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Should I insulate between outer block and inner stud walls?

indenturedcontractor | Posted in Energy Efficiency and Durability on

Like so many others, I have been given conflicting advice from “experts” in the insulation industry and I am now completely confused. I am building a two story home in zone 3 (montgomery, alabama). The external walls were done with concrete masonry units with cavity’s filled with pumped mortar. We then came to the inside and completely framed leaving 1/2 inch gap between framing and exterior block walls. We eventually plan to coat exterior of block walls with spray on portland, but that could be in all honesty at least a year down the road. We heat with an unvented gas fireplace and cool with window units for now. Future plans include Vogelzang wood burning firebox and minisplit HVAC system. ceiling heights are 10’4″ downstairs and 8’9″ upstairs with a 6/12 pitch aluminum roof laid over 3/4″ decking. Now to the actual question: Should I insulate any of the exterior framed walls with either batt or closed cell foam with or without a vapor barrier OR would I be better off just leaving a dead air space behind the sheetrock to allow the moisture to vapor out. Changing out the gas fireplace and all the moisture it produces is not an option.

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  1. GBA Editor
    Martin Holladay | | #1

    At the end of your post, you brought up issues related to your unvented gas fireplace. You mentioned that you want "to allow the moisture to vapor out" because of "the gas fireplace and all the moisture it produces."

    Here's my advice:

    1. The best way to proceed would be to remove the unvented gas fireplace and to install a different type of heater. Unvented gas appliances release moisture as well as combustion byproducts directly into your living room. These appliances are bad news. For more information, see these articles:

    Avoid Unvented Gas Heaters

    A Ventless Gas Fireplace Doesn’t Belong in Your Home

    In some U.S. jurisdictions, these appliances are illegal.

    2. If you disregard this advice and keep the unvented appliance, you still need to remove the moisture and combustion byproducts released by the fireplace, as you mentioned in the last two sentences of your post. The moisture and combustion byproducts aren't going to escape your house through your concrete-block walls. What you need is a ventilation system -- ideally one that brings fresh air into your house, and exhausts stale air.

    Here is a link to an article that explains what you need to know about mechanical ventilation systems: Designing a Good Ventilation System.

  2. GBA Editor
    Martin Holladay | | #2

    Now I'll address the question of the best way to insulate your home's walls.

    I am duty-bound to tell you (and other GBA readers) that you should have figured out a plan for wall insulation before you began to build your house. Now that your house is half built, it's a little late to address the issue. For more information on this issue, see Plan Ahead For Insulation.

    The best way to insulate a concrete-block wall is to install rigid foam on the exterior side of the wall. If you follow this advice, you can install any type of siding you want (usually on vertical furring strips screwed through the foam to the underlying concrete blocks), or you can install synthetic stucco (an approach called EIFS).

    Ideally, you would have planned ahead for this, because this approach may require different types of flashing than you chose to install at your windows.

    If you want to install insulation on the interior side of the walls, you should have installed a continuous layer of rigid foam on the interior before you framed up your stud walls.

    Now that your walls are framed up, your options are limited. The best approach would probably be to install spray foam (doing your best to make sure that the spray foam fills the 1/2-inch gap between the studs and the concrete blocks).

    Other approaches are possible, including the use of mineral wool batts or fiberglass batts, but they won't perform as well as the other approaches I mentioned.

  3. Expert Member
    Dana Dorsett | | #3

    IRC code min for zone 3 wall is 2x6/R20 or 2x4/R13 + R5 continuous insulation, or continuous R13 on the interior side of the CMU, or continuous R8 on the exterior side. Alternatively code can be met with a wall that calculates to be less than U0.098 (R10.2 "whole-wall", with all thermal bridging & R-values of each layer factored in, including interior & exterior air films.)

    Fiber insulation needs both interior exterior air-barriers, and using the CMU as the exterior air barrier is risky, since it wicks moisture. Open cell foam in lieu of fiber insulation might work, but it's extremely vapor open, which could create problems for the interior finish wall, especially during the cooling season. But you also need sufficient depth.

    If the framed walls are 2x6 you can get there with a flash half-inch of closed cell foam on the CMU to bring the total depth to 5.5" or less creating a non-wicking air barrier that is also vapor retardent (about 2 perms), then filling the framing with R20 batts.

    If they're 2x4s you're kind of stuck with insulating on the exterior to hit code-min, short of adding some Bonfiglioni insulating strips to the framing or a Mooney wall to get the extra depth.

    Leaving the cavities empty with the half-inch gap still in place violates fire codes in many areas. With the gap fire in the wall moves too rapidly to the attic unless there is a fire-block preventing it.

    For an all exterior approach to code-min, 2" of EPS and an EIFS finish is pretty straightforward, and a common enough method used in commercial building construction. But it may be comparable or cheaper to find a source of reclaimed 1.5-2" roofing polyisocyanurate, mounting it to the wall with 1x4 furring through-screwed to the CMU with 4" masonry screws, and mounting some sort of siding to the furring. Used foam is really cheap- 1/4 - 1/3 the cost of virgin stock goods, but the siding is more expensive than EIFS, and there may be a dearth of local people experienced with this approach.

