Kim Pressnail | August 03, 2016

Details // Concrete & Masonry, Insulation & Air Sealing, Drywall, Tile & Flooring

How to Insulate Old Masonry Buildings Without Causing Water Problems


Letting the brick dry evenly is the key for old masonry walls that want to leak less energy


Insulating solid masonry buildings can be tricky. If the brick can’t breathe, the insulation can cause spalling, efflorescence, and other moisture damage.

A fibrous drainage mat can create an airflow cavity between the brick and a layer of spray foam insulation. Drainage mat with built-in bug screen (such as Mortair Vent) is best, otherwise install insect screen against the brick first.


  • Fibrous drainage is then installed and held in place with masonry anchors. A layer of housewrap keeps the spray foam out of the drainage mat.
  • A framed wall set out from the housewrap will allow the insulation to be continuous along the wall and leave room within the studs for wiring.
  • Of course, drywall goes on the inside of the studs.
  • For any water that is driven into the walls from rain and sun, 1/4 inch holes are drilled angling down to the outside to provide drainage at the floor level.
  • At the top of the wall, insulation should completely cover the rim joist.


Multiple layers of breathable sheet goods provide a ventilation space, drainage, and a surface for a spray foam to make an energy-efficient masonry wall that thumbs its nose at moisture.


—This detail comes from research presented by Dr. Kim Pressnail of the University of Toronto, Department of Civil Engineering at the Westford Symposium on Building Science in August 2015. 


Slide show and download



How is the house wrap secures/installed to the wall or drainage mat so it is taught enough to be sprayed and support the spray foam? 

Daniel Morrison's picture


I asked Kim how they did it and he said this:

The Tyvek was temporarily taped together with the mortaire vent, and then the two products were mechanically fastened to the masonry wall.

The foam insulation was supported by the adjacent 2x6’wood framing… so the mechanical fastenihng really only served to hold the mortaire vent and Tyvek in place…

Hope this helps.

(I drew a metal stud wall in the detail, which is what I thought he had specified.)
Additionally, you could use cap-nails to fasten house wrap to the floor framing/subfloor at the top and bottom of the wall.

Thanks for visiting ProTradeCraft,


Timely article.

I signed a renovation contract for a 1930 commercial truck garage conversion to a Capitol Hill House

The structure is double and single wythe brick with a concrete skeleton.
Did I say it’s in the hysterical district?

My intent was to build the exterior stud walls inside the masonry and then spray a flash and batt insulation system directly onto the existing brick.

From this article, I get the impression that might not be such a good idea.   

Can someone clarify? 

Daniel Morrison's picture


I forwarded your questions to Kim Pressnaillast night and he responded pretty quickly:


This builder is using the conventional approach to interior insulation, that has been used successfully in a variety of climates for many years.

A better approach in  net drying climates is to incorporate a vented drying airspace.  It offers the advantage of  improved drying in the event of wetting, and  it is reversible.  Improved drying means that salt and freeze-thaw spalling are less likely to occur.  In a relatively mild climate such as Washington DC, reducing the freeze-thaw risk  may not represent a significant advantage over the conventional approach.

Hope this helps.

I also dug into the literature a bit to expand on Kim's response, here's some more:
One problem that people have experienced applying insulation to masonry walls is that it changes the vapor profile—meaning that the wall can only dry to the outside.  Because an insulated brick wall will be colder than an uninsulated brick wall, the brick will stay wet longer. 
If the floor framing is buried in the brick (common), then the ends of the joists can rot prematurely — though the interior wall can become a bearing wall to carry the floor above...
In general:
  • More rain makes the assembly more risky
  • Colder climate makes the assembly more risky
  • More insulation makes the assembly more risky
  • Lower quality bricks make the assembly more risky (often the good bricks were used for the front and the less-good bricks were used on the sides and back).
During freeze/thaw seasons, the extra wettness can cause damage to the brick.
The first thing to do is look at the outside and inside of the walls closely for any preexisting water damage that may point to a risky proposition. (If water is already a problem, adding more water may be a bad idea).
John Straube, of Building Science Labs, suggest that looking at the parapet walls on the roof can give you a good idea of how an insulated assembly will work—because there is no heat flowing through the parapet, just like there will be (much less) heat flowing through an insulated wall. If the parapet doesn't show signs of decay, you are probably safe to insulate. 
Though providing drainage and ventilation is a pretty good back up on an important historic building. (Not sure how 'important' your building is). 
They also suggest making sure all of the exterior flashings are in excellent shape—no leaks or dribbles.
One other aspect that may matter a lot is the local historical society(which you hinted at)—they often want things like spray foam to be (at least theoretically) reversible. This was the situation for Kim, who tested this assembly on the oldest and most beautiful masonry home on the campus of U of Toronto, among others. Because the spray foam is against Tyvek, rather than historic brick, it is (at least theoretically) removable, so the historical society was happy. 
Take a lot of pictures and video and post them to ProTradeCraft, so we can all watch the project unfold in real time.
Thanks for contributing,

How important is doing this in a dry climate, like Montana?

