Insulating a double bricked house
October 29, 2014 7:43 AM   Subscribe

Is it a bad idea to fill the air gap in between double brick walls? Our 100 year old house is in need of some insulation so we had a couple of companies give us a quote. One company recommended spraying Air Krete (a kind of foam insulation) in the gap between our double brick walls. The other company said this was a horrible idea and could damage our house! They said to rip out the plaster & lathe and install interior insulation.

The second company said that the gap in between double brick walls can serve to wick moisture away from the brick and that filling it could cause damage over the years. Is this true or is he just trying to scare off the competition?
posted by krunk to Home & Garden (14 answers total) 1 user marked this as a favorite
One question is your wiring - is it knob and tube (or was it ever) and is it in between the two brick walls? Often when re-wiring old houses the old wiring is left in place and isn't all disconnected. If that's the case you could have a fire hazard with blown in stuff depending on what it is. I am not an expert but did find live wires years after we had our 100+ year old house re-wired.
posted by leslies at 7:46 AM on October 29, 2014 [1 favorite]

Response by poster: Our house used to be knob and tube, but it's not in-between the brick walls. As far as I know, the air gap is empty and all wiring was done in the interior walls.
posted by krunk at 7:55 AM on October 29, 2014

As anecdata - my parents had some kind of foam insulation put into the cavity between the double brick layers and no problems ensued in the following 20 odd years.
posted by crocomancer at 7:57 AM on October 29, 2014

Not a mason but I do have a passing familiarity with this, thanks to also owning a 100 year old brick structure.

That airspace between the two courses of bricks is there to provide circulation and moisture control. Bricks are porous and will absorb moisture (from outside when it's humid out and inside the other months). Filling that space in doesn't allow the brick to ever dry and will cause problems with degrading brick and mortar and leave you with large repair bills down the road. Insulating behind the plaster lathe is the right, albeit more expensive solution.
posted by moitz at 7:57 AM on October 29, 2014 [8 favorites]

Response by poster: Moitz: thanks, that's exactly what the 2nd contractor said and is my main fear. How is it allowed if there is such potential for brick degredation?
posted by krunk at 7:59 AM on October 29, 2014

Another answer. Cavity wall insulation is ubiquitous and unproblematic round where I live, but your circumstances may vary.
posted by Segundus at 8:04 AM on October 29, 2014 [2 favorites]

I should have qualified my answer with saying that if there's some insulation which provides moisture wicking capabilities (think like a base layer for athletic clothing), it might be usable there. There may however be code issues requiring air space, so I'd check on that also.
posted by moitz at 8:04 AM on October 29, 2014 [1 favorite]

Not a fire hazard. Air Krete is non-combustible.
posted by Bruce H. at 8:12 AM on October 29, 2014

Response by poster: Yeah, we had a house fire last year so the non-combustibility of Air Krete is a big win for us. I'm more concerned about the moisture damaging the brick.
posted by krunk at 8:22 AM on October 29, 2014

You shouldn't have to rip out the plaster. You can blow in cellulose insulation through interior plaster walls. The holes are drilled in each bay, top and bottom. Patching old plaster is a learned skill however.
posted by Gungho at 8:49 AM on October 29, 2014 [1 favorite]

Best answer: krunk,

Unfortunately, the short answer is "it all depends". Condition of brick, condition of mortar, density of brick and mortar, wall orientation, solar access, shading and amount of eaves and drop on the house can all factor into whether or not an old brick wall will suffer from the addition of insulation.

In simple terms, moisture follows the movement of heat. In your climate that is mostly from inside to outside, and there is a fair amount of heat flow given the uninsulated walls. This drives moisture back out of the wall, your bricks are happy. Adding any insulation, at any location in the wall, can potentially change this. So, you are assessing degrees of risk. A relatively modest amount of insulation on the very inside of the wall is lower risk than that same insulation in the middle of the wall because it is less likely to trap moisture being pushed in through the outer brick (from driving rain, capillary action or sun on the wall on a cool day). This moisture can theoretically hit the air space, diffuse and then be pushed back out of the wall later. A quick look at AirKrete's literature shows a perm rating of 0.15 which is about 20% of that of brick and even more different from mortar, so in theory, water could form on the cold side of it and freeze. Whether this would happen in practice depends on the many factors I listed above. Amounts of insulation matter too. For example, it could be that a 1" AirKrete installation is not as bad as a 3" foam insulation on the inside in terms of risk (although it would also be 1/3 the total insulated value).

There is a process for knowing this with greater certainty called hygrothermal modeling, but it is highly unlikely contractors could employ it. I'd suggest you ask for local example projects of the same construction type over five years old and go out and really poke around at them a bit. Or, ask about for a local architect or engineer who can help you make this choice.
posted by meinvt at 8:59 AM on October 29, 2014 [4 favorites]

What the second contractor said can definitely be true.

If you live in a cold climate, the greatest worry for you is moisture transport during the winter. Moisture from the warm interior of your house will slowly migrate through the wall assembly toward the drier cold exterior. Ideally, this will be slowed down by a vapor barrier (some type of plastic or foil layer) on the inside surface of your walls. If not, it may condense in the bricks, or on the exterior surface of the interior brick layer.

The double bricked construction you mention is especially effective in warm, wet climates, where moisture outside tries to migrate inward. Ideally, it condenses on the outside surface of the inner brick wall (especially if there's a vapor barrier installed there), falls downward, and drains out a weep hole at the base of the wall.

One last consideration is physical moisture movement (by rain and wind). Wind can push rain or snow through any cracks in the exterior brick wall. If there is no cavity and weeps at the bottom, the water gets trapped in the wall, and can start to degrade its interior.

So I can't say for sure whether your specific house would be alright with the air gap filled in, but in general, it's not a great idea, moisture control-wise. One added benefit of installing insulation in the interior would be that this will often include a vapor barrier, which, in a cold climate, would be ideally positioned to help control moisture even further.
posted by BevosAngryGhost at 8:59 AM on October 29, 2014 [2 favorites]

Response by poster: Thanks for the answers! I'll try to find a local building scientist who has worked on similar houses. Getting the cavity filled is much cheaper than interior insulation, but not if it causes long-term degradation.
posted by krunk at 9:05 AM on October 29, 2014

Put a price on what it is insulating the walls will do for you. Than consider if you feel the cavity, (which is there for a reason,) you are not going to be able to unfill it if thinks go south. The house is a hundred years old and hasn't had its walls insulated in all that time. I find that it often makes sense to sit on your hands and resist the urge to do something. Apparently most people move every 7 years, maybe just insulate the roof.
posted by Pembquist at 6:37 PM on October 29, 2014

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