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Community and Q&A

Insulating and Air-Sealing Concrete-Block Wall and Roof

acwei | Posted in Energy Efficiency and Durability on

We recently purchased an old concrete block building in Hillsborough, NC (zone 4A) and will be remodeling it for our retirement home. It is currently an empty shell with 10’ tall concrete block walls, concrete floor, and wood truss roof (photo attached). We would like to approach the remodel with “net-zero” energy in mind.  Obviously, we want to do this sensibly and as cost efficiently as possible.  I have spent many hours reading GBA and BSC articles but still have questions and would like confirmation that we are on right track.  As we are in the design phase, we would hugely appreciate your thoughts & experiences on the following:

Walls/Siding:  We would like to use exterior insulation to keep thermal mass inside.  The exterior will be combination of stucco (or EIFS) and some cement board siding.

2015 IECC required R-8 for mass wall and R-20 for wood walls.  Joe Lstiburek in his article “BSI-081: Zeroing In” suggests R-40 walls.  Since our walls are “mass wall” (concrete block), it seems like 2” of exterior foam (2 layers of 1”) will meet the IECC requirement.  It just doesn’t feel like enough though… your thoughts?

·       Does it make sense install 4″ exterior foam boards since I think the maximum I can reasonably attach to the masonry blocks is about 4” due to fasteners?

·       Traditional stucco and cement board are heavy.  Are there any problems having this weight hanging out 4+” (4” foam + furring strip)  from wall since attachment are just some sort of long fastener?  Which fastener would be the most economical to buy and easiest to install?  Would screws like Tapcon pull out or bend in this application?

·       Would EIFS be a better choice for siding since it is lighter weight?

·       Has anyone done any cost/durability comparisons between EIFS and traditional stucco?  Traditional stucco seems to be more durable and repairable, but I am concerned that the stucco might crack due to movement since it is hanging out 4” (or more) from the block wall.

·       If we use traditional stucco or cement board siding, I presume that we will need to attach furring strips on top of the foam to provide a nailing surface.  Are 1×4’s sufficient? Will the furring strips need to be treated since they would be in contact with the water absorbing cement board or stucco? Would steel furring strips be a better choice?

·       Parts of the block wall exterior have been parged & painted and parts of it is just bare block. What would you recommend for the air/vapor barrier to be applied to this wall exterior?

·       Will another water barrier be required on top of the foam, or will the gap created by the furring strips be enough of a rain screen?

Roof:  The current roof is asphalt shingles on top of plywood on top of traditional 2’ O.C. 8/12 pitch wood trusses (commons and scissors with 12-16” overhang).  Attic is vented with no ceiling nor insulation; the roof is shot.  We would like to replace with steel roof.  Since half of house will have vaulted ceilings from existing scissor trusses, seems like the right thing to do is to un-vent the roof, seal up the attic, and make it conditioned space.  This will be good for the ductwork that will have to be in the attic.  2015 IECC code is R-49 for roof and 3 ach @50 Pa.  Joe Lstiburek suggests R-60 roof insulation and 1.5 ach.

·       What are your suggestions for the most practical and cost-effective way to achieve this level of insulation and air tightness?

·       So far, my current thinking is to put 4” (2 layers of 2”) foam board on top of plywood decking, cover it with water control membrane, attach furring strips on top of that (for roof attachment & ventilation air gap), then install the metal roof. On the inside, spray 2” closed cell foam on bottom of roof and to seal the roof/wall gap.  Then get the remaining R-value based on lowest cost through either (1) more spray foam, (2) blown in cellulose, fiberglass or rock wool, or (3) install batts of fiberglass or rock wool.   Does this approach seem sensible?  Am I  missing any other considerations?

·       I am relatively indifferent to which approach seems “best”.  Should cost then be the only decision factor?

·       I am a bit concerned with spray foam due to potential offgas and unknown long term health effects.  Also, because of cellulose settling, the initial lower cost may not be real .  Am I over thinking this?

·       If I do not use spray foam, to improve the sealing for the transition from wall to roof, should I cut off the overhangs (“monopoly” house) and then air seal and insulate the wall and roof together so that the roof & wall seals together  and insulation are joined; then build the overhang/lookout above the roof deck?  Or would it be more cost effective and simpler to just just seal up the overhang area with spray foam?  Again, Am I over thinking this?  Can anyone comment based on experience?

This ended up being much longer than anticipated.  Thanks in advance for any feedback/help/advice you can provide.

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  1. Expert Member
    NICK KEENAN | | #1

    I would insulate on the inside. Concrete is very conductive and trying to keep it inside the building envelope you're going to be fighting a lot of thermal bridging.

