Insulating Historic Masonry Buildings Safely: How WUFI Can Help

Insulating Historic Masonry Buildings Safely: How WUFI Can Help

With the rising popularity of Passive House and EnerPHit renovation in brownstone and brick masonry renovations in the US and Canada, the question comes up time and time again: What insulation levels are needed to create an efficient structure and how far should and can we push interior insulation levels without running into issues? One of the tools we can use is WUFI hygrothermal modeling software. 475 can provide WUFI analysis for your projects utilizing the complete Pro Clima airtightness system.

Airtightness First*

But before WUFI, the first element to understand is that properly and durably airsealing the building envelope makes these building much more energy efficient (adjust the blowerdoor test results in PHPP to see what effect this has...). More importantly, interior airtightness prevents humid air from entering the insulation and causing issues when it encounters cold surfaces (ie the brick wall). This is the reason that the details that 475 has developed and published for interior insulation of historic masonry construction rely on an interior air barrier made with ProClima's smart vapor retarder INTELLO, along with TESCON VANA tape, to keep conditioned air where it should be - on the inside, keeping the insulation dry and the interior comfortable. And using a service cavity to assure that airtight results are achieved in a practical way.

* Of course shedding the water by restoring cornices, drips, leaders etc... to avoid saturation of brick is the top priority - but airtightness comes before insulation.

The Variables: Climate, Brick, Insulation

There are a multitude of variables in WUFI (making the proper training of WUFI users extremely important). But here are some of the basic variables for the examples that follow. The location is Albany, NY, selected because it is solidly in climate zone 5. We use the worst case wall (north facing, so no real direct solar radiation available to dry wall inwards, and most exposed to precipitation), moderate interior humidity in winter (30-40% per EN 15026), and a small interior air leak in each assembly.

Printing from WUFI®

The wall is three-wythe brick. The face brick is in most cases relatively non-absorptive and durable, and was placed there by conscientious architects/builders that wanted their buildings to last. One can observe this in the field - unheated buildings without damages are a good example. This is further underwritten by publications that have determined the same (Badami, 2011, Ananian, 2014). Now this is not to say that loading the walls with exterior moisture and/or interior humidity won't lead to issues. Good drainage, overhangs, cornices, etc. normally take care of exterior elements in historic buildings if they are kept in good operating condition. The outer face of the brick probably freezes several times a year - but since face brick is typically good quality, making it colder more often is most likely not going to lead to more freeze-thaw damage. Of larger concern is the interior fill brick - this brick is normally not as high quality, would become colder because of the interior insulation and if interior humidity is not controlled, and can be prone to condensation. Hence any additional moisture can increase both the chance of mold growth and freeze thaw.

The depth and type of insulation can also play an important role as we'll examine below. We recommend you don't use foam because Foam Fails - however, fiberglass, mineral wool and cellulose are all viable options.

WUFI and Safety Thresholds

There are safety thresholds on which there is general consensus - and these thresholds should not be crossed to be sure that the assembly maintains drying reserves and greater resilience in case further unforeseen wetting occurs.

To prevent mold issues we refer to the following thresholds:

  1. ProClima's recommendation is to keep relative humidity at the condensing surface below 92% at all times.
  2. ASHRAE 160P establishes a criteria that for a given 30 day running average the relative humidity cannot be above 80% while the temperature is above 41 degrees Fahrenheit.

WUFI allows us to comparatively understand if any one assembly poses greater risk or less risk in regards to these thresholds and the resilience of the enclosure. The correlation between each safety threshold shows that using either leads to similar conclusions, and by adhering to both one should have certainty that the assembly works (given that correct material, climate and orientation inputs are used).

Fiberglass with "Airtight Drywall Approach"

A large number of buildings for code reasons are insulated on the interior, often using fiberglass, and most have no issues. This is not a surprise as the enclosure typically leaks so much air that the leaks undermine the insulation's value and the brick remains warmer. However, if one tries to use ADA/airtight drywall approach to air seal the insulation (see this blogpost for reasons why ADA is ineffective), we would observe the following WUFI simulation:


This graph shows that in the first cold fall days, the moisture starts loading the brick. This moisture loading tops out when the free water saturation point of the brick is reached, or approximately 95% RH -as opposed to higher saturation level achievable in a vacuum. This means that a substantial amount of moisture loading from the interior is possible. (**See note on Scrit at bottom of post.)

(Note: both this wall and the wall below are not code compliant/legal in Climate Zone 5, since an interior vapor retarder (Class I or II) is required per IRC R702.7 for vapor retarders).


Cellulose is hygroscopic. It can buffer a certain amount of moisture that would otherwise condense and accumulate on the first cold condensing surface. Nonetheless it is not a magic cure-all for building enclosures. When used, one needs to consider that the cellulose is vapor open, and although it can redistribute moisture loads much better than other fibrous insulations - questions remain about how much better and what is sufficient in heating dominated climates at various insulation levels. WUFI shows that with 4" of cellulose inboard of ('airtight') sheetrock, moisture levels exceed both the 92% ProClima threshold as well as the ASHRAE 30-day running average maximum of 80% RH and 41F.