Daniel Morrison's picture

Drier is safer, but much of the danger is in temp fluxuations: Freeze/thaw cycles that cause wet bricks to become damaged wet bricks.

Insulation decreases a brick's ability to dry (because there is less energy exchange) so a little water turns into a little bit more water. A lot of water turns into a boatload of water.

Big rainstorms in autumn, could mean moist bricks all winter, regardless of how dry it is in July and August. A little ventilation channel, as depicted in this detail, can be a big insurance policy.


Further Reading:



I don't understand how the one inch drain holes will pick up any moisture on the inside unless the water happens to be directly over the holes.  Is there meant to be some sort of flashing to guide the water to these holes or are they just vapour holes for high humidity?

Daniel Morrison's picture

The holes allow water to drain out. Yes, there would have to be enough water to dribble over to the hole.

The holes also create a ventilation channel, so that small accumulations of moisture can dry before becoming a puddle that can flow over to the hole.


Key points to remember is how these old (100yr +) brick masonry buildings have survived so long in harsh climates without much or any maintenance.

1. The masonry absorbed water all summer (wet season) and if some moisture showed up inside it didn't matter for these industrial buildings.

2. Once the interior heat was turned on in the cold dry season the masonry dried to both sides. 

3. When the interior heat was on the freeze line was typically at the inside edge of the outer wythe.  Exterior brick is typically hard fired expensive while inner wall brick is typically softer and cheaper brick and is not freeze/thaw resistant. 

So walls get wet all summer and dry out to the low RH winter side and low RH heated inside.  If we change this then bad things happen.  Interior insulation against the masonry moves the freeze line inward to the insulation resulting in the entire thickness of masonry freezing. 

If there is water contained in this masonry we now have expanding ice causing damage to the inner soft brick and mortar.  Plus, as mentioned above, wood beams set into the outer walls are now wet all year. 

I am very reluctant to insulate solid masonry walls.

Anyone have any ideas....other that tuckpointing the grout ?


The video says to drill 1" drain holes while the text below the video calls for 1/4" holes, so which is it?  And how close should the holes be along the horizontal length of the wall???   How would/could a hollow block wall best be vented????

Daniel Morrison's picture

I'm sorry for the confusion.

It should be 1/4 inch holes. I forgot to update the audio in the video after getting comments from Dr. Pressnail.

Will do soon.


Our c.1957 brick house was damaged by 30" of floodwater recently and when we removed wet drywall, there was no insulation in the walls. The house is small (1100 sq ft), has always been comfortable and heating/cooling bills are affordable. I hesitate to insulate for fear of moisture problems. We live in a very humid climate. Am I making a big mistake by not insulating?

Daniel Morrison's picture


Most of the freeze-thaw damage due to newly insulated brick structures occurs in colder climates. You don't say where you live, but that it is very humid, so I speculate that it is a warmer climate.

Whenever you open up walls or a roof, it is a once-in-a-lifetime opportunity to significantly improve the energy performance of a home. Once you close them up, you're unlikely to open them again. It is also the cheapest chance, because they are already opened up and they are already going to get closed.

Now, if you're gutting and redoing the interior finishes, the building department will most likely require insulation, so it is a matter of making the best choice for your climate, construction, and budget. 

I'm sorry to hear about your house; hopefully, insurance will cover the cost of the renovations.



I insulated my home with 2" closed cell boards against the brick and a 2x4 stud wall with fiberglass. The walls had been stripped of plaster. This house had been sitting un-heated for 50 years and the walls showed little damage from freeze thaw. At the time their was little data on how one might insulate and most historic preservation sites were against the idea. But with the idea of something being able to be undone and a history of beinging un-heated we bit the bullet and insulated the boards are foamed together from floor to ceiling. The joist framed into the walls were insulated with just the fiberglass to allow moisture to dry out around them, and are also resuported on the new stud wall. Foam fire rated and a 2x6 at the top extending over the board act as fire stop. Cost was a factor and labor was not so the board insul was also a good deal cheeper.

I recently purchased a 1400 sq ft, 2- story foursquare home in very poor condition. It was built about 1910 and is in a historic overlay district. The 1st floor is solid, brick masonry construction, above the beltcourse is wood cladding. The exterior brick is painted but flaking. The house has plaster over the interior brick walls with outlets & switches currently standing proud.

I live in  Louisville, KY. Our winters are short, but can get into the single digits. Summers are warm & humid. The house is built into a hillside and the back wall is several feet below grade. I plan on installing french drains, gutters, overhangs, etc. Would you recommend the Mortair Vent in this situation? .25" or .40'? Should I use open or closed cell spray foam? How thick? Would the permeance be the deciding factor, as I have read elsewhere?

 The exterior brick actually has a certain charm with it's uneven appearance if I could get rid of obvious flakes. If not, is it safe to paint with a good quality peel bonding primer or will this just compound the moisture trapped in the brick issue?

Thanks. I am in way over my head!


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