Above-grade block wall insulation: Exterior continuous foamboard. Add unfaced batts inside, too?

Motorapido

House is concrete block on slab on grade. Zone 5. Total renovation work underway. I'm adding a continuous layer of 2 inch XPS foam panels to the exterior before applying siding, with furring strips sitting on top of the continuous foam boards. I will frame out interior walls with 2x3s (interior walls are just bare concrete block now) and finish the interior walls with tongue and groove pine planks. With the exterior 2 inch insulation foam boards strapped tightly to the exterior concrete block and with insulation panel seams sealed and taped all around, I have created a wall assembly that MUST dry to the inside, since the foam boards outside act as a vapor barrier/retarder.
Question: would adding unfaced fiberglass batts on the interior 2x3-framed walls help or hurt? By adding the foam insulation to the exterior of the block walls, I have brought the concrete block into the conditioned space and added thermal retention mass from the concrete blocks. If I use unfaced fiberglass batts inside, I don't assume that I would harm the wall's ability to dry to the inside. Adding the batts would in theory add about another r9, and the exterior foam boards are giving me about r10, so by adding the interior batts, I assume my total wall assembly would yield r19, not counting the r-1 that the blocks themselves are said to be good for. So should I keep the interior 2x3 walls empty or fill them with unfaced fiberglass batts?

Jamie Hall

Interesting question... without a simple answer, at least in my mind.

Let's consider, though, two things: first, the necessity for the wall assembly -- specifically the concrete block -- to dry, if at all, to the inside. Second, what will the temperature profile and moisture profile across the wall be?

The first consideration means that the the concrete block must stay above the dewpoint on the outside face of the block.

So... the question is the temperature profile in the wall -- and that, for once, is simple. You are interested in the temperature of the outside face of the concrete block. You have a total of about R10 insulation inside of it, if you use the batts, and another R10 or so outside. To a good first approximation, then, the temperature of the outside face of the block will be very close to the average or the interior wall surface temperature and the exterior wall surface temperature. Simplifying a little, in other words -- half way between the interior space temperature and the exterior air temperature.

So you need to know three values: the design outdoor air temperature, the design indoor air temperature, and the design relative humidity indoors which gives you the design dewpoint indoors. If ;when you crunch those numbers your indoor dewpoint is lower than halfway between the indoor temperature and outdoor temperature, you're good to go. Otherwise, skip the batts.

Motorapido

Thanks, Jamie. Design outdoor air temperature winter, let's say 10 degrees, and summer 90 degrees. Indoor relative humidity in winter about 35%. Summer 60% and upward. Property is located in Northern central Pennsylvania, about 15 miles south of the New York border. Gaines, PA, to be precise.

Jamie Hall

The winter dew point of your indoor air with those figures is about 41 F. Assuming 70 F indoor air, the outdoor face of the concrete will be around 40. OK... yeah, but too close to comfort for me to be happy with it. Wouldn't take much extra humidity to give you a problem.

hot_rod

A colorful, to say the least speaker, Joe seems to be one of the folks with the best research and data on that topic. You might sniff around his site.

https://www.buildingscience.com/users/joseph-lstiburek

Larry Weingarten

Hi, I'm wondering if you want the added insulation, why not add another 2" of foam outside? This would help with dewpoint concerns and then with no insulation inside, the block is better thermally coupled to the interior space, making the temperature swings less. Also, there would be less impediment to drying.

Yours, Larry
ps, If you add more foam outside, make sure you stagger the seams.

Motorapido

Thanks, Larry. I will be hanging LP smartside engineered wood lap siding. With 2 inches of foam and the 2x furring/strapping strips, my Tapcons are already cantilevered out 3 3/4" from the block, and I don't want to risk adding more insulation thickness, which would increase the cantilevering of the tapcons. And Jamie, is your bottom line indicating that I should not add interior insulation? And without any interior insulation, based on my numbers am I OK to proceed with the 2 inches of foam on the exterior and nothing at all on the interior?

toeknee

Does only 2" of XPS constitute an actual vapor barrier? I Thought you needed to have a certain thickness to reduce the permeability numbers of the insulation in order for it to be considered an effective vapor barrier. Im not sure what that thickness is for XPS though. And for this setup, I think you need sufficient thickness of XPS to ensure that the point in the cross section of the wall where the dew point is reached is in the XPS insulation itself. The thicker the insulation on the outside, the further the dew point is to the outside of the wall. XPS is thought to handle moisture pretty well. EPS being the best for moisture.

