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Because of our budget, there's some hard decisions and trade-offs to be made. Grateful for any constructive comments! Remaining garage size It's a double-length garage - 9m. We could just convert half of it, leaving the front part as a regular-sized garage, but that will leave us tight on space. Instead we're leaning towards taking around 5.5m, making the front part just a workshop and store (and hopefully, micro-brewery ?. Floor will be raised by 100mm using PIR with 18mm chipboard on top, and external walls with timber frame and PIR. Roof to be raised by our tame builder. Shower location Due to the 3-gabled chalet roof, there's not many places where a shower can be put upstairs. There's no scope for an en-suite that we can see (unless we shelled out for a dormer, which is likely to be beyond our budget). The only option for an upstairs shower seems to be to move the bathroom wall into the 3rd bedroom, making it smaller still. The other option is to have a downstairs shower room in the garage, but that's then eating into kitchen/dining room space. Kitchen/dining room configuration The existing kitchen is long and narrow, but we aim to widen the room by taking out the built-in cupboards and moving the door back slighting into the hallway We'll then knock through the current utility room The new space is actually larger than the existing kitchen. Do we move the kitchen into the new space and have the dining room where the kitchen is? Could seem a little odd to walk through the dining room to the kitchen, plus we'd have to install a new kitchen pronto as we'd be wrecking the orginal (and that's not in our short-term budget) Would like big (possibly 4m, 5 door) bi-folds onto garden, and a big (3m x 1.5m) roof lantern in new converted space on flat garage roof. Lowest u-values we can afford. Leaning towards keeping the kitchen location where it is, sink relocated to window, and with a small utility room in old garage space Insulation Uninsulated concrete ground floor - which might be tricky to raise and too expensive to lower for insulation. VIPs too expensive to do throughout (but considering 10mm for kitchen) Cavity walls are already insulated Roof insulation needs topping up, and lap vents installing in roof fabric Heating and hot water Existing system is warm air, with electric immersion and cylinder for hot water. Boiler replaced within last 12 months, but the system can't easily be extended into the new conversion space. So we'd need a combi-boiler just for the conversion. Floor drop in garage isn't enough for the insulation that would be recommended for UFH House isn't well-enough insulated for ASHP As a result we're leaning strongly towards simply a new modern combi-boiler and wet rads throughout

From @JSHarris and others I have learnt about the importance of decrement delay as a characteristic of the fabric of a house in providing internal comfort. Background. My build may well be a single-floored, flat-roofed building of contemporary design. As a rear-garden plot surrounded by other dwellings, it has precious few sight lines and instead will have a profusion of roof lights to let in light. As a (near) Passive House, the roof will be thick and there is a concern that the roof lights will give the impression from inside of a house deep underground. One suggestion to address this is to thin the roof, to use high-prefromance insulation such as vacuum panels in the roof to improve the aesthetic look instead of cellulose filled I-beams. So far so good but I worry that a roof with vacuum panels will have a low decrement-delay factor. So to the subject decrement delay. As I have learnt, a cellulose-filled roof would have a welcome decrement factor. But what about vacuum panels? I have done a little google-ing and came across the following summarised from here: Thus to my question: how can Vacuum Insulated Panels have a decrement delay of 0 hr? I understand that the decrement-delay factor is product of λ (lambda), which is very low for vacuum panels. But is also related to specific heat capacity and the density of the material concerned and I do not understand how to consider these two for a vacuum insulated panel. I wonder, can anyone enlighten me? (For those interested in learning more about decrement delay factor, I found this explanation a help: http://www.greenspec.co.uk/building-design/decrement-delay/)