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Roof going on soon, I need to decide which insulation material/system to use. It is included in main contract but spec says “160mm K7 or equivalent to give 0.20 W/m2K”. I am aware this is min building regs so will be upgrading ... Builder wants to use Actis (Hybris and H Control) to save labour cost. Kind of like the H Control as a continuous layer but not at all sure about Hybris! I don’t want to push for PIR because I know it will end up with loads of gaps and I can’t be there to supervise. Would 200mm loft roll be easy enough to install from the underside, ie. self supporting until plaster boarded or would we need to use batts? I have plenty of space for insulation between and below rafters; raised tie trusses with 360mm rafters! What is the best balance of benefit vs cost?

The build up of the loft space where my MVHR and hot water cylinder are going was going to be like this: Steico Flex has been in short supply for months. We placed the order at the beginning of August and were told it was a 10-12 week lead time but we would have it in October. Today my builder got a call from the supplier who cancelled the order. Calling round today we are getting even longer lead times of 12-14 weeks (From now). My builder is in a rush and wants to use PIR instead of the 140mm Steico Flex. I'm not keen on this at all because this build up was designed (together with the external layers of the roof such as the sarking board) to give us a decrement delay of about 8.5 to 9.5 hours. Not as good as the ideal 11h, but still much much better for keeping the house cool than PIR. My builder says it's just the MVHR and hot water cylinder room so it doesn't need to be kept cool. There is not much linear metres of MVHR pipe in that room, as although all the pipes start and end there, they leave the MVHR room within 3m or 4m at most. We have enough Steico flex for the the actual living spaces in the roof (where the build up is 160mm of steico instead of 140mm and we also have additional 40mm of hard wood fibre, so 200mm in total). What should I do: just make the builder's life easy and go with PIR for the section of the roof that encloses the MVHR and cylinder room, or try and find an alternative supplier or alternative product? (I've already called 4 suppliers any nobody has Steico flex in supply.)

Apologies if this is the wrong forum. Am looking at a timber frame (probably) build in the future. Although one requirement will be retaining heat, another one, possibly as important, will be not overheating in the summer. From what I've read then decrement delay can help with this so maybe looking at something like wood fibre insulation or similar. Has anyone experience of this, or designing for it. Alternatively, would you recommend other methods, e.g. MVHR with summer bypass, MVHR with cooling, UFH with cooling, AirCon, solar shades, anything else. Thanks.

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/)