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Brick house insulation

First of all, I wanted to wish everyone Happy and Prosperous New Year!
I would like to ask for an advice regarding a wall cavity composition. I am in Westchester County, 30 miles north of New York City, climate zone 4. I am about to acquire a brick house (tiny brick, maybe 2-3 inches thick). Single wythe, I believe. I think that brick is just decorative and is not load bearing at all. Only cinder blocks in the basement support the floor joists. Second floor ceiling joists are supported by wood framing.

I was trying to educate myself as much as possible on the subject. I have read http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-old-br... and other publications regarding the subject. I know that the best method is to insulate outside, but this does not seem to be an option as of now. As per inside, I understand that the best method is closed spray foam, but I would like to avoid that as well.

I think I will own a building with low risk factors, and may decide to go ahead without hiring a consultant. The building has no signs of exterior water damage, without any deteriorating bricks, and with flashings that do a good job of keeping rainwater off the building.

It will be total demolition inside only. After demo phase is completed, I would like to insulate as well as possible. I am not 100% sure, but as far as I was able to see walls are made of 2x6 studs. I think studs might be in direct contact with the brick, or there might be a very tiny space between (the house built around 1950). I know that according to recent research wood should not come into direct contact with brick, but it is a retrofit, and I will not be able to redo the framing, so whatever is there stays (if there is more space between studs and wall, I would slide insulation behind studs. I was planning to adhere 2 inch thick Polyisocyanurate (Thermax) R13 boards (due to all horror stories about respiratory problems, even when jobs were done professionally, I would prefer not to have spray foam in the walls, as far as I read Polyisocyanurate does not contain the same chemicals as spray foam, please correct me if I am wrong), cut them to snug fit, adhere them between 2x6 studs, tape them etc.. In case there are any cavities left, I would fill them with a canned spray foam (minimal quantities are ok). Then I was planning to use Roxul Comfortbatt R15 on the top of the board. Total R value (2 inch board, and 4 3.5 inch roxul) should be about 28, minus whatever I lose due to thermal bridging etc. I am fine with that. I plan to finish the wall cavity with a drywall.

• Does my idea make sense? Would you plan the wall cavity differently? If so, how?
• Should I apply liquid-applied air barrier, on the inside, to the brick first? Like a brick sealant or so? I believe I read that GBA does not recommend this. Is this correct?
• Is Thermax Polyisocyanurate appropriate material for my application? If I look at “Calculating the Minimum Thickness of Rigid Foam Sheathing” it seems that its thickness is well enough for my climate zone, however in places where studs are, it may not be thick enough. Can this cause moisture problems? It seems that the wall would be water impermeable, perhaps except where the studs meet the bricks, or “almost meet”? I admit this is pretty confusing, and I understand why closed cell foam is the recommended solution here.
• Should Polyisocyanurate foil face be installed towards inside or outside of the wall? As far I was able to understand, it should be on the inside. Is this correct?
• What about the vapor barrier underneath the drywall? Would my Polyisocyanurate create a vapor barrier on the outside side of the wall? I read that it is not advisable to have two forms of vapor barrier (the rigid foam and the poly) with any kind of cavity or other material in between because condensation will build up between them. I was thinking about installing MemBrain Smart Vapor Retarder & Air Barrier Film underneath the drywall. Is this a good choice for my application? Would I create aforementioned problems? Perhaps I should skip it?
• What about the vapor barrier in the ceiling? Should the vapor barrier be installed on the ceiling too (ceiling will be insulated with R30 Roxul, and I may add cellulose on the top of it later, no boards here)? I guess in this case double vapor barrier problem would not exist so I could use MemBrain Smart Vapor Retarder & Air Barrier Film underneath the drywall without worries?
• Can I use the same wall cavity composition in the basement? 50% of basement walls are above the ground, so it is like a half basement, as the house is built on a slope. The basement walls are made of cinder blocks. I would be able to redo the farming here (on some walls it does not exist at all), and adhere boards to cinder blocks in continuous manner and thickens without any problems. Assuming correct board thickness, I see that vapor barrier is not recommended in basement applications, correct?
• Are there any special requirements when it comes to bathroom walls? There is about 5 feet wide outside bathroom wall. Should I insulate this part differently? If so, how?
• What material should be installed inside of internal bathroom walls? Some sort of faced insulation?
• If all else fails, and I have to go with spray foam, would 2 inches thick, complemented with 3.5 inches of roxul be sufficient?

Any question and suggestions are very welcome. Any help is greatly appreciated. If I got it all wrong, please tell me what would be a recommended wall cavity composition in my situation. I am trying to achieve the highest R value, as well as I prefer not to use a spray foam and cannot insulate from the outside. Big thank You!

