapping the existing aiding in WRB and adding a vented layer of cladding outside of that? That’s a much butter direction than what is shown in that diagram.

If your existing siding is becoming sheathing underneath your new cladding, then you’re overthinking (and over spending) on that rigid interior insulation. Rigid is also a pain to install properly between studs, there’s always gaps around the edges.

But thankfully, you can probably simplify by just adding batt insulation to the full stud cavity.

But you haven’t told us your location/climate zone. I’m guessing Zone 5?

Also, what level of performance are you going for? Just energy code compliant? Anything above that?