why not insulate the roof trusses with the multi layered foil insulation?
I’m considering doing that at some point (when I can get someone to do it for me)…
IIRC the Strawbridges did that in their attic in the Château.
The sort of stuff I’m talking about -
The problem with blown insulation is that you need full knowledge of where there might be noggins or other obstacles in the joist structure below the attic floor. You can make an educated guess but you could easily miss sections &/or cut a lot of holes, which would negate your trying to keep the floor nice.
If you have the height to do it you might be better off building a false floor above the existing one & insulating the new void with your choice of insulation (my pet product is Steicoflex wood fibre panels) which would also allow you make a cleaner, more level storage area.
A more extreme (& messier) answer is to take down the ceilings below, insulate & reboard…
@graham’s idea of a foil product directly under the roof structure could be used in addition to this, or at least a proper membrane to reduce cooling draughts in the attic space.
If you are not using the loft as habitation then I’d put all my money into insulating the ceilings of the floor below i.e. close to where you want to keep heat, rather than losing it into an unused but albeit insulated loft.
Your membrane below the slates may well not be draught proof (joints not taped for example). An additional one (or even a cheap foil insulation, but that doesn’t breathe), both properly sealed will keep the loft a bit warmer & cleaner.
You’d be better off putting another 200 on top of the existing 200. If you do go down the multi couch route and seal off all ventilation in the loft beware of humidity / dampness and all the consequences that go with them.
You could always reclassify the existing loft floor as being firewood. Chop it out by cutting alongside the joists with a circular saw, lay some nice new and cheap glasswool insulation between the joists (you can probably re-use the old stuff as well to help keep cost down as long as it isn’t infested with anything), and then lay a new Stirling Board (OSB) floor over the top of the remains of the old floor that sits on top of the joists.
This will enable you to have a nice flat floor, and also allow you to inspect and treat the joists with insecticide during the process.
Better using rockwool the increase in cost is worth it as glassfibre colapses to 50% after a few years and the reusing of old is not really viable. Water runs off rockwool but soaks glassfibre. Over the years moisture get to glass fibre that why they use an extra support fibre in earthwool etc.
Here’s one I did earlier. Ancient glass wool was laid on the 25mm x 50mm joists and top side of the ceilings of the rooms below. Removing the old lagging, during a heatwave, was one of the worst jobs I have ever done. At thaat time there was no opening in the room and no light.
The joists were so weedy that a board across three made the ceiling bow like crazy.
I added an 800mm x 50mm joist for each of the originals - officially, for the 4m span, they should have been 850’s but I gave 50mm ‘credit’ to the originals. They had, after all, held the ceiling up for years.
A passing roofing gang did me a Velux for 100 notes, my Velux.
Then the rockwool. I found it not at all ‘irritating’ to handle
I put plasterboard covers over the ceiling spots
Doubled the size of the access hatch
Bathroom grade chipboard.
Bob’s yer relative…
PS. After creating that nice spacious area I had to fill it up with my late mum’s stuff. I just boxed it all up and shoved it in the attic, including the contents of her kitchen cupboards.
That created a free cafeteria for mice. Whole bags of sugar and rice and flour and cereals …
After trying various baits - carrot, cuke, apple etc I was tipped off that they can’t resist peanut butter. The first afternoon I tried it, having bought more Little Victors from B & Q, I got 13! It was like someone playing castanets up there!