I'm planning a house extension. I know that U value regs are going down all the time, and perhaps that is why people use the dreaded thermal blocks for the inner leaf of cavity walls. But there is no way I'm using them. My house is all internal brick walls, very solid, and I need to use denser concrete blocks that you can fix to. What cavity / insulation will be required when not using thermal blocks. Will I need a wider cavity, different insulating batts etc ? Simon.
There's nothing dreaded about thermal blocks. Excellent things, superior to dense block in almost every way. No trouble mounting to them, although use rawlbolts if you're hanging heavy stuff like kitchen cabinets.
If you use dense blocks, you might need insulation between the drylining and the wall to prevent excessive cavity width. You may also need deeper foundations due to the additional weight. You may find that the BCO doesn't permit them to close the cavity at window and door openings due to cold bridging.
You need to / must acheive the required U values, using less thermally efficient blocks, means you need more insulation in the cavity, or on the outside of the blocks. The blocks will act as a heat store, and you will get lag when heating / cooling the room.
You can keep the cavity narrow, by using more expensive insulation. However kingspan and the like are not "full fill" so you can't just shove 100mm of kingspan in there, insted of 100mm of full fill rockwall.
To make life real hard, you have to calculate the heat loss via the wall ties. Also once the cavity goes over 100mm standard wall ties are no good, and you have to use specalist ones, and these are expensive.
They are 3.5N per mm2, what does this mean ? On a 100mm x 100mm square of block, you need 35,000 N to crush it, or approx 3500 KG at sea leavel on Planet earth. Assuming you load it evenly.
The 7N dence blocks are twice this. You can also get 3.5N concrete blocks, which is what I belive the wickes ones are.
The 3.5N thermal blocks are so so much easier to work with - I used 6 lorries full of blocks on my build, 3 of each, and I never wanna see a
7N dence block again.
Th roof in my house is held up by the internal walls, which are 3.5N thermal blocks, with 7N blocks as padstones under lintels. The main roof weighs in at 4000KG, when there is no wind. The upper story is held up with these blocks.
The thermal blocks are not suitable for places where they will get wet a lot, like underground work. Also if you build with them when they are wet, when they dry you will get 1 very fine crack in a 10m run of blocks. This may be the same with the dence 7N blocks, I have no idea, mine are burried, they made a steel re-infoced retaining wall.
I guess if that's the way things are built these days, I will just go along with it. Inside will be dry-lined. Although I do worry about thermal blocks + full fill insulation and water getting through to the thermal blocks which do funny things when wet. If they take the regs further and futher, there will be so many conflicts that it will be impossible to build anything pratical. Simon.
Yep. They are rated to the same strength as dense concrete blocks. They are usually available in 3.5N, 7N and even 10N versions and can be used as the main structural element in 4 storey buildings. The standard 3.5N blocks are usually good for 3 storeys. Your structural engineer will advise.
On 4 Oct 2005 05:33:59 -0700, a particular chimpanzee named sm snipped-for-privacy@hotmail.com randomly hit the keyboard and produced:
So we've gathered ;-)
With super-duper, top-of-the-range aerated concrete blocks, you would need 65mm mineral fibre full-fill cavity batts. With bog-standard aerated concrete it would need to be 75mm thick. With denser blocks,
85mm is needed. If you wanted to keep a clear cavity, a PIR insulation (such as Celotex, Kingspan, Xtratherm, etc) would be best. Between 30-40mm with at least a 25mm clear cavity.
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