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Our house was built in the late 1980s, brick outside, 50mm cavity, thermal block. We get dampness and occasional mould on the top of the window openings, ie, under the lintel. The lintel is a perforated galvanised box, not a cavity lintel, ie, it runs straight across both skins. At present it has plasterboard on the underneath, which I plan to replace with insulated plasterboard. Is there anything I can do with the air void in the lintel, and is it worth it? The heat loss through the air is probably small compared to the loss by conduction through the top and bottom of the box.
Easiest thing to do might be fill the void with expanding foam?
That's effectively all a proper insulated box section lintel is.
Thanks, that was what I was thinking, but I can see some possible snags. I could drill out some of the holes to get the injector nozzle through, but I have no way to see how much foam I've put in, and I don't know if it will extrude, spaghetti-like, from the other holes. It's probably 20 or 30 litres, which is over £100 if I use the squirty cans, so I'd prefer not to just overfill it. It will be much cheaper not as an aerosol, but I don't know how I'd get it in. Maybe I'm over-thinking it, if anyone has done it and can advise that would be great.
A proper lintel these days isn't a box, the steel plate on the bottom does a zig-zag up into the cavity, but there's no way I can modify it to that.
I should imagine a single can of expanding foam, maybe two, would fill a 30 litre space. Or have I misunderstood what you are trying to do?
Yeah I was thinking that as well, you aren't just reading the number of litres written on the can are you, the whole point is that it's expanding foam 😂
Made me wonder how much you get in a can. If you look through the Q&A on the one linked below it says a 750ml can makes about 25 litres of foam.
You probably would need to overfill and then cut off the sticky worms that jump out of the gaps once they're set.
Not sure if one foam is better than another but I found those Soudal cans easier to use than the no nonsense ones when we were doing a load of insulation about 15 years ago.
https://www.screwfix.com/p/soudal-genius-insulation-foam-hand-held-750ml/25943
I'm not sure I'd use canned foam, pretty sure it absorbs moisture (and you may need to mist it with water when squirted into a cavity). Might cause it to rust?
I'd leave it open as air is a good insulator, and foaming it is not going to solve the two cold bridges created by the bottom and top of the box
PS gun grade foam is much easier to use, although the can needs to be upended so you may find you can't keep the can vertical enough.
If you value yur safety you should fire protect it anyway, 2 layers of 12mm fireproof plasterboard.
Based on the fact you have an external steel lintel face on the outside of the house and another face of the same lintel on the inside, insulating the inside of the box is going to achieve very little indeed. You have a thermal bridge on the top/bottom faces of the lintel. Remember basic thermodynamics - Metal is a far better conductor of heat than air.
It would be far better to insulate the lintel - certainly inside the house and preferably on the outside too, if possible.
Thanks for the responses. I didn't know gap filling and insulation foam are different. I read the label on a can and that suggested it can expand to 3 times, so I think that was gap filling foam. I'll have to do some calculations on heat transfer to see if it's worth it.