I have heard Joe Lstriburek twice say that a wall in this climate (Albany NY, almost 6,000 DD) should have 1/3 of the R-value on the outside of the structure and 2/3 in the cavities to prevent condensation inside the wall. So 3 questions:

1) Is there agreement that this generalization covers most bases?

2) I have a couple of ideas in mind, but has anyone come up with a wall they have had success with?

3) Would wet-spray cellulose in the cavity reduce the potential for condensation inside the wall?

Thanks
Ed

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A few quick thoughts:

For #1 - that is the minimum, he would say the optimal is all insulation on the outside (see his perfect wall) --- oh I am sure many will disagree especially when you get to NAHB, but yes that is a good rule of thumb.

#3 - as long as it dries out it would be better than batts, but with that the primary issue with condensation is air leakage. You stop the leaks...

#2 - see above, almost any type of wall or material can work if installed properly. Personally I like closed cell foam - doesn't allow vapor diffusion at all - feel free to follow that up with rockwool panels or foam outside & a rain screen detail & you have an almost bullet proof wall when it comes to moisture  

My quick and dirty guess is that you have the proportions backward. For 30 years, cold-climate builders used a little bit of exterior insulation, to reduce thermal bridging. But if you went overboard, the wall couldn't dry to outdoors, and rot ensued.

The REMOTE exterior wall insulation system turns that on it's head -- get enough insulation outside the sheathing AND GET GOOD FLASHING AND A GOOD AIR BARRIER OUT THERE, and the little bit of drying that you might need can all be done to the inside.

I am doing the calcs and planning for a retrofit REMOTE wall system in S. Wisconsin, for around 7,200 HDD. For my design, the MINIMUM number is 2/3rds of the insulation exterior to the sheating.

So, no -- #1 is wrong. I'd make an educated guess (no calcs) that 1/3 exterior - 2/3 cavity insulation would be a bad idea for any climate where one entire month has an average exterior temp below 32 degrees F. Maybe you could also make it work in an alpine climate with low ambient humidity and good interior ventilation. But the real answer is to do the physics and run the numbers.

#2 -- note that a REMOTE wall, with both exterior and cavity insulation is based on design temperature and design interior RH, not heating degree days. The critical element is sheating RH during the coldest month of the year -- you  have to keep the sheating "coupled" enough to interior heat and "de-coupled" enough from exterior cold to keep the sheathing from condensing.

Journal of Light Construction had an EXCELLENT article on REMOTE wall systems in their November, 2013 issue. That article provides some perfect detail drawings for critical junctures -- I believe they came from the Building America work done by Building Science Corp. (your tax $$ at work!) It also provides the calculation routine so you can run numbers for your design conditions.

Note that the details in JLC are really useful -- there are very few good materials that provide good R-value AND retain even minimal vapor permeability, so you just assume your insulation is impermeable, and put your air barrier at or just exterior to the sheathing. In fact, the material of choice is foil-faced polyiso foam bord, which has a functional perm rating of about zero. A REMOTE wall done this way dries ONLY to inside. That means that you quit worrying about an airtight interior surface -- in fact, you need a vapor-permeable interior surface so that the little bit of moisture that will get in can dry to the inside. Air seal at the outside surface of the sheathing, put enough insulation outside of the air barrier and you've got it all licked. But it also means you need to reduce bulk moisture loading to the bare minimum. Flashing failures are NOT good.

#3 -- no, wet-spray cellulose won't hurt, but it won't help you either. If the sheathing gets cold enough, vapor drive alone will pull enough moisture out of the interior air to the sheathing to mess up your wall. But the beauty is, if the sheathing is warm enough, no amount of interior air leakage into the cavity can do any damage.

How does this all work in the real world? Well, construction here to retrofit my 100-year old farmhouse should start in a couple of months -- ask me in a few years! But I do think the science is impeccable, and the Building America case studies are very persuasive. The few buildings I know of here in S. Wisconsin that have been done (new construction) appear to be performing well.

 

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