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Community and Q&A

Insulating an old cinder-block building

SOTASOTA | Posted in General Questions on

Hello! I have a 1975 building that I am renovating. It is primarily a cinder/masonry block building.

The blocks are standard 16″ x 10″. There is no insulation in the cavity (vermiculite) of the blocks.

I am in Penticton, BC Canada which is Climate Zone 5. I wish to properly and cost-effectively insulate the walls. We have a Building Permit issued for the renovation, which says we have to conform to 2012 BC Building Code rules. However, after much research, if I insulate based on the 2012 code, there stands a high probability that the walls will condense moisture and cause rot/mildew/mold.

I need to know the proper way to insulate while still adhering to 2012 Code.

I was thinking of the following in the order of assembly:

[Exterior Latex Paint over Cinder Blocks] [Cinder Block] [2″ XPS FOAMULAR R-10 Rigid Insulation glued to Cinder Block] [2×6 framing 24″ O.C.] [R-19 Unfaced Fiberglass Insulation Batts] [Drywall] [Latex Paint]

There will be NO vapour barrier to allow the assembly to breathe. This is based on Building Sciences article BSD-106: Understanding Vapor Barriers. Please see attached wall assembly.

The latex paint will act as a Class III Class III Vapor Retarder.

1) Is this a good approach?
2) It is not per the 2012 Code, but if I do not do it this way I can potentially create a sick building.
3) Is there a better way?
4) I do not want to use Closed-Cell Spray Foam as it is too expensive and toxic.
5) How can I convince the City Inspector that I must build the walls properly even though it does not meet 2012 Code?


Thanks in advance!!!
Russ 🙂

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  1. iLikeDirt | | #1

    If your cinder blocks are currently exposed to the elements, or if your plan was to leave them this way, it's much simpler and easier and probably cheaper too to simply wrap the exterior of the whole building in rigid foam or rigid mineral wool than it would be deal with complication of your plan. This will also avoid a huge amount of interior disruption, loss of floor space, expense of a big drywall job, and reduced durability from adding a whole bunch of wood, gypsum, and paper, all of which are far more moisture-sensitive than masonry.

  2. SOTASOTA | | #2

    Thanks but the outside block has to remain as is. The building is gutted from the inside, so no disruptions. So, I have to find a good interior solution. :-)

  3. GBA Editor
    Martin Holladay | | #3

    Nate is correct: exterior insulation is always preferable to interior insulation.

    If you are forced by circumstances to insulate on the interior, your can. If you want to install 2 inches of XPS insulation, just tell your building official that you have a vapor retarder. The 2 inches of XPS has a permeance of 0.55 perm, making it a Class II vapor retarder.

  4. SOTASOTA | | #4

    Thanks Martin! That really helps. :-)
    Now, is it superfluous to add additional fiberglass insulation? Is the 2 inches of XPS better since the cinder block's temperature would be higher, helping to avoid freeze/thaw and condensation?

    Or, OK to add the extra fiberglass insulation for lower heating/cooling costs?

  5. Expert Member
    Dana Dorsett | | #5

    It would be much greener to use 1.5"-2" foil faced polyiso rather than 2" XPS, and use R21 fiberglass or R23 rock wool to hit similar or better performance.

    R19 batts only perform at R18 when compressed into 5.5" stud cavities, and aren't very air retardent. R21s & R23 rock wool are more than 2x the density, and perform as-labeled and are far more air retardent. R23 rock wool is more fire-proof than R21 fiberglass, and is a generally better product on multiple grounds.

    XPS uses more polymer per R and has a huge greenhouse gas footprint due to the HFC134a blowing agent (1000x CO2), compared to polyiso, which is blown at lower density using pentane (~7x CO2.) The labeled R value of 2" polyiso is about R12 R13, but in winter will only perform at about R10-R11 in your climate. But even 1.5" is sufficient for dew point control in 2x6 framing in your climate. Polyiso also has a higher kindling temperature, and chars in place rather than melting into pools of flaming plastic the way polystyrene does.

    Normally when the exterior is masonry exposed to the elements it's best to leave a gap between the masonry and the next layer as a capillary break and a space for the block to dry into, venting it to the exterior with weep holes at the bottom and small vents at the top. The gap can be achieved gluing 2" x 2" blocks of 1/2" foam board to the larger sheets of foam in a 16" o.c. grid using dabs of foam board adhesive then gluing the whole thing to the CMU.

    Without the air gap the moisture content of the CMU will be higher, and although the Okanogan valley is a lot drier than on the foggy-dew western slopes getting far less direct wetting, a higher moisture content in the CMU can cause the exterior paint to fail.

    Your stackup as-described exceeds IRC 2012 chapters 11 and 7 for climate zone 5 (R19+ 10 is considerably better then R13+5 code-min), but using higher-R batts and 1.5" of polyiso would meet it on both the letter of code and performance basis.

    With 2" polyiso and R20+ batts it would meet both the letter and U-factor performance for zone 6.

  6. SOTASOTA | | #6

    Wow! Wonderful information Dana!!
    Questions as I digest the post...the 2012 Code stipulates a vapour barrier. I am hoping to get the Inspector to acquiesce and allow the XPS to be the Class II vapour barrier.
    1) Does polyiso provide any kind of a vapour barrier?
    2) If I "have to" use 6 mil polyethylene for the vapour barrier between the batt insulation and the drywall, is this a bad idea? Again, I have to convince the Inspector of the science behind what I am proposing.
    3) With regards to the weep holes, is there a size and spacing for these?

    Thanks so much!!

  7. Dana1 | | #7

    The foil facer on foil-faced polyiso is a Class-I vapor barrier, as vapor retardent as 10 mil polyethylene. You have to have a sufficient R-ratio of foam-R/total-R to rely only on the interior latex paint as the vapor retarder, and the minimum foam-R is spelled out in the table in Chapter 7 of the IRC under R702.7.1 :

    In Zone 5 with 2x6 framing it only takes R7.5 (minimum) on the exterior side to use latex as the vapor retarder. With 1.5" of polyiso you have 9-ish (7.5-8-ish really, derated for mid-winter temperatures, but enough). With 2" of polyiso you'd have R12-R13 (R10-R11-ish fully derated).

    With 2" of XPS you'd have R10 nominal, which increases to R11+ at the mid-winter average, but decreases over a few decades to about R8.5-ish nominal as it loses it's blowing agent.

    Weep holes and vent holse can be 1/4" round holes in the mortar lines every 2-3 feet, or cored 1-1.5" holes with screened plugs every 5-6'. The weep holes shouldn't be on the very bottom horizonal mortar line, since they're more likely to get plugged by debris there. An inch or two up on a vertical mortar line works. Even though "weep" is the name, and it does allow an escape path for liquid water, it's primary function is as an air inlet to allow convection to continuously displace the cavity air.

  8. SOTASOTA | | #8

    You are an amazing resource Dana. I feel like I should be paying you! :-)
    Please let me know if I need to!
    1) OK, with the air gap, how would I attach the 2x6s to the foam and make the 2x6 framing? Without the air gap, I can see driving concrete nails though 2x4 braces and attach the 2x6' to the braces. But with the air gap, I cannot do this as the foam would deform and break as there would be no solid surface behind it.
    2) Are there any pros/cons to using steel-studs over wood?
    3) With the foil-faces polyiso as a Class I VB, do I seal all joints? With tuck-tape, silicone/caulking?

    Seriously Dana, I am happy to pay for your time if I ask too many questions. Let me know please!

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