I was following the below referenced document: Measure Guideline: Installing Rigid Foam Insulation on the Interior of Existing Brick Walls H. Natarajan, S. Klocke, and S. Puttagunta

Asked by Sebastian Smith
Posted Dec 29, 2017 10:18 AM ET


15 Answers

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You have posted a long thread with lots of questions. It will be necessary to answer your questions in chunks, a few at a time.

Your first misconception is that you have a brick house. Your second misconception is that my article ("Insulating Old Brick Buildings") has anything to do with your house. It does not.

You have a wood framed building. The brick veneer on your house is equivalent to siding. It is not structural. The guidelines in my article do not apply to houses like yours.

Answered by Martin Holladay
Posted Dec 29, 2017 1:19 PM ET
Edited Dec 29, 2017 1:20 PM ET.


Before we can advise you on the best way to insulate your walls, we need to know whether the existing 2x6 studs are touching the brick veneer, or whether there is a gap between the studs and the bricks. If there is a gap, we need to know the width of the gap.

(I'm assuming that your house has no sheathing.)

Answered by Martin Holladay
Posted Dec 29, 2017 1:22 PM ET


Thank you for your response.

I guess it is good news. Bad news is that the house is not mine yet, and it will take 6-8 weeks until it becomes mine. I just wanted to get ready to hit the ground running with renovations as soon as the transaction is completed.

There is a very small part of a wall exposed next to a pipe chase, and in there studs touch the brick. However, I cannot tell if this is the case for the entire house. If this detail significantly changes responses to my questions, I will wait and come back here when the house is mine and when I open the walls. If it is not a big deal we could analyze both variants. However, I do not want to stretch anyone’s generosity here, so probably the best option is to refresh the topic when details are known. Once again, thank you!

Answered by Sebastian Smith
Posted Dec 29, 2017 1:59 PM ET


If the studs are touching the brick, and the walls have no sheathing, I would proceed with one of the methods recommended in this article: Insulating Walls in an Old House With No Sheathing.

Answered by Martin Holladay
Posted Dec 29, 2017 3:34 PM ET


I looked through the article and comments. Seems that my approach with 2 inches polyiso (airtight) and the rest filled with Roxul should work. My idea with membrain product should be fine too, so if needed the wall may dry to the inside.

If studs do not touch the brick, would I follow the same guidelines with the exception that I would not have to install spacers crating the air gap? As putting polyiso flush with the back of studs would take care of it?
Thank You!

Answered by Sebastian Smith
Posted Dec 30, 2017 3:51 PM ET


You don't want the rigid foam to touch the bricks. So whether the studs touch the bricks or not, you will still need 1" x 1" sticks to provide an air space between the bricks and the rigid foam.

Answered by Martin Holladay
Posted Dec 31, 2017 5:56 AM ET


Using 1x1" sticks of cut up foam board for spacers is even better!

For foil facer to have any thermal benefit it must face an air space.

Building out 2x6 studs and cutting up foam to fit between them is a mistake. The performance of the high R/inch foam is severely undercut by the very low R/inch framing. A the same wall thickness it's significantly higher performance (and easier to build) to use 2" of continuous foam and a 2x4 wall.

In zone 4A you don't need a vapor barrier in any wall or ceiling assembly. But if you DO install a vapor barrier (such as foil-faced foam), take care to design the assembly so that the interior side of the assembly can dry toward the interior, and the exterior side to the exterior. (No interior side vapor barriers if there is exterior side foam.)

The foundation wall needs to be assessed for moisture before deciding how it needs to be insulated. Worst case it may need a dimple-mat between the CMU foundation and the foam layer, ideally draining into a perimeter drain cut into the slab, but that's a worst case- most don't need to go that far. Code minimum for foundations in zone 4A is R10 continuous insulation, which can be achieved with 1.5" of Thermax, or 2" of reclaimed roofing polyiso, at a fraction of the cost of Thermax. There are multiple vendors trading in used & factory seconds rigid foam. eg:




Keep the cut bottom edges of polyiso off the slab, since polyiso can wick moisture. An inch of EPS on the slab extending from the foundation wall under the wall foam and bottom plate of the finish studwall as a capillary and thermal break works pretty well. Give the EPS a 1/4" gap from the wall for drainage if there is no dimple mat.

Answered by Dana Dorsett
Posted Dec 31, 2017 9:47 AM ET


Thank you all.

Yes Dana, you are right about building a wall of studs vs foam and studs. The problem is that the wall already exists. At this point I cannot insulate outside, so I am looking for whatever best I can do inside. If I had a comfort of building it from scratch, I would definitely follow all recommendations to insulate outside. In my situation, it seems like Cut-and-Cobble Insulation is my best option, despite its pretty significant disadvantages.

Thank you for all suggestions regarding the basement, I will follow them, as well as I will try to use second hand materials. Vapor barrier situation has become clear as well. I will drop it entirely, or will use intelligent membrain products.

