My dining room extension is under construction. The builder has completed it except for decoration but due to a missing window sill I can still see the construction method of the wall. It appears to be
– a thin outer cladding of stonework to match the external appearance of the rest of the property
– a very small air gap or approx. 2cm (clogged with quite a bit of debris in between)
– an inner wall of single thickness blockwork
– inner plasterboard dry lining.

I am no building expert but I thought there had to be a layer of insulation in addition to the blockwork and that given the standard construction here this would normally be expanded foam or the like sandwiched between 2 layers of blockwork. Don’t building regs demand a good layer of insulation?

I am also concerned that the small gap between the outer skin of stone cladding and the blockwork needs to be completely free of debris bridging the gap so as to ensure no water transmission?

I want to challenge the builder on these points but would like some expert advice first so I can feel more confident on these points.

Many thanks,


  • Adam


    You are right to question whether this is the correct method of construction. Never forget that you are the client, you pay the money and that you expect the work to be carried out to meet current building regulations!

    You are correct in your feeling that there should be an insulating layer between the walls. Most modern extensions and builds use rigid board foam and you want to see that it is hugging the "warm side" ie held against the internal blockwork to maximise its effectiveness in operation.

    The "cavity" (space between the bricks) should be around 100mm (although this is by no means gospel and is subject to changes depending on the situation/design/construction method) This allows for the 50mm or 70mm insulating boards to fit and also provide a gap that helps prevent the transmission of water between inner and outer skins.

    It might be the case that the builder has ‘closed off’ the cavity around the window opening and this is why you can only see a small gap. There are various ways of doing this and without photo’s I can’t comment on whether it has been done correctly.

    The most important question I can ask is whether the project had prior building regulations approval for the construction plans and have you had visits from the Building Control Officer as the build progressed?

    If there have been regular site visits from the BCO then there is unlikely to be anything untoward going on. They’re involvement is solely to ensure that the build is being properly and safely constructed to current regulations.

    That said if there is absolutely no insulation present at all then there is indeed a problem. A big one. Rigid foam insulation is expensive and can be a good way for a builder to cut corners and save costs.

    Engage the builder in a friendly chat and ask him to explain his design choice for closing the cavity. You might find you get all you need from that conversation and have your mind put to rest. If you don’t you know you can start to ask more probing questions.

    Remember, it’s your project and you get to ask as many questions as you like. If you need to you can speak to the building control case officer who is overseeing the work. He will have a written log of the works he has seen. He may be able to allay your fears.

    Good luck!

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