I moved into a new house earlier this year (typical early 20th century with solid brick walls, old uPVC double glazing, uninsulated floors etc). My plan is to improve the thermal efficiency over time as funds allow and potentially put in an ASHP at some point. The loft is currently unboarded and due to the need for alot more space to store stuff this is where I'm going to have to start! I'm pretty new to this but have already picked up lots of useful info from reading old posts on this forum - no doubt I'll have more questions in due course...
I'm trying to decide between insulating between and above the ceiling joists (to give a total of ~300mm mineral wool insulation) and insulating between and below the rafters (giving a total of 150mm foam board insulation).
There is clearly a difference in material costs (£800 for the ceiling joist option including plastic legs to support the new boards vs ~£2,500 for rigid foam insulation boards to the rafters).
However, pretty much everything else seems favour insulating the roof (more usable space, boarding out first will make it easier installing the insulation, don't have to worry about insulating the loft hatch, and ventilation perhaps less critical?).
Are there other considerations I should be aware of? I saw a previous post about roof insulation boards shrinking over time - is that a significant risk?
Many thanks in advance

I think loft floor 400mm quilt 500 ventilation then any proposed deck.

Doing the rafter option increases the heated volume and hence heat losses and heating bills

Worse is that the loft will almost certainly be sit leaky, costing even more on unnecessary heat losses

Yes sorry 50mm ventilation

Not a problem to store stuff in the cold

I don’t think that 50mm is required under loft boarding?
The roof itself, yes.

Posted By: Arf measuresThe rooms below the loft are 2 bedrooms and a stair landing, all of which have lath and plaster ceilings (the loft hatch is above the stair landing). The only other penetrations in the ceiling are the cables for the lights which can be sealed well. I've seen people talking about adding a vapour barrier above the ceiling and joists. How important is this? I would have thought that the loft hatch is always going to be a much more significant source of air and water vapour leakage with its less than ideal seal

Plaster is pretty transparent to water vapour, though not to air. That's one reason people add a vapour barrier or a VCL. But some paints are pretty good vapour barriers themselves (e.g. latex paints tend to be barriers, clay paint is transparent). Loft hatches are similarly variable, ranging from very leaky to airtight. Opening the hatch won't matter much as long as it isn't left open for periods. So everything depends on the particular circumstances.

“Fig. 6.7: The loft insulated and the stools and rails in place ready for the raised floor with 50mm of ventilation space under it. This picture is from much later in the build process. ”

— Adventures of an eco builder: Designing and building a low energy home by Tony Cowling
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