Going back to some of my original questions:
• As far as liquid-applied air barrier, like a brick sealant on the inside of the brick, I should not do it, correct?
• Are there any special requirements when it comes to bathroom walls? There is about 5 feet wide outside bathroom wall. Should I insulate this part differently? If so, how?
• What material should be installed inside of internal bathroom walls? Some sort of faced insulation?

Thank You,

Answered by Sebastian Smith
Posted Dec 31, 2017 11:27 AM ET


It would be unusual for a house of this age at least in most parts of the US to be built with 2x6 walls, and brick veneer is usually around 4 - 41/2" thick. I think you may have been misled by a small area of plumbing wall built extra thick to accommodate a pipe chase. In any case you should wait until you take possession and can inspect the wall construction properly, and you should definitely seek professional input from a local consultant or contractor before potentially making expensive mistakes with your energy upgrade plans. There is a wealth of first rate information and advice to be found on GBA but remember it covers a vast geographical range of climate types and building practices and it should always be applied through the lens of local knowledge and experience.

Answered by James Morgan
Posted Dec 31, 2017 12:45 PM ET


James Morgan has it right- until you take possession and can do some exploratory demolition to assess the wall stack-up most of this is going to be mere speculation. Almost all brick veneer houses built in the 1950s had a 1" or greater cavity between the brick and the next layer, and the structural wall was a wood (shiplap plank or plywood), sometimes fiberboard sheathed milled lumber (1.5" x 3.5") 2x4 wall, sometimes with R11 rock wool or fiberglass but often empty. A 2x6 wall of that era would be rare- sometimes the sound of pounding hooves really is a zebra, but that's not what to expect.

The brick doesn't need to be an air barrier if it's facing a vent cavity, and adding sealants might even increase the risk of freeze/thaw spalling damage. The brick needs to be able to dry freely into the cavity to keep it's moisture content lower, reducing freeze damage risk. (I have seen freeze thaw spalling on a circa 1960-ish house in Westchester county, but that brick stayed wet primarily from excessive splash-back wetting at the eaves.)

Rather than cut'n'cobbled foam board, adding polyiso edge strips to the framing held in place with 1x furring sufficient to thickening up the cavity depth to 5.5" and using R23 rock wool or R21 fiberglass does more for performance than cut'n'cobbled foam between the studs. Most circa 1950 homes were framed with milled 3.5" x 1.5" 2x4s, so 1" thick polyiso edge strips (R6-ish) and 1x furring brings that out to 5.25".

An R23 compressed to 5.25" will still perform at R22, a compressed R21 would be still be at least R20. Alternatively, blown cellulose would deliver R19.5-R20, and would automatically have a near-perfect fit.

If it's full dimension rough 2x4s backing off to 3/4" polyiso edge strips.

This is pretty much the Bonfigilioni methodology:


Polyiso between the studs has the same performance problems as closed cell foam, and it's far more time consuming. Even with 3.5" of Thermax (R24 - R25-ish) packed in there only adds R2 to the "whole-wall-R". Adding even a half-inch of polyiso thermal break on the framing edges and (0.5" + 0.75" furring=)1.25" of fiber depth adds more performance than that. See:


Answered by Dana Dorsett
Posted Dec 31, 2017 6:07 PM ET


There is probably a great chance that you are right regarding the walls thickness. They might be 2x4. Regardless what is there, I will bring it to 2x6. Either by simply adding 2x2 furring strips or by using Bonfigilioni methodology (thank you for that). What I am trying to achieve now is the best possible insulation in the 2x6 wall cavity. I planned to install 2 inch of polyiso board R13 (cut and cobble method) and 3.5 inches of Roxul R15, so total wall R would be 28, minus whatever (might be a lot) I lose due to thermal bridging, imperfectness of cut and cobble method etc.

I read the following document “The-Economic-Thickness-of-Thermal-Insulation-Dec08.pdf”. The document basically indicates, that the first inch of insulation represents 80% of the heat flow reduction in the system, the next inch 9%, the next inch 3%, the next inch 2%, the next two inches 1% each and the next four inches only 1%! So in the first 2″ you are getting 89% of the total performance realized in your insulation assembly. If the document is correct, then the key for success in insulation is to do a thorough job, sealing all the openings in your building envelope. Again the best done from the outside, no questions about it.

In my situation, I thought that first two inches done with polyiso, even keeping in mind its disadvantages and poor method, is still the best I can do in order to build as good as possible air barrier.

Answered by Sebastian Smith
Posted Jan 1, 2018 8:45 AM ET


There is a VERY significant difference in thermal performance between adding wood furring v.s. Bonfiglioni style insulated strips. An inch of polyiso edge strip cuts the thermal losses of the framing by half compared to adding a 2x2.

