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22 March 2021

Considering Thermal Insulation for Concrete Block Walls

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Recently I was involved in various discussions regarding the possibilities for providing thermal insulation to the solid concrete, and concrete block, external and retaining walls of several residential buildings. For lightweight framed and panel wall construction, the thermal performance of the insulation is the primary consideration. With heavyweight wall construction (normally concrete), the ‘thermal mass’ performance needs to be given equal thought along with the insulation’s contribution. This performance can be both positive and negative, and even then these aspects can reverse as dictated by time, and by static or dynamic temperature differences. Also differing orientations (and over-shadowing) of the same construction detail will give different thermal performances. [NB. Overlaying all this is the equally important aspect of condensation control, but is set aside for the purpose of this blog, even so it must always be considered as part of the overall construction detail design.]

There is common knowledge that exposed concrete floors are good at tempering the internal temperatures of houses, and internal concrete walls can do the same if positioned intelligently. Block or solid concrete walls are part of the thermal envelope of buildings and so are exposed to the dynamic exterior environment, and yet their fabric cannot be continually adapting to the constantly changing conditions. Consequently when designing the construction details, compromises must be made.

For the majority of buildings and locations in the country the various standard details being used for thermal mass construction are generally satisfactory even allowing that each project is unique where all design considerations should be given due consideration. It is not my purpose here to give solutions but rather to stimulate thought for when more than the ‘usual solution’ is desirable. Below I have set out two scenarios that can arise. The first is when alterations have unintended consequences, and the second a unique extreme situation where in my opinion standard solutions must be seriously questioned. Although the setting is extreme there are many more moderate situations where the circumstances would benefit from lateral thinking.

In the first example, the inner face of a concrete block wall became significantly invaded with surface mould after many years without any problem, and reappeared after each cleaning. From questioning, it emerged that the bathroom window had been replaced but the existing fan had not been refitted to the new pane. Previously the internal moisture levels were too low for mould to develop but without the extract fan conditions became suitable. By giving thought to matters beyond just replacing the glass, the potential for mould growth would have been easily anticipated. Without any change to the block wall itself it became a negative to the building.

The second scenario illustrates the importance of checking if there are unusual factors which should be explored even when, taking a narrow view, the standard insulation solution seems to be OK. Some time ago the problem of how to insulate the concrete block exterior walls of a ski lodge was raised. Although this type of building is ‘residential’ when occupied, it is not continually lived-in as a normal house and so a different priority of the importance of the various construction detail design parameters, (some as mentioned above), is needed. A common solution for urban areas is to insulate the exterior so as to maximise the benefits of the available thermal mass, but is it appropriate in this distinctive situation? Contrary to a normal position, I suggested that the thermal mass is a negative and so should be minimised for the primary reason that the building is occupied on an intermittent basis in a below freezing environment.

There is a lot of thermal mass in concrete so that when the occupants arrive at the cold building the heating energy will first disperse into the concrete blocks, rather than being available to raise the air temperature, and therefore the insulation should be internal. Also by bringing the insulation inside a whole range of significant construction and life-long maintenance problems are removed, such as: achieving perfect application in the hostile external environment; wind/sand erosion of protective coatings; and the importance of all water being kept out over the insulation’s lifetime, as R-value is reduced and ice-heave damage results if wet etc, etc. Rather than, as in urban areas, using thermal mass to passively moderate the internal environment, consideration should be given to elimination of the weak points in the insulation envelope of which the windows are the primary one (properly hung multi-layer curtains would best continue the wall insulation zone).

Combining thermal insulation with thermal mass in a thoughtful and carefully constructed way can reward the dwelling’s occupants with long-term economic passive thermal comfort.

Through EcoRate Ltd – Architect I provide objective independent passive solar thermal performance analysis and advice on sustainability matters, to Architects, Designers, Builders, Manufacturers, and others in the construction industry, included those proposing to build a new home. I am also a Homestar Assessor.

For more information feel free to contact Keith at EcoRate Ltd on 021 890 251, keith@settlement.co.nz, or our website www.settlement.co.nz

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