Concrete blocks: medium density or aerated type?

[Lobster]

I'm currently bricking up two doorways in a 4" party wall between two terraced houses, as part of a project to reinstate the original dwellings. I did the first one using medium density blocks, which I lugged back from Wickes myself, nearly stuffing both my back and the car suspension in the process... a joiner who later surveyed my handiwork said I could/should have used the much lighter aerated type instead. Is that right? If so I could at least use those for the second doorway!

When we had an extension built a few years ago, with a cavity wall, I remember the builder used heavy blocks for the outer wall and much lighter ones for the inner - I assumed I'd need the heavier ones for the doorway.

I've just noticed in the latest Wickes catalogue (page 36) both types of block are described as "3.5N" - would that be their load rating?

Thanks David

[Grunff]

That is their load rating. I would go for 7N high density aerated blocks. These are slightly more expensive than standard ones, but well worth it. Any builders merchant will be able to get them for you. FWIW I used the Thermalite ones, and they were very good.

[Grunff]

I believe the term high density is used to describe aerated blocks, to distinguish between the 7N ones and the 3.5N (lower density) ones. Not to be confused with concrete blocks, which are /really/ high density!

They weigh about 30-40% more. They are still very easy to handle, much easier than concrete blocks. You'll see the difference when you cut one

- the 7N have much smaller bubbles, a much tighter structure.

[IMM]

If between two dwellings, use high density concrete blocks for sound insulation. Houses normally have lightweight high insulated blocks on the outer walls and the cheapest blocks available on the inner walls. It is best to have dense concrete blocks on inner walls to give a higher thermals amass.

[Lobster]

Thanks. 'High density' and 'aerated' looks like an oxymoron according to Wickes' terminology but obviously I've misunderstood! Are these 7N blocks going to be heavier than both of Wicke' offerings? David

[Derek]

Did he say why? Was it due to the load on a concrete slab below?

[David]

No, it was more that he thought my blocks were overkill for the job, and that I'd subjected my spine and car suspension to unecessary grief!

David