10" Block or Cavity Wall - Joist floor or concrete

[ianm75]

I am currently in the process of receiving building quotes for an extension and have some questions about the differences in building estimates from different builders and would like to get some feedback on peoples experience of thoughts:

1. The plans specified cavity wall but one builder wants to use 10" block, is this better for insulation and deisgn or just cheaper for them?

2. Should interior walls be plastered directly or be plasterboarded first? One builder states plasterboarding is a must?

3. The whole house has joist flooring however most builders are specifying New Concrete floor with ducting for the exisiting air bricks. Would it not be easier for pipework and cabing to continue the joist flooring and insulte properly?

Thanks

Ian

 

[^woody^]

1 This must surely refer to the foundations, and if so its acceptable. Otherwise you are looking at a render finish external and insulation board internal

2. Plasterboard is for builder's who can't plaster, but sometimes external walls may require boarding to achieve the required u value

3. A concrete floor will perform better than a timber one, be cheaper and less onerous long term

 

[ianm75]

Thanks Woody,

The Extension is to have pebbledash render to match exisiting house so i suppose thats why they are stipulating 10" block with all internal walls to be plasterboarded and then plastered?

Is this not a cheap way to build an extension and should i worry about things like damp etc. Would i not be better off to stick with the original plans and have cavity celcon Solar block walls with celotex tuff-r insulation in between?

Thanks

Ian

 

[^woody^]

In a severe exposure location then a solid wall could potentially experience penetrating dampness. The render will protect it, but over time if the render does crack, then problems can occur.

A solid wall in aerated blocks, may be more susceptible to thermal cracking too

Otherwise both walls should perform the same.

Its just seems odd why a builder would choose to build it in solid block by default

 

[noseall]

if i were building my own extension then i would prefer a cavity wall construction with 100mm dritherm cavity batt insulation.

i would not mind paying more for this method of building either.

 

[ianm75]

Thanks Noseall and Woody for your help.

I live in a 1940's semi with all joist flooring. Are you guys against an extension with a joist floor so matching original? Or as Woody says it will just be less onerous and cheaper in the long term. As long as its insulated properly are joist floors that bad?

The reason my hesitation about a concrete floor is because i have a friend who purchased a brand new ground floor flat in a block of 6 which is about 3 years old with concrete floors. They are having a problem with damp and when the builder was called to come and look at the issue he says its very common in ground floor flats and normally occurs due to an imbalance between hot and cold. Need to keep window trickle vents open. They builder has a really good reputation and the flats are really good quality. Does this sound right?