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Do I need interior ventilation holes in hollow concrete block walls?

Motorapido Member Posts: 237
My renovation contractor recommended that I drill vent holes along the top perimeter of the house’s concrete block (CMU) walls from the inside of the house, penetrating just the inside of the concrete blocks, to allow interior air to enter the block cores for ventilation. In the renovation, 2 inches of foam board insulation was applied to the exterior of the entire house’s walls, caulked and taped at all seams, with furring strips on top and wood lap siding installed on top of the furring strips. The interior walls are currently bare concrete block, but the contractor will be 2x4 framing out interior walls and installing shiplap boards on the interior perimeter walls. With the continuous foam exterior insulation, the building must dry to the interior, and there will be no interior wall insulation (or interior vapor barrier, of course). The contractor suggested that as moist interior air migrates into the concrete blocks, it will become trapped in the cores and precipitate, causing moisture buildup inside the concrete block cores. He suggested drilling holes between each joist at the top of the concrete block walls on the interior of the walls, to allow any trapped moisture in the block cores to dry out, and also to create air flow upward into the roof and out the ridge vent.

The house location is northern Pennsylvania, deep in the woods, about 10 miles south of the NY state line, Zone 6 I believe -- so cold winters and warm/hot/humid summers.

I question the contractor's recommendation. I have so very carefully air sealed all penetrations through the block walls before this construction began and I can’t imagine drilling all these holes, which will allow my conditioned interior air to get sucked into the block wall cores and get sucked up and out of the ridge vent. Goodbye to my carefully air-sealed envelope!

However, I am an amateur, so I am seeking expert advice. The contractor does great work, but he is not a green building scientist, so I’m double checking his recommendation.


  • Jamie Hall
    Jamie Hall Member Posts: 17,132
    Your contractor has a very good point. He may not be a green building scientist, but he's a good contractor. The problem is moisture -- as he has pointed out. Concrete block (or worse, cinder block -- but both) is not vapour tight, nor can it be made vapour tight without a continuous membrane vapour barrier. Therefore moisture from the house will migrate, albeit slowly, through the interior finishes and the interior side of the concrete block -- where it will quite happily condense and accumulate ether on the inside of the cavity on the cold side, or continue to migrate through the block in warmer months and accumulate on the outside of the block under the insulation.

    This you do not want.

    But there is a potentially related matter: you want to have at least two and preferably four air changes per hour from outside air into the conditioned space. Any less and your indoor air quality will be somewhere between very bad and truly awful. I hope that you have made provision for that. The amount of air which will go our through that block might be regarded as a useful contribution to that.

    An alternative which is often used is to fill the cavities of the block with insulation which will not absorb moisture. Vermiculite is not a bad choice.

    Be very careful to ensure that the bottom row of block is separated from the foundation with an impermeable membrane, and that if any of the block is below grade that that is sealed against any soil also with an impermeable membrane.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,322
    I think you should completely seal the basement and crawlspace, vapor barrier and all.
    Then you should use a whole house dehumidifier along with your AC to control the moisture.
    The whole house humidifier should have a duct going to the outside, to allow your fresh air exchanges needed, and should have a return in the part of the house where the most humidity will reside.
    When the AC isn't running, and the humidity goes up (shoulder seasons, rainy mild summer days) the dehumidifier will run, keeping your humidity under 50%.
    The dehumidifier only runs when the AC doesn't. A separate return, plus returns on all your duct from the AC unit, go into the dehumidifier. The output of the dehumidifier is upstream of your AC supply. You don't run the dehumidifier and push the air over the AC coil.
    The control strategy is pretty standard.
    rick in Alaskakcopp
  • Zman
    Zman Member Posts: 7,001
    Your exterior insulation is likely closed cell, so it is a vapor barrier. 2" foam board is probably R-10 so you should be concerned about the dew point on the interior surface of the foam. I am not sure if you need the holes or if the block will breathe enough to remove moisture, I don't see any harm in the holes. Do not put another vapor barrier on the inside of the wall or you will surely have an issue. If you want to control air movement on the interior surface, use a breathable membrane
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • heathead
    heathead Member Posts: 160
    I would worry about the holes in the top block and radon gas.
  • Jamie Hall
    Jamie Hall Member Posts: 17,132
    heathead said:

    I would worry about the holes in the top block and radon gas.

    Radon from concrete or cement block would be VERY unlikely, unless someone decides to use granite for the aggregate --which would be really dim.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • heathead
    heathead Member Posts: 160
    I am talking about radon coming from ground under slab and ground soil around house. In radon mitigation systems they plug all the holes in block walls and do a sub slab suction. Just a thought on why I would not poke holes in the block.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,291
    Hi, Building Science has a number of searchable articles here: I'd be surprised if they didn't have something to say about this. o:)

    Yours, Larry
  • rick in Alaska
    rick in Alaska Member Posts: 1,244
    This seems very confusing to me. If you don't want moisture from inside the house to get in to the wall, then install a sealed vapor barrier on it. In almost all houses here, it is typically a solid poured wall using foam forms that are R10 on the inside and outside, with a vapor barrier on the inside, and the outside has a breathable house wrap on it.
    You can go on the Alaska Cold Climate research site and check out some drawings they have put together. They have a lot of scenarios to be used that might match what you are using.
  • Motorapido
    Motorapido Member Posts: 237
    Thanks for all the comments and insights. Here is a little more info on the overall project that might provide helpful context.

