Property

We are buying an older house, what are the best insulation options?

Improvement of thermal performance needs to be looked at holistically or there can be unwanted side effects

Until you have secured a property and had it surveyed, you will not know the full parameters or restrictions in terms of what is required to improve insulation

Thu Mar 8 2018 - 06:00

Question: I am buying my first home early next year. With my limited budget, it looks like the house that I buy is going to be an older type that needs a bit of work. I rented a house similar to this for the last five years and it was absolutely freezing. The BER rating was F – and that was after they had insulated it with the foam bricks on the outside. For that reason, I'm sceptical about this external insulation lark; when it was done on this rented house all it caused was mould – in fact, the house was colder during the summer afterwards.

As far as I know, older houses have solid blocks rather than cavity blocks, and I assume that means I can’t have any sort of cavity insulation. Does this mean that I am left with only two options; external insulation and internal dry-lining? I am happy to put money into it, rather than wake up in a cold house again.

Answer: When it comes to improvement of thermal performance by insulating, you essentially have three choices. You have alluded to these in your question but for the benefit of other readers, they are the provision of insulation on the outer face of the external wall, provision of insulation within the thickness of the wall (cavity insulation) or insulating the internal face of the wall (dry-lining).

Your experience of externally applied insulation has not been good. Your assertion is that the situation was not improved significantly by the introduction of the insulation. In fact, you suggest that conditions within the house worsened, leading to condensation and mould.

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There are many reasons why the retrofitting of insulation may not be successful. For example, the property in question may be draughty. Insulation will not improve this as the problem relates to the amount of air changes in the house. Most people notice that their home is harder to heat on windy days. This is because heated air is being lost more rapidly through chimneys, wall vents, underfloor vents, gaps around windows etc. The number of air changes in a property increases on a windy day, with a resultant additional requirement for heating. The location of insulation within the external fabric will not have any effect on this phenomenon. Therefore, if you are upgrading insulation to any property, this needs to be looked at holistically. Draught proofing should also be improved.

New properties are now required to be “airtight”. This is an extreme which was not required in older properties. Limiting air changes can lead to condensation without the correct use of air-conditioning equipment. Condensation can lead to mould, as you have already experienced. Air-conditioning removes excessive moisture from the recirculated air therefore reducing condensation and mould. The requirement for some form of controlled ventilation system may be needed as part of a solution.

You are hoping to buy an older property. Until you have actually secured a property and had it surveyed, you will not know the full parameters or restrictions in terms of what is required to improve insulation. So, let’s imagine that you do buy an older property in need of work to include improvement of thermal performance.

Generally in older houses, the walls will either be of solid brick or of hollow concrete block. This means, as you correctly point out, that there will be no cavity capable of being insulated. I have seen cavity blocks where insulation has been pumped into the wall. This concept is beyond futile, as unless each individual block was drilled in two places, insulation cannot travel to all of the voids within the wall thickness. It is important therefore to fully establish the actual make-up of the external wall to help inform the decision as to which is the best route forward.

Where cavity blocks have been used, or where two leafs of blockwork exist and the void between the inner and outer masonry leaf is thin, these walls will not be capable of receiving any significant insulation. You will be left therefore with either the option of internal dry-lining, or the application of external insulation.

If you choose internal dry-lining, you will typically lose internal skirting boards, covings, window boards and electrical sockets on the inner face of the external walls. With older properties, this provides an opportunity to improve the number of electrical sockets in tandem with dry-lining but some historic features may be lost. Room size will also be impacted upon. Central light fittings on ceilings will be off centre, but if you’re rewiring this can be addressed. Chimney breasts can be awkward to accommodate and can leave a substantial cold bridge to the external. The potential for condensation forming behind linings on the external walls must also be assessed.

If you choose external insulation, the insulation is applied to the outer face of the existing walls. You will lose the original external finish with this method, be it brick or dashed render and you must also accommodate downpipes, soil stacks, alarm boxes, permanent vents, subfloor vents, window sills, inlet gullies and external taps. The application of external insulation will also mean that windows will be recessed quite deeply into the eventual thickness of the wall. This can appear unattractive. With semidetached or terraced houses, there will be a step at the party line and finishes will typically be unmatching. It is also important that ventilation paths to the attic are not obstructed where roof soffits project across the external walls.

When assessing the most suitable method of retrofitting insulation, the potential for air movements in the structure on the warm side of the insulation must be considered. If a small cavity exists between the inner and outer leaf and insulation is fitted externally, there is potential for air movement to continue within this cavity. This is termed a ‘thermal loop’. If one imagines an overcoat that allows air to circulate within, removing warmth. A thermal loop will interfere with the correct performance of the insulation, making it inefficient.

Retrofitting of insulation and in particular, externally applied insulation, has become extremely popular. One size does not fit all, however. It is important that improvement of thermal performance is looked at holistically or there can be unwanted side effects. Therefore, the original form of construction, the potential for thermal loop, condensation and draught proofing must also be examined when planning your insulation upgrade.

You should seek the assistance of a building surveyor to firstly establish the actual construction of your property and then to advise on available options and impact of the chosen solution.

Noel Larkin is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie

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