block thickness for timber frame

[GCarnegi]

My Architect has specified 140mm blockwork over the top of my 6" timber kit extension. My builder says this is crazy and that we should use only 100mm blocks. He tells me that he knows the BCO and will talk to him to see if it's okay to use 100mm blocks.

The garage we are knocking down to make the extension is made from single skin 140mm block, with an internal 100mm block wall between garage and utility. To me, the 100mm block loks helluva thin to make a two storey extension, and all our comlpeted (approved) plans and heat loss calcs say 140mm.

Is it normal practice to use 100mm block on a timber build, and is a "word with the BCO" all we need to make the change. I'm guessing this would probably impact the heat loss calcs as well as the plans.

Apparently this could save us a couple of thousand quid (switching from 140mm to 100mm block), but the last thing I need is to build it with the wrong blocks.

Obviously the 140mm is more substantial, and I'm curious if it would give us better noise insulation as we are next to a busy street and the rooms will be bedrooms. I'm in two minds whether to pay the extra and use the wider block, or save the couple of grand and use it elsewhere in the project.

thanks

Any advice greatly appreciated.

 

[tkmax]

When you say "over the top" do you mean the blockwork is to be used for the external skin of the timber frame walls? If so then 100mm thick is usually fine for 2 storey (assuming no wind resistance issues) and it shouldnt affect the thermal performance significantly if the original spec was for 140mm dense blocks.

 

[^woody^]

Presumably, this has come to light before the footings have been laid?

You will get marginal additional sound insulation from wider blocks of a similar material, but it all depends on what has been specified

Likewise, it all depends on the specification for the wall as a whole, as it may be that 140 blocks are required to meet the required thermal insulation values, rather than 100mm blocks and a different timber frame or insualtion

So, make sure that your £2k saving on blocks does not mean a £2.5k increase in other areas