Loft Insulation

hamant · 11 Aug 2010

Hi,

I'm looking into re-insulating and boarding my loft floor. After getting a few quotes I've decided to save some money and do it myself.

The joists in my loft are only 100mm high, so I thought about using 100mm Glass Mineral Wool insulation and covering that with some Aluminium Foil insulation type stuff and finally boarding over the entire lot.

So I went up earlier today to have a look under the existing flooring and noticed that the gap between the joists is inconsistent. I measure 4 different gaps and they ranged from 370mm to 420mm.

So my 2 questions are;

1) How sound is my plan of using 100mm Glass Mineral Wool insulation with Aluminium Foil insulation on top all boarded over?

2) How to I get a roll of insulation that is 400mm wide to fit snugly in those varied width gaps? I don't really want to trim long rolls of insulation.

Any help is greatly appreciated.

Thanks.

Deluks · 11 Aug 2010

Standard advice given round here is to first lay lengths of timber at right angles to the existing joists. Then board over this.
It will not only strengthen the structure a little bit, but will enable you to run cables easier, give you a deeper area for insulation, plus you can space them to suit your loft boarding width. (600mm centre to centre minimum)

2x2 is a safe bet, fix down with long screws instead of nails, or you'll end up with cracks in the ceiling below.

mointainwalker · 11 Aug 2010

So my 2 questions are;

1) How sound is my plan of using 100mm Glass Mineral Wool insulation with Aluminium Foil insulation on top all boarded over?

2) How to I get a roll of insulation that is 400mm wide to fit snugly in those varied width gaps? I don't really want to trim long rolls of insulation

.

The aluminium foil insultion will add very little, IMO nothing worthwhile to compare with the cost.

As for uneven joists, no magic here, Either squash the roll ( which doesn't improve insullation because of greater density ) or cut where necessary, which is a pain, and use off-cuts to fill gaps in the 420 mm joist-spacing.

Don't forget that it is absolutely ESSENTIAL to lay a vapour-barrier (plastic-sheet) over the whole ceiling area before laying the insulation.

hamant · 11 Aug 2010

Deluks said:
Standard advice given round here is to first lay lengths of timber at right angles to the existing joists. Then board over this.
It will not only strengthen the structure a little bit, but will enable you to run cables easier, give you a deeper area for insulation, plus you can space them to suit your loft boarding width. (600mm centre to centre minimum)

2x2 is a safe bet, fix down with long screws instead of nails, or you'll end up with cracks in the ceiling below.

I can't really raise the level of the joists as the height of the roof is pretty low as it is and I want to use the area for storage. Would it be worth insulating between the rafters using some kind of foam insulation boards?

mointainwalker said:
Don't forget that it is absolutely ESSENTIAL to lay a vapour-barrier (plastic-sheet) over the whole ceiling area before laying the insulation.

I've never heard of laying a plastic sheet down before insulating. What would be the purpose of this? There's nothing under my existing insulation at the moment and my loft is pretty well ventilated.

freddiemercurystwin · 11 Aug 2010

mointainwalker";p="1699008 said:
Don't forget that it is absolutely ESSENTIAL to lay a vapour-barrier (plastic-sheet) over the whole ceiling area before laying the insulation.

Click to expand...

mointainwalker, why the need for a barrier? Do you mean sitting on top of the plasterboard under the insulation?

And surely your user name should be mountainwalker?

mointainwalker · 11 Aug 2010

hamant

The air in your house is warm and damp because you are breathing (, cooking, showering etc )

The warm air will penetrate the pb ceiling and at a certain point in the thickness of your insulation, due to the effect of cold air above, the dew-point will be reached and condensation will occur in your insulation.

If this happens, you willl not only have soggy insulation, but you lose the insulation value as wet fibre-wool has very little insulating property.

Laying a vapour barrier on the loft-side of the ceiling prevents the moist air passing through and prevents the condensation.

What type and what thickness is current insulation ? Why are you thinking of changing it ?

Expanded polystyrene only insulates as well as fibre-wool but costs double. Extruded polystyrene or PIR boards cost 3 to 5 times more for 40 % to 60% better insulation.

@FMT

mointainwalker, why the need for a barrier? Do you mean sitting on top of the plasterboard under the insulation?

Yes Freddy, that's why i wrote

lay a vapour-barrier (plastic-sheet) over the whole ceiling area before laying the insulation

.

Yes Freddy, but I wasn't paying 100% attention and u+i keys are adjacent and , your Honour, before I knew it, it had happened and there was nothing I could do

freddiemercurystwin · 11 Aug 2010

mointainwalker, why is there no requirement under Building Regs for a vapour barrier to be installed where you say? Why is the insulation in my loft bone dry when according to your good self it ought to be soggy as you put it. Seems odd that millions of householders across the land that have insulation at ceiling level yet no vapour barrier are not complaining of soggy insulation too or have I missed something?

freddiemercurystwin · 12 Aug 2010

@ mointainwalker

mointainwalker · 13 Aug 2010

Patience Freddy, patience. been spending my waking hours laying a path and terrace . Got dirt under my nails - traumatising !

Surprised at your question. I thought I remembered your participation in threads this winter when I - and others - went through the same thing.

I have laid out some reasoning below for the OP : the physics is exemplified by the example of the cold drink can in a warm room collecting a complete coating of water-drops.

Pb will let water-vapour through. If there isn't a barrier ( foil-coating, plastic-sheet, full covering of kraft paper ) the vapour will rise through the fibres and if the conditions are correct ( RH, temp diff interior/exterior) condensation will occur.

Why is your loft not soggy ?

Well, to use one of your favourites

Did you install it yourself ?
Maybe it has one of the protective features listed ?
Maybe you heat/breathe very little

Maybe you are in one of those Devonian temperate zones where palm-trees grow and it never gets very cold ?

As for Building-Regs. I didn't know they didn't and I don't know why, but I;m sure you have your own experiences of government regulation not always making sense .

Some time ago I had to remove some pb im my kitchen and was very surprised to find it very wet in a top-corner against an external wall where I had been cavalier about cutting and securing the kraft-paper. There was no water-ingress.

My roofer told me that he had frequently taken down cladding hiding an external wall and the g-f is a soggy lump lying in the bottom of the cavity

EDIT
Sorry - for some reason the fortune-teller hasn't materialised on the page . Can you see what I have done wrong ?

freddiemercurystwin · 13 Aug 2010

Well I guess I shall continue to breath little! Physics and theory aside I and millions of others across the land do not have soggy insulation above the ceiling! The plasterboard is ordinary board and is not foil backed.

You've not pasted the whole link in your photo link BTW.

mointainwalker · 13 Aug 2010

freddy

Which bit is missing ? I really thought/think it was the whole link but obviously not. You must know since you use it a lot,

freddiemercurystwin · 13 Aug 2010

Well if you paste your link minus the img bits obviously into explorer it should throw up the image but your linky does not work ergo it must be missing something.

Was this what you were after?

mointainwalker · 14 Aug 2010

Yes

You must be a mind-reader !

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