Guidance on insulation for new kitchen extension

newbie23

We bought our 1940's mid-terraced house in 2009, it was in poor state and we renovated it before we moved in, apart from it being in poor condition there seemed to be no problems of damp probably due to the ventilation as all 4 fireplaces remain in the house. We put in B rated Low-E windows and doors, rewired, A-rated boiler with 3 zone heating and thermostatic radiator valves and insulated the attic. We did what we could with a small budget as we were not eligible for grants for anything, we tried to future proof our small house . We also put in 50mm insulation backed plasterboard (37.5mm + 12.5mm). This ceilings were bonded at the top for application of boards and the boards were fixed directly to the poured concrete walls with mushroom fixings. This insulation had been chased out by 22mm for conduits for cables, for backboxes and for batons for radiators and curtain poles. All joints between boards and ceilings were covered with scrim tape and plastered, the expansion gap was sealed with silicone and skirtings fixed with glue. Our house is very cosy and costs very little to heat in comparison to our friends or parents heating bills and we pay about 30% below the national average for gas.

We are now building a 23 sq.m kitchen extension. We had an engineer design the structure of the extension i.e foundations, blockwork, roof and structural steel. Due to our small budget we cannot afford to pay an arch or archtech to design the rest as we have a small budget. We are trying to compromise on targeted u-value vs. price to bring the build in on budget so design is getting done as building is going up . I would like to achieve an easy to heat house and not ruin what we have at the moment, don't want to achieve a passive house as that would require a total refurb of the entire house. I'm using the advise of our contractors and the technicians at the insulation manufacturers to help make the right decisions.

I've put in 100mm insulation into the floor below the concrete slab. Our roof is going on this coming week and I plan to use 100mm insulation between the rafters, a gap for services with 50mm boards (37.5mm + 12.5mm) below the rafters. I'm stumped on the types and constructions of internal wall insulation :eek:. Our extension walls are 215mm hollow block construction. Installing our insulation, I would prefer to put 102.5mm (90mm + 12.5mm) boards fixed directly to the wall as before as we had no problems and chase out for conduits, batons for kitchen presses and rads but our contractor is advising that we install 90mm between batons at 400mm spaces, a gap for services and plasterboard on top.

I've learned alot through my investigations in 2009 and now again. These batons will cause amounts of thermal bridging and to be honest I can't afford to have the same construction as the roof with 2 layers of boards

Anyone any thoughts???