This graph indicates that the wall will get additional moisture load (92% plus spikes) from the interior because it lacks a code required vapor retarder, and is also exposed to humidity levels that could result in embedded wood members such as nailers, blocking and joist to deteriorate (RH over 80% when 30 day temperature is over 41F).

Cellulose with INTELLO Plus - Pro Clima intelligent airtight system

When we introduce ProClima's smart vapor retarder INTELLO on the interior of the insulation, these issues disappear because the humidity produced on the interior in winter (breathing, cooking, showering, etc.) is retained on the interior. This leads to comfortable interior relative humidity of 35%+ even with a heat recovery ventilator (HRV) running. Just as important, the humid air is kept away from any cold condensing surfaces on the other side of the insulation, as the INTELLO forms a durable air barrier and vapor retarding layer.


With INTELLO and cellulose how high can we safely go?

A question we often hear is: We are using windows with a U-value of 0.15 or better (>R-7), so wouldn't it be beneficial to space the studs further from the brick and increase the insulation levels too? So let's see what happens if we increase the insulation to 6" of cellulose:


This can still be considered ok - there are no spikes over 92% or long periods over 80% RH when the wall is also not below 32F. Note that this only happens after the initial construction moisture has dried inwards in the first spring. This is done to show a worst case scenario, in which we start the WUFI calculations in October with all materials at 80%RH - right when the cold weather starts.

Going to 8" cellulose (R-30) leads to the following graph:


Now we do exceed 80% for longer periods of time, but only when the interior face of the brick is below freezing. This is not an immediate red flag, but we are starting to use up the reserves of the walls, and careful consideration and investigation is required to assure that this approach is indeed safe and durable. The assembly also very briefly peaks above 92%, although this peak dininishes over the years. To determine if this amount of insulation can be recommended, the following should be undertaken: lab-testing of the brick, more modeling of hygrothermal behaviour of the wall for each orientation, and additional measures to keep the wall dry (eg increased airtight goals, overhangs, brick treatment).

Note that in row houses or compact/boxy freestanding buildings, far less than R-30 can be sufficient to reach EnerPHit certification if there are good windows, proper installation details, high efficiency HRV and no large thermal bridges. It is not worth risking the soundness of the assembly/structure/health of the occupants, just to increase the energy savings beyond safe levels of insulation.

Even so, the quest for better insulation values does remain for some owners/architects. We did model 12" of cellulose, which gets you a new construction suitable Passive House level of R-45. However, as the final graph shows below, humidity spikes now reach 92%. In addition, the relative humidity in spring stays above 80% while the wall in late spring exceeds 41F for several weeks, even in the fifth year. The reserves of this wall now clearly have been depleted, and any additional (unforeseen) humidity or moisture ingress from the interior or exterior will lead to situations that no longer can be mitigated either by cellulose buffering or by inward or outward drying. This is too risky in our opinion.



Historic buildings cannot take a pass on addressing climate change mitigation. We can - and should - safely make our historic masonry walls more energy efficient. WUFI is a great tool to insulate with acceptable risk levels, in combination with a comprehensive approach to upgrading the enclosure. Our free downloadable Smart Enclosure eBook, Historic Masonry Retrofits, is another useful tool. But always proceed with caution.

**Note on Scrit

Keeping moisture loading from the interior low will also prevent the brickwork from reaching dangerous humidity levels that could result in freeze thaw - not only in the outer wythe, but additionally in the the coldest (outer) part of the fill brick. This fill brick will be slightly warmer and less exposed to rain, but because less hard brick was used for fill, these values are lower than for face brick. This critical moisture level is called Scrit, and is the moisture level compared to vacuum saturation of the brick. If for specific brick this threshold is exceeded (see this ASHRAE article), and the temperature cycles below 23F, freeze thaw damage would likely occur. If the moisture level stays below this level, the brick can freeze without exhibiting damage. For historic face brick that have do shown any damage, especially if buildings where unheated for a while, or for brick that has been tested for Scrit, these values can be as high as 0.80. For fill brick, the values can be much lower and be as low as 0.4 or 0.3 . This test of brickwork is a much larger undertaking than proper waterproofing, visual inspection, karsten tube testing etc. and is justified when damage is present, higher insulation values are sought, other concerns about the structure are present, or a combination of such factors.

As shown in the graphs below, the brick moisture content of the brick in heavily influenced by the type of insulation used, it's thickness and if ProClima's smart vapor retarder INTELLO has been installed. If the Scritis exceeded depends on brick type, but it is clear that moisture loading of the wall from the interior can add significant amounts of moisture into historic walls and increase the chance for freeze thaw damage. In this graph the moisture content of the outer 3/8" layer of the fill brick is shown.

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