    Forget about "thermal mass." It's not going to do you any good.

    I don' t know why mass walls only require R8, but I have a suspicion. Lstiburek talks a lot about how things in the code are often driven by practicality. I don't believe that mass walls have any superior thermal properties, but they are hard to insulate. R8 is two inches of foam, which is about the practical limit without elaborate fastening.

    On the roof, you don't need both exterior foam and interior spray foam. Foam is relatively expensive. I'd go with exterior foam board and then fluffy insulation filling the rafters.

    The thing to be careful of is keeping your building envelope continuous. The entire building should have four layers around it: rain barrier, air barrier, vapor barrier and thermal barrier. Each layer should be continuous around the whole building, you should be able to trace it on a plan of the building without lifting your pencil off of the page. The tricky parts are where surfaces meet -- where walls meet the roof and where walls meet the foundation.

  2. Expert Member
    Akos | | #2

    Net zero remodel generally means high levels of insulation. 2" is defiantly not there, 4" is probably still on the low side.

    Thick assemblies with foam get complicated and expensive. I'm with DC on this either insulated from the inside with batts/dense pack or from the outside with Larsen truss type of setup. Since you don't have big overhangs, you can also split the insulation a bit and install enough rigid insulation on the outside for condensation control and the rest as batts on the inside.

    If you are insulating from the inside make sure to treat the CMU as a masonry veneer construction. This means a gap between the CMU and the framing with a WRB in between.

    It is hard to say how well the CMU is air sealed as a lot depends on how well it is build and if the cores are grouted. Generally you need to air seal CMUs, this can by parging it or installing an air barrier (specialty membrane, taped drywall, taped rigid insulation).

    Cheap high R value roofs generally mean blown in loose fill insulation with a vented roof above. Unvented roof can be built but getting similar R values gets very expensive and there is usually no ROI by insulating the roof above code minimum. Since you are in milder climate, you don't need all that much exterior rigid insulation, so a hybrid roof with 30% of your assembly R value as rigid above the deck and the rest as batts in the rafters underneath is pretty simple build. Skip the SPF over the whole roof, not much benefit in this case, you only need a little bit to seal the transition from your walls to the roof.

    EIFS or 3 coat stucco depends on local availability. Both can work great over rigid insulation, there are no issues with weight if the correct fastener schedule is used.

    Both can be installed with bad details that can cause lot of issues and be very expensive to fix. Either way you go, make sure your contractor pays attention to water management details and follows all manufacturer details precisely. Lot of time something simple like incorrect flashing around a window or a missing weep screed can cause the entire wall to fail, not something to be taken lightly.

    1. acwei | | #5

      Akos & DC, thank you for pointing me in the right direction. For the walls, I think the least cost approach will be dense pack cellulose on the inside. To get depth, I will be installing an interior 2x4 @ 24"OC wall (need it anyways for hanging drywall), leave a 3-1/2" gap, and then fill all 7" with cellulose. How would you prevent moisture from getting into the cellulose (from concrete block, or wicked up from concrete floor)? You mentioned WRB in the gap...what WRB material would you recommend? Would it make sense to use foam for this purpose?

      1. Expert Member
        Akos | | #6

        I would read through this:

        Since you are in warm enough climate, you can get away with all fluffy on the inside as long as you deal with bulk water on the exterior.

        Going with a hybrid insulation assembly, either bit of spray foam on the inside or exterior rigid on the outside combined with dense pack is generally a more robust assembly. Just make sure you keep the ratio of rigid/SPF to fluffy as per the table here:

  3. acwei | | #3

    DC & Amos, thank you very much for your comments. I would like to avoid inside wall insulation in order to minimize interior space and the nominal benefit of wall mass. The insulated Larsen truss approach is interesting... any thoughts on cost/hassle differences between Larsen (with R20) vs. 4" XPS (R20)? Would it make economic sense to go to Polyiso in order to get the additional R4 on outside? Would EIFS then be the best option in order to avoid the additional cost of exterior wall framing or additional fasteners to support the weight of Hardie or traditional stucco?

  4. Expert Member
    NICK KEENAN | | #4

    I would think of the concrete wall not in terms of mass, but bridging. There really isn't any efficiency benefit to mass. When you do a Manual J, they don't ask you to put your house on a scale and enter its weight.

    There is however a real cost to bridging. Masonry is an excellent conductor of heat, if some of it is going to be inside the envelope all of it needs to be inside the envelope. Which means the entire exterior needs to be insulated. This makes places like where the wall intersects the roof tricky, especially if you're dealing with existing construction.

    For a net-zero house I would think you want to be closer to R-40 than R-20.

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