Following up on the previous post, common brick, like in our building, is very pervious to water. If adding insulation to a storage or mass wall system, such as our brick shell, one of the issues is condensation and potential freeze-thaw damage to the brick work. Insulation on the inside of the building can lead to condensation and interrupts the heat transfer into the masonry shell during the cold season. As such it is very likely that the shell will go through more freeze-thaw cycles than ever before.

“Freeze-thaw damage […] require the material to be at or near capillary saturation (100% Relative Humidity)”

Reference: Building Science Digest 138 (Moisture and Materials)

“Driving rain is typically the largest source of moisture for the above-grade building enclosure”

Reference: Building Science Digest 013 (Rain Control in Buildings)

The removal of water from our brick shell, or keeping the rain out, is – let’s say – rather important to its performance and integrity. It comes down to a balance of moisture storage capacity versus drying capacity. So, how does water or moisture move through a brick wall during wetting or drying?

I had to re-learn some basics of water and its various states (solid, liquid, vapor and adsorbed). The liquid and vapor states appear the most relevant. The former is pretty straight forward – or for that matter straight downward, controlled by gravity (water flows/drains downhill).

Good detailing can prevent rain from entering the storage or mass wall system and help with the balance between wetting and drying. This includes tuck pointing, drip edges, and the right flashing details, particularly around windows and in corners. Rain deflection through overhangs is another strategy.

“It should be clear that drainage is not sufficient for this purpose since it will leave large amount of saturated (100% Relative Humidity) material. Capillary and absorbed moisture can only be dried by evaporation followed by diffusion.”

Reference: Building Science Digest 138 (Moisture and Materials)

We are now talking about water vapor. Its movement is governed by three rules:

1. Water vapor in the air moves from high pressure to low pressure areas.
2. Water vapor diffuses through permeable materials from warm to cold.
3. Water vapor diffuses through permeable materials from areas of higher concentrations to areas of lower concentrations.

As part of the drying mechanisms, I have to allow water vapor to diffuse out of the wall, whether towards the inside or outside.

In the summer, water in the brick tends to diffuse to the inside of the building, following the thermal and concentration gradient. Whatever kind of insulation system I decide to use, it is important that it allows for that inward driven moisture (water vapor) to pass.

“Rule Number One: Never install a vapor barrier on the inside of a wall assembly, which has a moisture reservoir cladding…”

Reference: Building Science Digest 108 (Investigating and Diagnosing Moisture Problems)

If moisture cannot pass, or if I add a vapor barrier to the interior wall assembly, summer moisture will condense in the wall assembly. That is a perfect recipe for water damage and mold growth – something our building has already seen in its past.

During winter, vapor tends to diffuse to the outside – again – along the thermal and concentration gradient. That vapor may have its origins in high Relative Humidity levels in the living spaces. The insulation I plan to add to the inside will reduce the temperature along the interior masonry shell. Inside air diffusing outwards and coming into contact with the cold masonry face could condense.

“Given sufficient air leakage and sufficiently high indoor Relative Humidity this condensate can accumulate faster than it can dry, and the interior face of the masonry will become saturated.”

Reference: Building Science Digest 114 (Interior Insulation Retrofits of Load-Bearing Masonry Walls in Cold Climates)

This in turn can create the potential for freeze-thaw damage.

Using insulation that can eliminate any air gap or air leakage with an airtight layer along the masonry shell should prevent condensation and subsequent freeze-thaw damage. But it still has to allow for vapor diffusion during summer (see above).

Confused? I don’t blame you! It took me a while to wrap my head around this. Eventually, I was able to boil it down to two key principles that I can use:

1. The insulation will need to eliminate air gaps and needs to be air tight to eliminate air leakage driven condensation in the winter.
2. The insulation will need to be permeable to water vapor for inward diffusion and drying during summer.

Tags: drainage, insulation, research, water

This entry was posted on Friday, October 23rd, 2009 at 10:49 pm and is filed under design. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.