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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/)