Insulation & Boarding

[bazman]

I've read through the comments on this forum and I have to say that it's been the most helpful place on the internet for what I'm looking for - namely for getting advice on Insulating and Boarding my 3 bed semi in Surbiton, Surrey. The previous occupants put no insulation in and I am currently using the loft for boxes, suitcases, etc. but I want to bring it up to 270mm of insulation and board most of the loft. The problem I have is that the joists are 100mm, so I'm looking to raise the joists and insulate to the full 270mm.

I used the advice on this site to write a spec in an email which I sent to a load of traders advertising on a website

Anyway, I spec'd filling the existing joists to 100mm with rock wool and raising the joists by 170mm at 90 degrees with battens, with 170mm of rock wool added topped off with board. The responses I've had from the builders has been interesting.

Firstly, they advise not to batten at 90 degrees to the existing joists as the battens will not be supported at the ends - this could cause problems as the structure is not designed to support weight in this way. I took myself down to Champion Timber, New Malden, today and discovered just how heavy a 17cm deep joist is - a 4.5m length is very heavy! The builders have advised raising the joists inline with the existing joists, allowing the weight to bear down on the existing supports, ie the internal brick walls.

Having tested the weight of the timber I was planning to add I think the sensible option is to not raise the entire joist, just add sufficient timber to the joists to support the boarding, so maybe add support across 25% of the joist length. Otherwise my lath and plaster ceilings (house built in 1937) might start cracking up, if not collapse!

I should also mention that some of the builders have mentioned using Super 10 Multifoil products in the rafters. These foil products have recently (Oct 2006) been shown to not have an effective U-value equivalent to rock wool. This is due to the fact that the accreditation of the foils was not done using the hot-box method and has not been shown to be as good as rock wool and mineral wool. One builder I've asked has advised putting Super 10 in the rafters and filling the joists with 100mm of rock wool. Another builder I have asked is evidently not up to date with current building regs as he hasn't heard of the 270mm recommended depth of rock wool insulation and has said that 100mm topped off with 18mm of boarding is sufficient.

I have two more builders visiting my property next week to give their opinions and quotes and I will report back what they have to say in due course.

I had no idea that insulating and boarding my loft would be so complicated!

Bazman

[markie]

If you are using your loft for storage only ( and u are not going to put a boat load of stuff in it ) then putting lets say 3x2/4x2 timber's at 90% angles to the exsisting rafters it will be ok. If you walk on the rafters if it's latt/plaster or even plasterboards you may crack the plaster, so the only way not to crack the plaster is not go up at-all. But to get the job done you have to go in the loft so it's your call, go up or not. Up to a few years ago you could use rock wool to insulate between the rafters ( roof ) but as you want it for storage rock wool will be fine as long as you put a vapour barrier ( TPS ) on before you plaster board.

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