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Tricks on insulating an already finished wall

giantsean | Posted in General Questions on


Last summer I sought some opinions on whether or not it was worth demoing existing plaster walls while we renovated an upstairs bedroom (the last room of a lengthy total reno, which saw us insulate or re-insulate almost all non/poorly insulated spaces). At that time we were both very tired of renovation and just wanted to “git ‘er done” and went ahead and finished it. Now that it’s cold, and I can see how well it plays with a newer efficient forced hydro air system that works fine for the rest of the house, I regret the decision.

The two walls in question are a masonry gable end wall, and a 2×4 wood dormer wall. The dormer wall is full of 50’s era Balsam Wool. It’s a little chilly but not horiffic. The big problem is the brick block, which from outside is solid brick, 4″ cinder block, 3/4″ air gap via furring strips, and an inch or so of gypsum board and brown/finish plaster. When it gets below 30 it becomes a refrigerator, and you can almost feel a draft blowing in through it. With the door open the upstairs air mixes in and keeps it manageable, but close the door and suddenly penguins show up asking to be let in :).

I figure the dormer wall can be just left alone or worst case I could punch some holes, dense pack, and patch. The masonry wall however is a real challenge. The room is DONE so wrecking the wall would be a significant undertaking, including re-trimming the finished windows. I could probably get a supplemental space heater to help keep it in check, but it seems like such a shame after all the work. I have also read that filling the air gaps with perlite from above may be somewhat effective, but I only have 3/4″ to work with – and I’d have to hunt and peck to find the gaps through a top plate over the masonry in the attic. Finally, I could just leave the plaster and frame a wall in front of it and insulate and re-sheetrock/trim (lots of work) but I am not sure if that will create any condensation issues.

So I guess my only question is whether anyone has an opinion on the best approach – maybe something I have not even thought of yet? The only thing that won’t fly is to insulate from the outside – the brick is part of the look of the house. At this point I’m open to anything else that would help w/o too much destruction.

Thanks as always!

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    G S,
    Possible solution #1: Improve your heat distribution system so that the room in question gets enough heat to make the room comfortable. It sounds like the duct serving the room is undersized.

    Possible solution #2: Install a continuous layer of rigid foam, followed by new drywall, on the cold wall. For information on this approach, see Walls With Interior Rigid Foam.

  2. Dana1 | | #2

    What is the source fuel & equipment heating the water for the hydro-air? What are your local fuel options & rates?

  3. giantsean | | #3

    Thanks Gents. The hydro system, while very poorly installed and able to be greatly improved (a whole other topic), isn't working badly everywhere else. Water is heated via a Navien combi unit and the upstairs has a dedicated Trane air handler which is good for 60K BTU by itself. The trunk is metal but most of the takeoffs are flex, and horribly routed and sealed to boot... it is a wonder it works as well as it does. Dana, I don't have a gas bill at the moment but electric here is about 15c per Kwh, pretty average.

    @ Martin - if I do decide to go with foam, do I have to demo the plaster wall or can I just leave it and use glue?

    Thx again!

  4. GBA Editor
    Martin Holladay | | #4

    G S,
    You don't have to demo the plaster wall, as long as it is (more or less) coplanar. If I were you, I would want to screw the rigid foam and drywall to the lath or furring strips, so you'll need to do some investigation to figure out the location of the wall's wooden components.

    The main reason you don't want to depend entirely on adhesives is that the plaster might deteriorate as it ages.

  5. giantsean | | #5

    Thank you Martin. I will work on improving the HVAC and air-sealing the top of this wall at the attic, as well as insulating the entire room. If I cannot maintain comfort I agree rigid is the way to go. Still kicking myself for not sucking it up and addressing it at the time I had the rest of the room apart, but them's the breaks I guess :)

  6. Expert Member
    Dana Dorsett | | #6

    It's sometime tough but not impossible to reasonably micro-zone with a Navien combi boiler. With a hydro-air I assume you're running it at a fixed output temperature, not using an outdoor reset curve (?). With a panel radiator capable of delivering at least 75% of the minimum-fire output of the Navien's heating side at whatever your fixed temperature happens to be it won't short cycle on the micro-zone call when you've dialed it in. Panel radiators on the "cold" wall would increase the mean radiant temperature of the room, thereby increasing comfort.

    The NCB-240 puts out about 17,000 BTU/hr at min-fire. Say it's set up for the hydro-air with 140F output with 110F return for good condensing efficiency and not-too-tepid air at the farthest duct register, running a panel radiator zone with an average water temp (AWT) of 125-130F you can get there with something like a pair of 24" H x 79"W x ~4"D Myson T621-6-20 panel rads and still be condensing. They would't deliver the full 17K, the specfied output at 140F AWT is nearly 14K and they'd be putting out more than 12K @ 130F AWT, with ~90lbs water equivalent (water + steel for the pair) to not short-cycle. Between the rads and UK standard threaded valves and an ECM drive pump it would set you back about USD$1.25 K in hardware as a DIY. There are others- this was just a quick & dirty example.

    At a higher water temp you could do it with smaller radiators, but it would be out of the condensing zone, and smaller radiators=less thermal mass, which means the mean radiant temperature would fluctuate more quickly. A chill followed by a hot flash followed by a chill then another hot-flash may have the same mean radiant temperature when time-averaged, but it's not the same as "comfort". Lower radiator temperatures and more thermal mass matters, from a comfort point of view. With a condensing tank type water heater this would be a bit easier, since you wouldn't have to up-size the rads to not short-cycle the burner, due to the far more substantial thermal mass of the tank.

    Installing perlite in the air gap between the masonry and wallboard is somewhat risky, since the air gap is a capillary break, and perlite would create a capillary path. Adding 3/4" of perlite thermally broken by 1x furring adds at best R1 to the stackup, though it would improve the mean radiant temperature somewhat. Martin's concept of installing rigid foam over the existing drywall is a much safer approach, and can add quite a bit more performance, at the cost of some interior space.

  7. giantsean | | #7

    Yeah I think mess aside, rigid foam is the way to go. Let me add to the well of tears by saying that I spent a good 10 man hours crack patching and smoothing the plaster on this wall. Probably generating about the same amount of dust in the process than if I had just demo'ed the thing :(

    As to my setup, if you are the same Dana that posts in the HVAC forums you know of my sad tale - I'm the poor sap in CT w/ the counterflow exchangers plumbed backwards and the flex tubes made of open fiberglass lol. I was running 140 until it got below 30F but have to crank it up to max (185) when it's very cold, because the air can't get there. I will replace all the flex w/ metal in the spring, and probably add some booster fans. As stated the heat is there, it's just not getting far enough.

    Rads are another good idea but as this wall is block and the house is buttoned up now I have no good way to route hot water w/o more mess. If it were up to me I would have kept our old cast iron old school rads but my wife wasn't having it. Now she can suffer :P

    Thanks as always for all the advice everyone!

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