Also, fiberglass batts handle moisture terribly. Certainly dont want those in an area where they encounter a lot of moisture.

toeknee

If it were me I'd use thinner furring strip, longer fastener and at least 1 more inch of XPS. Fasteners may need to be spaced closer together

Motorapido

toeknee said:

If it were me I'd use thinner furring strip, longer fastener and at least 1 more inch of XPS. Fasteners may need to be spaced closer together

I hear you, but LP Smartside wants the fasteners to bite into 1.5 inches of framing/furring, so I have to use 2x.

toeknee

how about countersinking holes into the 2x material where the tapcons go

toeknee

with your current 3 3/4" tapcons, you would already have to counter sink as it is with 2" foam

Edit: sorry, I initially read that post wrong. I now understand, you have 3 3/4 of cantilever. Your fastener is longer than that

Jamie Hall

Motorapido said:

Thanks, Larry. I will be hanging LP smartside engineered wood lap siding. With 2 inches of foam and the 2x furring/strapping strips, my Tapcons are already cantilevered out 3 3/4" from the block, and I don't want to risk adding more insulation thickness, which would increase the cantilevering of the tapcons. And Jamie, is your bottom line indicating that I should not add interior insulation? And without any interior insulation, based on my numbers am I OK to proceed with the 2 inches of foam on the exterior and nothing at all on the interior?

Yes -- sort of. I'm not really keen on insulation without an interior vapour barrier (except for closed cell foam), but in this instance it shouldn't really be a problem over a reasonable term.

toeknee

How about something like this. Birds eye view

hot_rod

Foam adhesive also?
I saw a demo of a Tapcon competitor over on IG that everyone is raving about, I'll look for the brand name.

Canucker

Motorapido said:

Thanks, Larry. I will be hanging LP smartside engineered wood lap siding. With 2 inches of foam and the 2x furring/strapping strips, my Tapcons are already cantilevered out 3 3/4" from the block, and I don't want to risk adding more insulation thickness, which would increase the cantilevering of the tapcons. And Jamie, is your bottom line indicating that I should not add interior insulation? And without any interior insulation, based on my numbers am I OK to proceed with the 2 inches of foam on the exterior and nothing at all on the interior?

I wouldn't worry about your fasteners cantilevering. If you attach them with the proper depth and spacing it won't be a shear force on the assembly. If there is too much weight, that set up wants to pull out of the wall. There is a good write up about that type of assembly testing on Building science and GBA websites

neilc

this guy has a LOT of good videos also
https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

Motorapido

Reviving this thread. The contractor who will be framing out the interior walls insists that unfaced batt insulation on the soon-to-be-created interior walls is very good, with no downside. But this contractor does not seem to be thoroughly schooled in the physics of insulation and dewpoints, so I want to proceed with caution. Suppose the interior batts are unfaced rockwool -- would adding it be beneficial and without a potential down-side? The extra R-value would be nice, the additional cost would be minimal, but I don't want to mess up and create dewpoint problems. What's the bottom-line consensus here -- add rockwool inside the wall framing being added inside, or keep that cavity empty and rely just on the 2 inches of foam insulation on the exterior for all my r-value?

Canucker

If you have enough foam board to keep the inside wall above the dew point, you'll be fine with unfaced batts. Vapour will get through it but if it stays warm, you're fine. What level of humidity does the space get to in the winter?

Motorapido

I bought the place in February and wasn't measuring relative humidity inside at that point. I'm assuming the indoor relative humidity won't exceed about 40 percent during winter. Providing everybody uses the bathroom exhaust fans and the kitchen stove exhaust fan, there won't be much introduction of humidity indoors. The structure is two-story concrete block, slab on grade. First floor sits on sleepers on the slab. During renovation, the existing subfloor and sleepers will be removed so I can install a thick vapor barrier on the slab before the new floor goes in, so that will reduce indoor humidity coming up through the slab (property built in 1950, so no insulation under the slab, for sure, and no vapor barrier currently between the slab and the sleepers/sub-floor)

Canucker

Keep the air sealed outside insulation. It will only help. If that's in place, I would use unfaced interior insulation. If you get rid of the outside insulation, I would use a well sealed air barrier to keep my moist indoor air off my very cold block walls in the winter. I'm willing to bet your contractors design has never been taken apart to see if it's growing mould. It's a risky assembly

mattmia2

Motorapido said:

Reviving this thread. The contractor who will be framing out the interior walls insists that unfaced batt insulation on the soon-to-be-created interior walls is very good, with no downside. But this contractor does not seem to be thoroughly schooled in the physics of insulation and dewpoints, so I want to proceed with caution. Suppose the interior batts are unfaced rockwool -- would adding it be beneficial and without a potential down-side? The extra R-value would be nice, the additional cost would be minimal, but I don't want to mess up and create dewpoint problems. What's the bottom-line consensus here -- add rockwool inside the wall framing being added inside, or keep that cavity empty and rely just on the 2 inches of foam insulation on the exterior for all my r-value?

that may be a sign that this is not the right contractor for the job.

You will want to carefully wrap the bottom in metal so that rodents can't get in.