1995 build, felt under conc tiles, how best to insulate between/beneath the 120mm deep rafters.

MarcusTheHat.

As above, to add I have sufficient 1.1mm thick Lamilux fiberglass sheeting to line underneath my rafters in the roofspace, this would be in addition to any insulation placed between and perhaps also reluctantly beneath the rafters, the Lamilux is tough, impervious and wipeclean, and best of all it is FREE.

I already have glasswool(hairy molly?) between the ceiling joists with standard 12mm chipboard flooring already insitu.

I am not looking for any particular insulation value, and I am aware of the 50mm airgap recommended beneath the felt, though I was probably intending to use 75mm Rafterlok and "risk" the 5mm reduction in the airgap.

Thoughts please.

muffler

This is the 2nd thread started by this new user on the same topic and both mentioning this free fibreglass product.

MarcusTheHat.

Apologies Muffler, I am not very computer competent(perhaps some form of computer dyslexia?) and inadvertently posted twice, because I could not find my first post, so I imagined it had not got posted, me being very new on here an all..

My reference to the fibreglass product was simply to flag up (i) that I would not need plasterboard or other internal finish, and (ii) just in case somebody had a reason NOT to use it for this purpose.

It is offcuts from bus roof production that I get to take away absolutely legitimately, otherwise they got to pay somebody to dispose of it. There is no side or angle to my mention of this product.

Cheers,

Marcus,theHat

MicktheMan

OP, what are you trying to achieve?

It would appear that your thermal envelope runs horizontally across your attic floor (between/over the ceiling joists).

Doing anything between and/or under your rafters is a waste of effort unless you are planning on completely redesigning your thermal envelope (not just insulation but also air tightness and ventilation.

Mellor

https://www.boards.ie/discussion/comment/121872843#Comment_121872843

I don’t think having rafters and joists insulated is as useless as people make. But it’s a more complex approach.

I also imagine OP is not proposing a complex strategy and has just misunderstood the thermal envelope.

MarcusTheHat.

Mick and Mellor, you are both correct, in that I am only now starting to imagine that I might understand the potential complexity of my proposed undertaking.

Phew.

I do accept that I will need to fit vents in the soffit and the ridge for ventilation between the added insulation and the felt.

I also need to understand how I am going to insulate and seal off the awkward to reach area inboard of the wall plate, to both insulate the ceiling below(or ensure my thermal envelope is properly complete), while allowing unimpeded airflow up below the felt.

As I said I am not aiming for any particular insulation value target, merely wishing to improve the insulation of our home as best practical, while retaining a useful loft area.

Thank you

Marcus,the Hat

Edit,

To add that I got a proper set of stairs installed to access the roofspace a few years ago, located in a small room just off the landing, so additional insulation of the floor would be "wasted" since the cold air will simply roll down the stairs, into the body of the house, though there is obviously a door to the room. But still, on balance I rather insulate at rafter level, and being now retired, I intend to do it myself, so can afford to invest my time on "fiddly" bits of insulation detailing.