We want to upgrade the insulation on our 1970s home. What is the best way to do this?

We have a house that was built in 1971. It’s a four-bed semi built with cavity blocks. We dry-lined it over the years. We would like advice about wrapping. We have conflicting opinions about moisture, and are not sure what is true and what is not in this regard. Can you advise?

When getting ready for school on cold winter mornings my three children would sing: “When it’s cold outside, I wear my coat to stay warm, it keeps me comfy toasty in a winter storm.” External wall insulation is a bit like a warm winter coat wrapped around the outside of your home.

The experts will tell you that when retrofitting you should follow a “fabric-first” approach. This means you initially deal with draught proofing, improving attic, wall and floor insulation before progressing to the replacement of windows and external doors. The next stage would then be the improvement of heating systems by replacing old radiators, improving controls or full replacement of older, inefficient boilers with modern heat pumps and the provision of solar panels and the like.

When looking to improve external-wall insulation you essentially have three choices. Insulation can be placed internally in dry-lining; two-leaf walls will have cavities that can be pumped with insulation or insulation can be applied to the outer face of the external walls.

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There are pros and cons for each method. Dry-lining will be disruptive, but the external appearance of your house will not change. Wall cavities are easily filled but they may be of insufficient width to make the job worthwhile. External insulation means you can remain in the house, but the appearance of your house will change. Decorative render or finishes such as brick or stone will be lost to a new acrylic render coat applied over the new externally fixed insulation boards.

In your case you have a single-leaf wall built with cavity blocks. These are semi-solid concrete blocks with a void incorporated into the block during manufacture. The only options available to you are therefore to dry-line internally or insulate externally. I note you have already provided some dry-lining in the past. The main advantage of external-wall insulation is that there will be no disruption to the inside of the house, and you can remain in occupation. It is important that the top of the cavity block wall is sealed to avoid movement of air through the blocks as any air movement within the wall structure will reduce the efficiency of the external insulation by what is called “thermal loop”. Imagine if air could circulate between your body and your coat. Your coat would have no effect.

In terms of moisture this is sometimes held out as a negative when considering external-wall insulation. There have been failures in the past. This is usually as a result of poor workmanship and poor attention to detail in planning the work. However, the fact that the original blockwork acts as a “thermal mass” between the heated inside of the house and the new external insulation layer should reduce the likelihood of condensation. This is because the dew point is moved further out into the wall fabric and away from the internal surfaces. This reduces the likelihood that condensation will occur, with mould then forming as a result.

Providing adequate ventilation to remove moisture-laden air from the inside of the house is imperative to reduce the occurrence of condensation and mould forming. An air-tightness test should be carried out post-construction to ensure that your home is not airtight and there are adequate air changes every hour.

Make sure you deal with reputable installers and take professional advice from a chartered building surveyor. They will be able to advise with regard to mitigating measures to ensure dampness will not be an issue. They will also assess the original construction to make sure that external wrapping is the most appropriate method of improving the insulation of your home.

My children are all grown up now, but I see they still reach for their coats on stormy and chilly days.

Noel Larkin is a chartered building surveyor and a fellow of the Society of Chartered Surveyors Ireland

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