thermalite v's concrete blocks...?
Discussion
I've pretty much come to the conclusion that I'm going to be building my extension myself, but one conversation with a prospective builder has me a bit concerned.
I was talking about thermalite blocks, he said that they don't like using them on interior walls (due to their tendency to crack around reveals and windows) and don't like using them on the external wall as they tend to suck the moisture out of the render too quickly and leave a surface likely to crack.
What's the thoughts on this? Thermalite are obviously a hell of a lot easier to lug about and maneuver and if I'm doing it all myself I'd prefer not to damage my back much further.
I'm thinking of either plasterboarding the internal walls afterwards, or using a concrete lintel BELOW the window reveals as well as above to prevent the typical cracking.
I was talking about thermalite blocks, he said that they don't like using them on interior walls (due to their tendency to crack around reveals and windows) and don't like using them on the external wall as they tend to suck the moisture out of the render too quickly and leave a surface likely to crack.
What's the thoughts on this? Thermalite are obviously a hell of a lot easier to lug about and maneuver and if I'm doing it all myself I'd prefer not to damage my back much further.
I'm thinking of either plasterboarding the internal walls afterwards, or using a concrete lintel BELOW the window reveals as well as above to prevent the typical cracking.
Timmy33 said:
Mate of mine has thermite interior walls, he tried to bolt in a 60in plasma TV onto the wall........much amusement followed ( from us )
Thermalite 1) cracks, 2) doesn't have the greatest load bearing capacity.
has exceptional load bearing capacity for it's weight - lets face it, it supports the entire roof of your house without much effort Thermalite 1) cracks, 2) doesn't have the greatest load bearing capacity.
Just a case of putting a bit of thought into how you are loading it.
Davi said:
Timmy33 said:
Mate of mine has thermite interior walls, he tried to bolt in a 60in plasma TV onto the wall........much amusement followed ( from us )
Thermalite 1) cracks, 2) doesn't have the greatest load bearing capacity.
has exceptional load bearing capacity for it's weight - lets face it, it supports the entire roof of your house without much effort Thermalite 1) cracks, 2) doesn't have the greatest load bearing capacity.
Just a case of putting a bit of thought into how you are loading it.
Presumably this makes a difference.
Plotloss said:
It only has good load capacity in one dimension though.
Its a right f
king pain in the arse to hang tellies on.
Odd. Never had a problem myself using proper lightweight block fixings! I've got a huge solid oak bathroom unit with mirror screwed up to the wall no problem, weighs a hell of a lot more than a telly!Its a right f
king pain in the arse to hang tellies on.Admittedly never screwed a telly to the wall so maybe something I'm missing!
rsv gone! said:
It should be your structural engineer advising you of the density you require.
That said, my (modern) house is built of lightweight blocks and they seem to have the same consistency as meringue.
There are a couple of areas that'll need concrete block but the majority the S.E. said it's up to me. Just looking for thoughts on the builders comments really.That said, my (modern) house is built of lightweight blocks and they seem to have the same consistency as meringue.
Plotloss said:
Specifically cantilever arms that are a pain in the arse.
You also only find out its thermalite when you go through the topcoat, usually with the wrong size drill for montis, as you've gone in with the idea of sheild anchors.
Pain in the arse.
\You also only find out its thermalite when you go through the topcoat, usually with the wrong size drill for montis, as you've gone in with the idea of sheild anchors.
Pain in the arse.
I used to sometimes hang 32" CRTs on Thermalite walls, what a pain that was, just about do-able.
cjs said:
Thermalite blocks have great insulation characteristics (hence the name) Often building regs will allow them to be used in a single skin wall, no need for a cavity, so a lot cheaper and easier to build with.
Single skin and still getting a 0.30W/m^2K U-Value... they're good but not that good!! Unless you're planning on some sort of lining system which will jack the price way up 
Thermalite's have better insulation properties than a solid concrete block. If you swap to concrete commons then you may need to increase the size of your cavity insulation to achieve the same U-value. Your insulation manufacturer can supply U-value calcs for you. Don't see a problem with Thermalite's myself though, I spec lightweight blocks all the time.
Edited by benny.c on Monday 28th April 12:24
I built my own extension last year.Quite a big one at 20' by 17' 
I used thermalites but wish I hadn't.
1. Whilst they can support a surprising amount of vertical load they are weak in most other respects. Put it this way; you can cut through them with a plastic knife and embed a nail with the palm of your end!
2. I was very thorough in the rendering process and took every precaution against cracking, i.e. I used EML mesh fixed at every 300mm and blocks soaked before bonding. The render has cracked and blown!
3. As mentioned by others, they are useless for drilling and fixing into. When you consider extensions are often built to create a larger kitchen area with wall cabinets it becomes a real problem. I dry lined with 40mm thermal board as per Part L of regs, which means that anything substantial (kitchen cabinets?) I fix to the wall requires 10 x 120mm screw and plugs
4. I spent a lot of time and money making the structure comply with Part L regs. With cavity wall construction you just need to drop thermal batts in the cavity and that is it. No thermal boarding,rendering etc.
If I ever do another one I will pay the bit extra and have it all done as normal cavity walls. Much easier in the long run.
I used thermalites but wish I hadn't.
1. Whilst they can support a surprising amount of vertical load they are weak in most other respects. Put it this way; you can cut through them with a plastic knife and embed a nail with the palm of your end!
2. I was very thorough in the rendering process and took every precaution against cracking, i.e. I used EML mesh fixed at every 300mm and blocks soaked before bonding. The render has cracked and blown!
3. As mentioned by others, they are useless for drilling and fixing into. When you consider extensions are often built to create a larger kitchen area with wall cabinets it becomes a real problem. I dry lined with 40mm thermal board as per Part L of regs, which means that anything substantial (kitchen cabinets?) I fix to the wall requires 10 x 120mm screw and plugs
4. I spent a lot of time and money making the structure comply with Part L regs. With cavity wall construction you just need to drop thermal batts in the cavity and that is it. No thermal boarding,rendering etc.
If I ever do another one I will pay the bit extra and have it all done as normal cavity walls. Much easier in the long run.
It's going to be a cavity wall regardless - it's a fairly substantial double story extension of around 55sqm with exposure issues (currently zero insulation in the house at all, it can drop to outside temperature inside 10 minutes of the heating going off...) But the render issue is one that concerns me - the house is 14 courses of brick with render above to roofline so I really don't want it cracking up! Lining the interior with 9mm plasterboard dobbed on is obviously a cheap way to prevent that inside.
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