Change from 75mm Cavity to 100mm Cavity?

[russellfeeed]

Hi

My drawings were made and passed by Building Control pre 2010 Regs.
So the walls are drawn as 75mm Cavity, and the Spec requires 75mm Crown Dritherm 34 insulation. The leaves are standard 100mm Red Facing Bricks and 100mm Celcon Solar blockwork.

That was all over 3 years ago and my consent expired so I'm having to re-apply.

The new 2010 regs means we now need to hit. 0.28U instead of the old 0.30U.

BC have advised that I need to go to 100mm cavity and Dritherm 32.

Problem is, my architect is indisposed due to family health issues and I want to see if I can avoid having all the drawings re-done.

So, now to my questions:

1. Can I achieve 0.28U using a 75mm Cavity - probably not from what I've read, unless I add about 37.5-40mm of insulation backed plasterboard?

2. What's the knock on affect of going to 100mm cavity? Will it affect my roof? Structural calculations?

3. If I go for 100m cavity - can I avoid having everything re-drawn?

cheers in advance
Russell

 

[freddiemercurystwin]

Any competent builder will not need any changes to the drawings to be made assuming its a straightforward build. Everything just needs to be adjusted a bit on site accordingly.

 

[russellfeeed]

Thanks freddymercurystwin

I guess as long the outer dimensions stay the same and only the inner dimensions are reduced, that sounds sensible.

cheers
Russell

 

[freddiemercurystwin]

Yes, anything critical like floor joists or roof trusses etc would only ever (or should only ever) be measured from site dimensions not the drawn dimensions.

 

[^woody^]

You could use 41mm of Celotex in the cavity. Just change the text on the plans when you resubmit, or submit an additional paper with this written on