Self Building two in North Wiltshire

Can you still achieve the u values with 50mm insulation of is it different with is being above a basement?

Looks good

Those crash decks are really annoying, especially when you have small rooms / corridors to do.  I am sure it could be managed another way but the bean bags are not much better.  Did you go for beam and block for structural reasons?  I have done timber frame over a basement and did a timber floor, just to cut down on a contractor.

Oz07 - the basement has part of it as warm habitable space, and part is cold garage so the insulation is complex - the habitable space has 150mm in the basement floor, and the garage has (at least - it varies!) 100mm in the roof (under the B&B). This is all overlayed with 50mm above the B&B. So minimum is 150mm (between cold and warm) which achieves the U values. The reason its all 50mm above the B&B is to get a level floor at 1st floor level, and in the basement the 150mm in habitable vs none in the garage gives you the step down into the garage you need for fuel spill etc. Inevitably there are some cold bridges and less than optimum junctions but overall its not bad - and with all of this there are diminishing returns on refining those junctions. Passive House it aint - but pretty well insulated it is! And all PIR not EPS - though SAP reckons that using Phenolic would have made no difference.

 

Given the benefit of hindsight the overall design adds complexity at every turn in every element (different insulation, beam thicknesses, wall design etc etc) and before going to detail design we should have taken the opportunity to simplify it as much as possible. Some of the complexity comes from our basic requirements (like an underground garage dictated by the side topology and planning restrictions) plus the inevitable evolution of the design as it gets modified and refined, but after that we wish we had stood back and looked at it carefully - if we knew then what we know now! Having said that it all looks like it will work! If you want the really ugly details this is probably a blog post in its own right!

 

Mr Punter - B&B was mainly structural as there are some large spans (6+ m) - but it could probably have been done in timber pozi joists (doubled up) that might have made the insulation simpler. Our GW contractor was OK to do B&B plus masonry, but not sure MBC would have been up for a timber floor - so we did avoid an extra contractor. Working with only two was complex enough - cant imagine how hard it would have been with a third.  Another of those - 'wish we had known' things was that the B&B itself is complex in that it has different beam thickness's (140 and 215) running is different directions! which means that there are different heights of supporting walls to get to a flat floor. It might have been simpler to go to all 140mm thickness and throw some more steel in, or over engineer and go for thicker 215 beams everywhere.  That cost versus complexity trade off, plus a SE who didn't really grasp the concept of value engineering! Plus of course a huge dose of ignorance / naivety  on our part - ho-hum you learn the hard way! And its been really interesting learning, though everyone seems to assume we know what we are talking about, so come to us to resolve the inevitable issues - what the American term 'drinking from the fire hose'!

 

More on those crash decks in the next post!