Suitable Insulation - abnormal construction

[Paul-man]

As part of our garage conversion and extension, we are going to need to build an inner leaf to our single skin wall. Because this will also be supporting a steel and walls on the first floor, the inner leaf will be block.

Due to the construction of our floor (long story) we can only leave a maximum of a 75mm cavity if we build the inner leaf out of 100mm blocks. Will our U-Values be ok if we do the following:
102mm Facing Brick>25mm Cavity>50mm Celotex CG4000>100m Celcon Solar>Dot 'n'Dab Plasterboard>Skim

According to the Celcon site this produces a U-Value of:
Combined Method : 0.24 W/m²K

My initial instinct was to fill the 75mm cavity with rockwool, then use 25mm celotex type insulation on the internal face of the blocks. (between battens) before plasterboarding.

Which solution would you choose and why? or is there a better solution?

 

[^woody^]

Form a 100mm timber frame with no cavity (15mm gap) and have full fill celotex insulation in the frame, and this will comply.

A further 25mm celotex across the internal face will be better still.

This will give more room space

 

[Paul-man]

Thanks Woody. Admittedly I don't know much about timber frame buildings, but would that be able to support the required beams above and the roof?

I have uploaded some pictures showing the problem caused by the foundation detail. I have included the locations of beams etc.
This shows the existing structure.
View media item 33931
This one shows that if we built the new wall with a 100mm cavity, it would "overhang" the existing footings.

View media item 33932
This shows what I propose, but building with the 75mm cavity that was left when the house was built so that it sits firmly on the foundation. I would then need to ensure I chose the correct insulation.

View media item 33933

 

[^woody^]

A timber frame can support a steel beam, but it depends on loading - it may need steel post if a timber one is not enough

That 25mm oversail of the 100mm inner wall would not be a problem in this instance

There may also be options for spanning a beam in line with the inner wall to take that other beam (at 90 degrees to it), and then you may have more options with the new inner wall with would ten not be load bearing.

 

[Paul-man]

Thanks woody, the steel will be supporting a new external front wall (bringing the front wall forward from the location of that other beam. The joists are also designed / calculated to run parallel between the beams.

Our neighbours did something very similar and the BCO apparently was ok with the oversail. However, my designer/structural technician seems reluctant to put it on the plans that way so I've been looking at alternatives.