loft insulation - best option?

[John Richer]

I have a loft that I want to insulate better, but also use as storage space - so I don't want to lay deep insulation roll loosely on top.

It is currently fully boarded, with a bit of old fibreglass between the joists. These are about 4-5 inches deep, holding lath and plaster ceilings below. I have several LV lights in the ceilings below, which poke up into the void I want to insulate.

The boards are currently up for other reasons, and here are the options I have considered before putting them back down:

(1) Do nothing (but the fibreglass looks rather badly layed and isn't filling the space well)

(2) Replace the old fibreglass with new insulation and put the boards back. I guess Celotex blocks is the way to go? They could be about 4 inches deep. Can these rest on the lath and plaster ceilings or do they need their own support? Or is the fibreglass just as good? Would obviously cut around the LV lights.

(3) Make the joists deeper by adding 2x2s or 3x2s on top to get more insulation, then reboard. This is probably too hard, and I'm hoping 4 inches of insulation is enough.

(4) Add a new layer of in insulation on top of the existing boards, and lay new board on laths on top of this for my storage space.

A builder mentioned a new insulation product that was very thin but effective, but not sure what he was referring to.

Any thoughts?

[Dave Liquorice]

100mm is pretty thin by modern standards I think they recomend 200mm these days but that would be open on the exposed sides the boarding will have some insulating effect as well.

It's more faff to lay extra timbers but on the whole probably better.

You want to lay them across the existing timbers not along. Fill to the top of the existing ceiling joist with glass fibre or WHY. Lay the new timbers then fill to the top of them. This means that the only direct through timber path from the ceiling to flooring is the small areas where the timbers cross rather than the full area of each joist. Laying the new joists cross ways also helps to spread the load across the whole ceiling as well. Ceilings are not normally designed to take much weight...

If you use celotex or similar you need to arrange ventilation between it and the ceiling to avoid possible damp problems. Glass fibre is breathable.

[Hugo Nebula]

On 18 Jun 2004 15:42:44 +0100, a particular chimpanzee named John Richer randomly hit the keyboard and produced:

The best insulation is currently Celotex (Kingspan is equivalent). It's almost twice as effective as mineral wool. 100mm of mineral wool isn't particularly effective, and you should be looking for at least

200mm thick, or 100mm Celotex between the joists.

When you start getting to these thicknesses of insulation, the heat loss through the joists starts to become significant and you get better 'value for money' out of your insulation if you can cover your joists.

[Peter Taylor]

Dave Liquorice wrote

Dave, in "normal" conditions of up to say 75% RH and a comfortable temperature, vapour will always condense first on the cold side of the insulation, not on the warm side. That's why "cold" roofs need to be ventilated. If there is a vapour barrier it has to be on the warm side. Abnormal conditions would be something like a steam bath in an igloo! :)

Peter

[John Richer]

...but I think I am OK for load bearing in this case: it's a 3-storey Victorian house and the joists in the ceiling of the 2nd floor look closely spaced and large. The boards are also original Victorian in the main so I think it was designed to be a storage space.

So your interesting idea of cross laying new timbers and insulation is intriguing - also means it wouldn't be too hard to access the ceiling for maintenance. But maybe 100mm celotex is enough.

Thanks for raising this - but is it really an issue in this location - a ceiling void? I'll look at the celotex literature... If I really need a

50mm air gap there then I am only going to squeeze in 50mm of insulation...

[John Richer]

Thanks - the relative performance was just what I was looking for.

So this suggests that if I do get to 50mm celotex, then I have done enough. The attic rooms have sloping ceilings with loose fill insulation poured between slates and lath-and-plaster sloping walls (a previous owner's contribution to the house) so they will dominate the heat flow.

[Mike Mitchell]

Whatever you do, DO NOT use the shredded/treated newspaper loose fill that is blown in. The ruddy stuff gets everywhere. I wouldn't recommend it to my worst enemy. Earlier this year I removed all of it and put the standard roll-type down. This was not expensive at all (I got mine from B&Q).

MM

[Dave Liquorice]

Two benefits. Less heat loss through the timbers as mentioned by someone else with 200mm it starts to get significant. Spreads the load.

Your lath and plaster ceiling has no DPM so the warm damp air will be getting into a cooler area. Mr Taylor is correct that any condensation will be on the cold side of the insulation but unless you are covering the whole ceiling area there is going to be somewhere with less insulation or where the warm air gets into the roof void proper.

I personally think that fitting rigid insulation in the confines of loft will be a PITA...

[Hugo Nebula]

On 18 Jun 2004 20:52:59 +0100, a particular chimpanzee named John Richer randomly hit the keyboard and produced:

The conductivity ('k-value' or 'lambda-value') is the relative performance. AFAIR, the k-value for mineral wool is 0.037W/m^2K, whereas for Celotex it's 0.023W/m^2K

I think I suggested at least 100mm thick Celotex. Although something is better than nothing.

Your previous owner's contribution appears to be to create a high risk of condensation in your roof by blocking the ventilation path between the insulation and the roof felt.

[The Natural Philosopher]

Agreed. You should try and put the celotex down the sloping bits, with an air gap above.

Probaly means ripping out all the ceilings tho.

[Lobster]

Heard a brilliant tale about this stuff from a joiner the other day. Another customer had put the stuff down all over the roof space, then some months later needed to get up there again for something. He removed the access panel in the landing ceiling, and was promptly deluged with the tons of the stuff - apparently the loft had been very well ventilated and the prevailing wind had blown the whole lot down one end of the loft, where the hatch was. SWMBO was delighted, I'm told.

David

[John Richer]

Yes, I have had concerns about this, but the surveyor did not flag this up [1]; and there is no felt, just original welsh slate laid on battens, so perhaps there is enough air movement to keep things dry?

[1] I know, one shouldn't expect 750 pounds worth of surveyor's report to tell you anything useful.

[Lobster]

As my house has this same configuration, and comes with the original vintage 1913 insulation down the slopey bits (ie none) I've been wondering about doing this myself. However, not sure I have the stomach to pull down all that lath and plaster etc.

If I was to stuff Celotex slabs down the sloping 'slots' from above, how worthwhile would that be, given that the slabs would certainly not be a perfect fit between the rafters? Would the small airgaps I'd inevitably leave cause so much heat loss as to render the Celotex not cost effective?

In comparison, how effective would it be to attach that thermal-lined plasterboard(?) over the top of my existing ceilings?

Thanks David

[The Natural Philosopher]

Probably. To be honest.

If you have the room, FAR more effective IMHO.

[hudsterou]

Which particular celotex product? Looking on their website I can't see any "loft insulation"....

[hudsterou]

What would be the best way of attaching it?