ᐅ Additional insulation between the upper floor ceiling and the attic space

Hello,

I currently have an unheated roof with a crawl space 😀 and I would like to install rough-sawn boards as flooring to simply store unimportant items there, such as tiles, roofing shingles, etc.

At the moment, the insulation consists of the upper floor drywall ceiling -> vapor barrier -> mineral wool. So, when I’m in the attic, I’m basically looking at the wool between the rafters.

Now, I still have a lot of expanded polystyrene (EPS) insulation boards left and wonder if it makes sense to install them between the mineral wool and the rough-sawn floorboards. I would probably need to compress the wool a bit for that. The layering would then be upper floor ceiling -> vapor barrier -> mineral wool -> EPS -> rough-sawn boards.

Does that even make sense, or would it be better to stick with just upper floor ceiling -> vapor barrier -> mineral wool -> rough-sawn boards?

I hope I haven’t described it too badly and would appreciate any advice.

Thanks!

No, that doesn’t make sense. Compressing mineral wool reduces its insulation performance and can cause condensation within the mineral wool.
Also, don’t install the counter battens (vertical battens) so that they completely press against the mineral wool. If the rafters are not high enough, you will need to build them up. Air must be able to flow between the counter battens and the mineral wool to carry away moisture.

The only option you have is to add insulation on the roof itself, provided you still have enough space.

Ah, okay, thanks for the response. At the moment, there is about 3 cm (1.2 inches) of air space in between; is that enough? Currently, there is no insulation in the roof slopes, only a kind of thick gray membrane or fabric sheet (I’m not exactly sure how to describe it) with ventilation openings. I thought you would need to install some sort of vapor barrier there before adding insulation?

Or is just an air layer sufficient, followed by the polystyrene insulation? So, essentially from outside to inside: roof tiles -> that fabric membrane -> air space -> polystyrene, and then, if desired, interior cladding? Or is mineral wool required instead of polystyrene? I have some leftover mineral wool as well.

Thanks very much for the information!

I’m going to join the discussion here...
Our roof is being redone and the insulation improved. In the process, the old insulation will be removed. Between the upper floor and the attic (which is occupied), insulation will also be installed again, and I would like to know if a vapor barrier or vapor retarder is strictly necessary for this? The thermal envelope will be insulated once more.