    This is a renovation of a 70-year-old two-story cottage. It's concrete slab on grade. No crawlspace or basement. Just slab on grade. Exterior dirt ends a few inches below the top of the slab. Concrete block walls outside and inside. Blocks are hollow inside and were not filled with concrete or any insulation material.

    From outside-to-inside, the walls will have SP Smartside engineered wood lap siding, installed on a rain screen composed of 2x4s on top of 2 inch 4x8 foam boards, continuously applied flush to the entire exterior of the block walls, caulked and taped at seams.

    Inside, the contractor applied a 15 mil vapor barrier on the entire slab, generously overlapped and taped with vapor barrier tape at the seams, and stuck to the walls one inch up from the slab with butyl acoustic caulk/sealant. This vapor barrier will trap all moisture rising up through the slab and prevent it from entering the house.

    Interior walls will be 2x4 framed, with tongue and groove pine boards for the wall surfaces. The contractor is still debating if adding unfaced insulation batts in the walls would be beneficial or potentially harmful from the standpoint of where it would push the dewpoint. Seems that adding batts inside the framed walls would pull the dewpoint inward, potentially inside the hollow concrete block cores, which would not be good, since vapor would condense there and trickle inside the block cores.

    The second floor of the house is essentially a finished attic. The floor joists are 2x6, which travel into pockets cut out in the block walls. Where the joists enter the pocket holes in the blocks, the gaps have been carefully sealed with spray foam. The upstairs ceiling is currently just OSB panels. Above the OSB are faced fiberglass bats. The roof has a nice overhang with ventilated soffit panels and a ridge vent.

    The contractor's theory is that we need to open holes in the block walls (holes drilled from inside of house just into the block hollow core and not puncturing the exterior face of the block). These holes would be between each second floor joists and his theory is that this will create an opportunity in winter for warm, moist interior air to get pulled into the holes to be drilled into the block walls between the joists, and that air can then flow out into the roof overhang where it will get pulled up through the baffles in the insulation and vent out the ridge, thus preventing moisture build up and precipitation inside the blocks.

    What baffles me is why did we foam seal the gaps in the pockets where the second floor joists enter the block walls, only to then drill new holes in the block between those joists? And purposely creating holes for my warm air to exit the building seems like the opposite of what I would want to do. Even without drilling holes in the block, wouldn't any moist air trapped inside the cores still be able to flow upward and find ways to leak into the eaves and up and out the ridge vent?

    Please shed some light on my murky thinking.
  • Jamie Hall
    Jamie Hall Member Posts: 17,132
    One really needs to go right straight back to square one on these things -- which too many "green" building advisors don't seem to be able to do.

    Which means, what, exactly, are we trying to do here? If one is starting from scratch, it's pretty clear. One wants as much of the thermal mass as possible to be inside the envelope, and heavily insulated on the outside. One wants the vapour barrier to be as well sealed as possible, and on the inside -- the warm side (note that if one is in a hot humid environment, the rules are different). One wants an infiltration barrier which is breathable to moisture -- such as Tyvek -- on the outside, just inside the outside finish sheathing whatever that is. That must also be well sealed (window and door penetrations are always a problem -- but what usually gets left out is penetrations for outside faucets and power outlets, or air conditioning and heat pump lines and the like).

    Then one wants to make separate, controlled provision for two to four complete air changes per hour. Since one is trying for thermal efficiency, that means that the air changes are made through a heat recovery ventilation system (sensible heat only!).

    Now... you have an existing structure, which always makes life much more interesting. Though the principles are the same. In your situation, I would ask for a vapour barrier on the outside of the concrete block walls, but since you already an outside finish on there... not sure wat to do about that. Extending this up into the conditioned spaces of the second floor is going to be interesting, but you need to do it. You can put whatever finish wall assembly you want on that -- but no insulation. You want that concrete block to be on the conditioned side, to act as thermal mass. Your 2 inches of foam on the outside gives you an R value of about 10. Had I been doing it, it would have been 3 and a half inches -- almost R 20 -- but what is is what is, and you will have to live with that. Hopefully the Rain Shield to which you refer is also an infiltration barrier; in any event, pay close attention to how it is sealed at any penetrations.

    You want to continue the vapour barrier over the second floor ceiling OSB boards, then your fiberglass batts on top of that (don't count on the facing to be a vapour barrier -- it isn't). You need 10 inches of fiberglass. If parts of the second floor are up against the roof, put as much insulation outside the vapour barrier as you can, but you may be limited by the rafters, and be sure that there is space between the insulation and the roof for air circulation.

    That's sort of generic, but the general idea. I don't think holes in the block will make much difference.

    An alternative would have been to put all the insulation on the inside of the block -- at least R20, so 3 and a half inches (2x4 nominal) or better 5 and one quarter inches (2x6 nominal) -- but you already have insulation on the outside...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    rick in Alaska
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,322
    When it comes to this, your contractor is a dunce with his theories.
    You can either take the advice and look at the links provided, or do what he thinks is best.
    A local HVAC contractor who understands these things would be the one to hire, and who’s advice you should take.