Insulating Masonry Walls

It may look like a plain stone wall, but the internal working of stone masonry is pretty complex. Anyone considering adding insulation to stone walls needs to tread carefully.

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masonry wall insulation challenges MAAC Studio conservation retrofit

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There is a lot going on in stone masonry, mostly people are concerned about the wind and the rain on the outside and believe that if the maintain the outside of the wall, everything will be alright.

Under normal conditions, rain will penetrate into a masonry wall but only to a depth of 150-200mm and this close to the external surface it is easily evaporated by the wind and sun. Vapour on the other hand, passes right through the wall fabric, usually from inside to out but sometimes from outside to in. Vapour is a curious thing. Moisture in the air comes from many sources, from cooking, showering and just breathing.

We can’t see vapour, but a lot of people talk about breathing walls and understand that sealing the building fabric of a stone building will lead to problems. This is only partly correct.

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temperature and vapour gradient through insulated masonry wall MAAC Studio conservation retrofit

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High internal humidity levels are normally considered a problem in homes, but this can be overly simplistic. A wall that manages vapour and moisture will generally be dry and warm. However, it should be noted that if the internal and external temperatures are similar, the humidity levels will between inside and out will also converge until they reach an equilibrium. In Scotland external humidity levels are generally high even in Summer, which means that internal humidity level can also be high in summer. During the winter, internal temperatures are much higher than those outside and the RH can fall to low level.

The temperature of the wall core relative to the external environment is critical to the management of moisture. Insulation resists the transfer of heat from the inside a building to the wall fabric. Adding insulation to a stone wall will cause the temperature of the masonry to drop. The more insulation you add, the colder the wall will get and the higher the humidity levels within the wall will rise. This will happen even if a breathable insulation is used.

A low core temperature and persistently high humidity levels will cause a number of problems.

  • At high humidity levels, mould and rot can thrive.
  • The moisture will accumulate over time until the wall is saturated. A saturated wall has a much poorer thermal performance, so your wall will get colder over time.
  • High moisture levels can also lead to the break down of traditional lime mortars, which can wash out of the wall core, leaving voids that further accentuate the problem.

Some types of stone and methods of construction can be more susceptible to problems than others and the problems can be even more acute if a vapour impermeable foam-based insulation is used.

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Air infiltration MAAC Studio conservation retrofit

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There is a lot of pressure to retrofit homes with high levels of insulation on external walls but this could be counter productive and damaging to traditional properties. Every building is unique and needs to be considered in its own context, but in most instances, there is much greater heat loss from air infiltration through gaps in construction, than there is through the masonry walls, and it would much better to tackle this area.

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MAAC studio are accredited conservation architects, PAS 2035 Retrofit Co-ordinators and Principal Designers working with traditional buildings throughout Scotland. 

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