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The Weather Network
Oct. 22, 2020 | Thursday
Editorials and Opinions
Arch-i-text: They have to breathe
Spalling in historical brick field. (Supplied/Brian Marshall)

Last week we looked at one of the types of perhaps well-intentioned, but definitely uninformed alterations that result in damage to historic masonry. Continuing in that vein, and much more common during the past few decades, there’s been another "silent killer" introduced into these old houses: Improper insulation.

Now, before proceeding too far, allow me to state that insulation is a far greater threat to structural brick buildings than for 19th-century frame and wood-clad houses. In fact, when correctly installed by experts, historic wood-clad structures can be fully insulated with little or no detrimental effects. But the same cannot be said for masonry houses.

Nineteenth-century brick is not only softer than its current day counterpart, but it is also more porous. In short, that means it absorbs water when moisture is present. Then, in dry periods, aided by the drawing effect of lime mortar, the brick transpires to both the exterior and interior of the building.

This was a normal and expected process in these old houses. With no vapour barrier (the plaster was applied directly onto the interior brick walls), they were able to “breathe,” thereby remaining sound and whole over many, many decades.

Unfortunately, when insulation is installed in a typical fashion not only is the “breathing” significantly impaired, but it will also lower the temperature gradient across the masonry and reduce the difference in temperature between the masonry and the exterior air, further reducing its drying capacity.

Damage to the brick field, particularly during our freeze/thaw cycles, almost inevitably results.

While the thermal mass properties of structural brick buildings deliver heating/cooling benefits to the building’s interior and a brick wall three wythes (layers) thick is approximately R-5, it certainly doesn’t perform at the level of a modern house.

So, are you the proud owner of a historical house that must limp into the future with comparatively expensive heating and cooling bills?

Not necessarily. There are methods to both insulate and protect your old house from damage. For example, one solution presented by Dr. Kim Pressnail of the University of Toronto at the Westford Symposium on Building Science in 2015.

Now in use by several insulation companies specializing in historic buildings, it entails adding a mineral fibre blanket (like Mortair Vent) to act as a vented airspace between the old brick walls and a layer of spray foam insulation.

Before you insulate, speak to a specialist!