  4. indenturedcontractor | | #4

    thank you. I am aware of the problems that I "added" by going with the unvented propane logs, but due to location, it is next to impossible to exhaust. The only plus is mild winters (generally run less than 200 hours per winter) combined with 3500 square feet floor space (33,000 cubic ft. indoor air space) means there is not "excessive humidity" in indoor air. I mentioned it strictly to show that we currently do not have a heat source that actually dries out the interior air. This home has obviously not been built to modern energy efficient "tightly insulated" standards, and my greatest fear at this point is I don't want to compound the problem by trapping humidity indoors. We do have a whole house fan installed if needed and use it alot in the warmer months, but obviously would defeat the purpose of heating during the winter. If this additional information does not change your opinion, then i will stick with your advice of spray foam insulation on interior of block walls.

  5. GBA Editor
    Martin Holladay | | #5

    You wrote, "We do have a whole house fan installed if needed and use it a lot in the warmer months, but obviously would defeat the purpose of heating during the winter."

    A whole-house fan is used for cooling. It is not designed for ventilation. You're right, of course, that a whole-house fan would "defeat the purpose of heating during the winter" -- which is why you need a ventilation appliance like an HRV or ERV instead of a whole-house fan.

  6. Expert Member
    Dana Dorsett | | #6

    The humidity is the least problematic of the combustion products, but long as you have active ventilation sufficient to deal with the humidity and other combustion products of the ventless fireplace indoor air quality can still be maintained. In an air tight house it's best to use a balanced ventilation approach (ducted HRV or ERV, or few paired ceramic-core single room HRVs such as the Lunos system.)

    What is the rafter depth? Is there any rigid insulation above the roof deck? Is there a thin attic space up there? IRC code-min is R38 between rafters, which can work with a vented roof with 2x12s using high-density R38s. If they are 2x10s you can get there un-vented using a flash-inch of closed cell foam on the underside of the roof deck and a compressed low-density R38. If there's an attic space that has no ducts of sufficient depth the cheapest way to go would be blown cellulose on the attic floor.

  7. indenturedcontractor | | #7

    in response to dana dorsett (sorry, did not see your post until after i sent my last response) I half-***ed planned ahead for insulation. My mistakes are from a lack of knowledge, not effort. The indoor framing is 2 X 6 as I originally planned for closed cell insulation. the exterior finish, as near as I can explain, was to be a variance of the old PM-EIF but could be better classed as a hybrid one coat as it would entail using fibered portland with a liquid waterproofer added and applied as shotcrete (sp) directly to to cmu wall with physically attached mesh. the waterproofer additive claims to not interfere with bonding of the portland with the block wall. I realize this does not ensure complete water proofing and my understanding is that it does not add signifitantly to the R value. It was mainly aesthetic and to provide some protection from wind blown rain wicking through unfinished block. This was actually part of the same discussion as vapor barrier / no vapor barrier between block and framed wall. A local poured concrete home built in the early years before the invention of ICF forms put a latex membrane (thicker than paint) on their exterior along with batt insulation and created a sweatbox. Trying not to duplicate their mistake.

  8. Expert Member
    Dana Dorsett | | #8

    With 2x6 framing you can go ahead and give it a flash half-inch to inch of closed cell + batt approach,applying the closed cell foam to the CMU to where it leaves 5.5" or less of framing cavity depth, then compressing in some R20s (or finishing out the fill with open cell foam). It's greener to go with an HFO-blown closed cell foam rather than the more commonly use HFC blown foams, but a full cavity fill of closed cell is an expensive waste due to the thermal bridging undercutting it's high R/inch performance.

    Closed cell foam is waterproof to liquid water, open cell is not, and closed cell foam runs about 1-perm vapor retardency at 1" thickness, whereas 6" of open cell would be on the order of 10 perms, about 2x as vapor permeable as interior latex paint. While it might be tempting to just do it all with open cell foam all in one pass, it's riskier.

  9. indenturedcontractor | | #9

    Thank you both. Just looked up HRV system. Once again, ignorance on my part. didn't even know it existed. Looks like exactly what I need. In response to roof and attic questions, I will try my best. I do not lack for attic space. 32' interior span with trusses (this is from inside to inside of block walls. 6/12 pitch. Over 10' clearance at peak. 2' clearance where truss crosses interior wall framing. 2' X 2' gable vent at both ends of house (54' interior length) PLUS gables are also block and stairstepped to meet roof so I wound op with continuous venting where gable soffit meets gable wall. Sorry, I am not very good at explaining. Hope I gave you a clear picture. Will probably have to go with cellulose or blown in pink stuff but already plan on going as deep as possible. Underside of trusses have the styrofoam eggcrate barriers to provide air channels between each truss and separate it from attic proper. roof deck has no rigid insulation. Georgia Pacific 3/4" decking with ice and water shield applied and then metal roof attached. i am not even sure if I answered what you were asking.

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