Also note, a 2x2 is only 1.5" in depth, and would only bring a milled 2x4 out to 5". not the nominal 5.5" of a milled 2x6. The IRC code minimum performance (= the lousiest wall that is legal to build) for zone 4A is R20 when the studs are all wood. Compressing an R21 to 5" won't even clear that bar, but compressing an R23 rock wool batt to 5" would. But at 5" the heat going through the framing fraction is ~9% higher than through a 5.5" stud. With an R6 polyiso 1.25" deep Buonfiglioni strip the heat through the framing fraction is cut in half, and a compressed R21 would still hit R20, making the assembly performance measurably higher than code minimum. Despite the lower center-cavity R, an R19 compressed to 5.25" (about R16-R17) would still hit code performance on a U-factor basis (with margin to spare) with R6 Bonfiglioni edge strips but a higher density, more air-retardent fiber is going to be more forgiving of any air leaks that develop over time.

If the existing structural sheathing of the studwall is plywood it's dead-easy to make the sheathing an air barrier. If it isn't, there are easier and more reliable methods than cut'n' cobbled foam board for air sealing. Before diving into the details, let's find out what you have first.

Typical hemlock, fir or high density pine species used for framing in your region run about R1.2/inch, so 5" thick framing would be ~R6. Typical framing fractions for 16" on center studwalls are 25% of the total wall area, after measuring up all the top & bottom plates, window framing, jack studs etc.. A compressed R21 will perform at about R19 @ 5.0" for the other 75% of the wall area. So at any arbitrary temperature difference dT the amount of heat moving through the framing fraction per square foot over the average area of wall is:

0.25 x dT/R6= 0.0417 x dT per square foot of average wall area.

The amount moving through the insulation fraction is:

0.75 x dT/ R19= 0.0395 x dT per square foot of average wall area.

Note slightly more than HALF the heat is being transferred through the framing fraction(!). That means even doubling the R value of the cavity insulation to R38 (say, with exotic & expensive stuff like aerogel) would still deliver less of a performance enhancement than doubling the R value of the framing fraction with edge strips.

And that is why it's better to spend the foam budget (and cut'n'cobble labor budget) on edge strips, rather than between the studs. It's only ~1/3 as much foam, far less cutting & cobbling, with maximal performance benefit. Cutting 1" foil faced polyiso is very quick and easy with a 4" steel wallboard tape knife sharpened on the edges, as demonstrated in this quick vidi;


Answered by Dana Dorsett
Posted Jan 1, 2018 10:35 AM ET


Understood, so it seems that Bonfigilioni method is a must, otherwise loses are huge.

What about the rest of the wall if there is no sheeting? Do you do bats and let it dry to outside?

“If the existing structural sheathing of the stud wall is plywood it's dead-easy to make the sheathing an air barrier.” -- Do you mind listing some of the ways of crating the air barrier in this situation, without going into any details?

“If it isn't, there are easier and more reliable methods than cut'n' cobbled foam board for air sealing. Before diving into the details, let's find out what you have first.” Do you mind listing some of them, without going into any details?

It might be another misconception, but I am ‘locked’ into thinking that the only effective, externally facing air barrier is some type of rigid foam, so without the option of external installation I am back to cut'n' cobble. I believe that, even when I use Bonfigilioni to cut my loses, I still need foam. At the same time, if i can avoid this labor intensive process, and not lose much R value, I would be fully into it.

Assuming Bonfigilioni method to cut my loses, and 5.5 inch of insulation I can achieve R28 with Roxul and polyiso, and supposedly R23 with Roxul only. Question if R5 difference is worth the effort, and if there are any alternatives which can also achieve close to R28, like polyiso and Roxul combined.

Answered by Sebastian Smith
Posted Jan 1, 2018 12:06 PM ET


"I am ‘locked’ into thinking that the only effective, externally facing air barrier is some type of rigid foam"

Unlock that thinking. Foam often shrinks over time opening up gaps over time. Even when foam is a continuous layer without seams with a dissimilar material such as wood it takes quite a bit of detailing to make it air tight even initially, let alone air tight after 50 years.

Using a sheet woody material such as plywood, OSB, or fiberboard sealed together with polyurethane caulk has a much better shot over time. The details of how to do that matter too.

Enough speculation. Nobody has enough time to go over every possibility in detail, whereas going of the details of how to deal with exactly what you have (still an unknown) is usually pretty straightforward.

Bumping the center cavity R from R20 to R28 has only a negligible effect on the whole-assembly R. Bridging expensive & difficult to install high R/inch foam with R1.2/inch wood is really a waste of time and money.

Answered by Dana Dorsett
Posted Jan 1, 2018 12:41 PM ET


Understood, thank you, I will come back in a few weeks when I have more, and accurate information.

Answered by Sebastian Smith
Posted Jan 1, 2018 2:38 PM ET

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