Most efficient loft insulation?

vader1_2

Morning all,
I've recently been undertaking some work in my loft putting in downlights etc and in this process I've been pulling around the loft insulation (regular horrible fibreglass stuff).
Aware that it is not deep enough I'm about to embark on a project of properly insulating the loft ready for next winter. I figured I do it bit by bit each month.
The question is, what is the most efficient insulation to buy? In a few years time we are thinking of converting the loft to another bedroom and so my initial thought was to buy the Cellotex sheets that fit in between the ceiling joists - clean, easy and to my knowledge very thermally efficient. Is this correct?
If this is the case what depth would I need to go with these type of boards to achieve the recommended thermal efficiency?
Would it be advisable to go to the current depth of the joists and then line the roof?
Sorry, lots of questions! Thanks in advance.

Silver-Surfer_2

I wouldn't waste the money on cellotex, if you were to convert it in the future you wouldn't want the new room insulated from the rest of the house. Just go for what's on offer at the sheds and get it up to 270mm minimum. Few years ago I got it for £1 a roll from this site.

27col

Just use the fibreglass stuff that comes encased in plastic. Like a plastic bag that you don't have to remove. Easy to handle and lay..

globalds

Are you saying the plan is to re use the cellotex in the loft conversion ?

Might make sense long term

vader1_2

Great thanks all,
actually that's rectified my misunderstanding. For some unknown reason I thought that between a loft conversion floor and the existing ceiling they would put a material such as Cellotex - therefore as you say cutting it off from the rest of the house.
I presume nothing would go in that space to allow the heat from the rest of the house to get into your loft conversion? However, the roof, of course that would need the insulation.
Off to diy world I go then.

stator

If you do convert the loft it will lose a lot of heat, so there is no harm in using PIR board like celotex underneath the floor in the loft, it is the most effective type of insulation you can buy. Once converted you would only heat the loft room when you need it and for the rest of the day leave it to get cold. As for depth use the maximum amount that will fit between the joists. The other advantage is that you can put down some chipboard flooring in the loft over the top of the celotex and use it for storage. If you put down Rockwool etc then you can't put anything on top.
How easy celotex is to fit depends on your joists. Mine are all irregular widths and shapes, so it's been a nightmare to cut it down. It does cut easily using a thin snap-off knife, but individually cutting hundreds of pieces is annoying.
Bradfords (South West) was the cheapest place to get PIR board for me.
Wickes also sell it in 50mm thick small slabs that are very convenient if they are the right width for you and not expensive either.

System

If you are 100% going to convert the loft then you could install insulation at rafter level and give you a nice big useable loft in the meantime, you could just leave whatever you have at ceiling level which you could later upgrade to acoustic type insulation...

Furts

vader1 wrote: »

Great thanks all,
actually that's rectified my misunderstanding. For some unknown reason I thought that between a loft conversion floor and the existing ceiling they would put a material such as Cellotex - therefore as you say cutting it off from the rest of the house.
I presume nothing would go in that space to allow the heat from the rest of the house to get into your loft conversion? However, the roof, of course that would need the insulation.
Off to diy world I go then.

You are heading for the right answer, but perhaps for the wrong reasons!

In simple, practical terms there is a need for fibreglass insulation, or Rockwool, in the floor void under your new loft conversion. The reason is ceilings on the floor below should have a minimum density to achieve current standards of sound proofing. As well as heavier plasterboard sheets, or two layers of plasterboard, insulation is put in the floor to increase the density of the void. This also stops the void acting like a drum and amplifying sounds from the floor above or belo.

Do not think Celotex can be substituted in here - in simple, practical terms it cannot.