Loft insulation

[i_am_fubar]

[Note: Does anyone else think that with rising fuel costs, it may be worth having a section on generic home insulation and econimising?]

Anywho, I cleared my loft out the other day, and I found that the shoddy fiber glass was patchy and probably doing a REALLY bad job as insulating the top of the house.

In an attempt to econimise, I plan on re-insulating it and flooring it. I am torn between polystyrene sheets (50mm) and rocwool. I dont believe that the loft is really accessable enough to make spray foam an option.

Our loft joists are 3"x2", so idealy, no more than 3" of insulation would be used. (unless there are ways round this?)

From the information I've supplied, what would you guys (and girls?) recomend, for the most cost effective re-insulation?

Sam.

 

[i_am_fubar]

seems the only thermal comparison charts are published by insulation companies who have are biased to their own products...

 

[shenks]

270mm of rockwool insulation is the norm these days for a loft... this is usually achieved by using 170mm between joists and then 100mm cross lay.

This depth of insulation would achieve an approximate 0.12u value according to the rockwool website.

However since you intend to floor the loft then questions must be asked as to how you are going to do this? Placing boards directly on top of the existing ceiling joists is not usually a good idea as they are only there to hold the ceiling up - they are not suitable for much of an imposed load created by boards and materials being stored.

If any appreciable weight is going to be in your loft storage then you need to have new joists installed which span the required area and do not contact the existing ceiling joists. Joist hangers to centre leaf or gable, new trimmer(s) etc. need to be considered.

Either way flooring would reduce the amount of insulation you could place under the boards so you may also have to look at insulating the inside of the pitched roof. Whatever you do DO NOT consider the foams that are sprayed onto the reverse of the tiles/slates, there are plenty of threads on this forum about this being a 'bad idea'. Instead, use celotex/kingspan between rafters but pay attention to roof ventilation.

EDIT: forgot to mention... if you use rockwool insulation then do not have any mains cables passing through it on their way to any ceiling roses for lights as this reduces the capacity. In extreme cases overheating can result with an associated fire risk. All cables should be clipped back to joists, binders or trusses which then acts as a heatsink (or so my regular electrician tells me). Some of the sparkys on this forum will be able to clarify...

 

[Shytalkz]

PUR board, such as Kingspan/Celotex is the most efficient, but there's a cost implication.

 

[i_am_fubar]

oh dear, wires are an issue?

There are wires going everywhere through that loft, just draped over/trough insulation.

And as for raising the floor. I don't know how feasible this will be as the loft is small enough as it is.

It was one of those budget properties when made. crap quality.

Sam.

 

[shenks]

Just try and make sure all wires are clipped back against any timber. Best practice dictates that no wires shoud be covered by insulation but in reality I see it quite a lot - in essence, whilst I recommend you follow the advice previously given there isn't anything I (or anybody else) can do to force you to do so.

If your loft is quite small which precludes putting in new flooring, then you will have to realise that you can't put much weight up there... well you could but then you run the risk of ceiling sag.

 

[davelx]

In most cases if the wiring is only for lighting you should be OK. 1.5mm cable is rated at 19A which de-rates to 9.5A surrounded in insulation, and 1.0mm is rated at 15A which derated to 7.5A, so provided the lighting circuit is protected by a 6A MCB that is OK.

If the lighting MCB > 6A, or there are other circuits up there - especially an electrically heated shower - then you need advice from an electrician.