Roof insulation advice please

[lou]

Hi everyone,
We are just getting quotes for the complete overhaul of our slate roof. Currently there are just a few inches of cold space between the lath and plaster and the slates. The first quote has mentioned 50mm kinkspan or celotex insulation. We had hoped to have sheep's wool (this being Wales!) but have been warned off by two people who have had disastrous experiences with moth lavae eating not only the insulation but moving on to carpets and anything else woolly they had left lying around. What's are your views on the best insulation - preferably environmentally sound?

 

[MattW]

Currently there are just a few inches of cold space between the lath and plaster and the slates.

I suspect that is going to really reduce your options - a lot of the greener solutions that I looked at require a lot more space than this to reach the required U value. One of your options (non-green) is to put 120mm of Celotex or Kingspan between & over the rafters and lay the slates on top:



This will give you a U value of 0.18 approximately (depends on rafter spacing etc). I guess the balance you will have to make is energy efficiency of your home vs environmental qualities of the insulation product - clearly rigid urethane foam insulation is about as ungreen as you can get, but it is extremely effective. Look forward whether anyone else has better ideas that can get a <0.2 value.

 

[Stephen]

I agree with Matt that if you really want to make a difference in terms of U-values the wool option may not be appropriate, albeit it greener in terms of the material itself.

On the matter of Moths in wool I had heard this and it caused me some concern (as I have specified/recommended it on various jobs), but I am told that the manufacturers claim this is nonsense and are threatening nasty things if the rumour is spread (because of course it will damage their product sales).

It would be a major set back for environmentally sound products if the Moth story is true, but I think we need specific information before we start worrying those that have used or are considering using wool insulation.

 

[MdB]

50mm of Celotex would be too little for me. 100mm+ sounds a lot better but then I am currently sleeping in a bedroom with three external non insulated non cavity walls and about 10mm of rockwool in the flat roof. It is a tad chilly at the moment.

I would guess that the equivalent in wool would be double this and it wouldn't be the easiest thing to install. If you go for wool, check where it is sourced from and check its flammability.

 

[lou]

Thanks - very useful - I would have written off the wool/moth story but met someone at christmas who had actually experienced it - at least, that's what he told me. But there's unlikely to be room for a green and red (opposite of green?) solution. Makes me a bit concerned that even using that won't give me good insulation, particularly as the ceilings are all high and vaulted, if that's the right word - see pic - which means that there is plenty of space for the heat to rise and disappear.

Perhaps it might be possible to put a false ceiling in and insulate between that and the lath and plaster? Does this sound viable?

 

[skier-hughes]

I've got some wool and had it for 12 months and so far I've not seen any problems.
The other bits of my roof space has some recycled plastic insulation, £5 for two rolls, buy 1 get one free and 15% off for spending over £50 sounded very green to me!!
and the bit I haven't done yet still has old insualtion, sawdust, hay, straw, mouse/rat/squirrel droppings, birds nests, wasp nests adn whatever other rubbish is yet to be cleared I'd have thought this ripe for a breeding ground of nasties.
Besides that, I'm sure they treat it with stuff to stop it getting infected don't they?
Unless of course you mean plain sheeps wool direct from the sheep?

Graham

 

[AMc]

Kingspan, Celotex and Ecotherm etc. are all substantially more effective at stopping heat loss than wool - usually about 3 times better thickness for thickness.
The manufacturing process may not be as ecologically low impact as sheep farming but to call it ungreen is a short term view. The manufacturing has an environmental impact but the energy saving advantages over the lifetime of the product more than offset it IMHO and that of many others.

To balance the argument a little
http://www.kingspanpanels.com/Responsibility/Sustainability/Vision-Policy.aspx
http://www.celotex.co.uk/Corporate/Sustainability
http://www.ecotherm.co.uk/climate_/sustainable_construction.aspx

I might have used sheeps wool in my build if the construction had allowed it, but because Ecotherm is so much more efficient it was the only logical solution to comply with building regs and keep heating costs down in long thin rooms with a low roof.

Breathability may be a problem in some period homes but don't write off PIR insulation 'because it's not green' without making up your own mind.

 

[Flyfisher]

The other bits of my roof space has some recycled plastic insulation, £5 for two rolls, buy 1 get one free and 15% off for spending over £50 sounded very green to me!!

Am I the only one who thinks it curious that insulation based on recycled plastic waste can be so cheap compared to insulation based on recycled wool waste?

 

[Feltwell]

50mm of Celotex would be too little for me. 100mm+ sounds a lot better

Yep, I'd agree. We re-tiled our roof and put 50mm of Celotex inbetween the rafters, which was the very maximum we could use (needed air gap above insulation, under membrane and tiles and could not change roof height as we are a semi and have to tie into next door).

It's a big improvement over nothing, which was what was there before, but I'd rather have more. You also get a cold bridge effect through the rafters, plus with all the cutting required we will have some odd gaps I'm sure.

Only way forward for me is to underclad the sloping, "fixed to bottom of rafters" ceilings with more insulation and then create a new plasterboard ceiling fixed to that. Visually I can do it without changing the appearance of my attic rooms too much, but it will give me a headroom problem in one area. Conservation wise it's not the best, but I can at least leave the original ceilings intact under the new insulation.

I'd have loved to use wool but there's just no way I could have created the depth required to get any decent level of insulation value.

 

[Feltwell]

50mm of Celotex would be too little for me. 100mm+ sounds a lot better

Yep, I'd agree. We re-tiled our roof and put 50mm of Celotex inbetween the rafters, which was the very maximum we could use (needed air gap above insulation (under roofing membrane and tiles) and could not change roof height as we are a semi and have to tie into next door).

It's a big improvement over nothing, which was what was there before, but I'd rather have more. You also get a cold bridge effect through the rafters, plus with all the cutting required we will have some odd gaps I'm sure.

Only way forward for me is to underclad the sloping, "fixed to bottom of rafters" ceilings with more insulation and then create a new plasterboard ceiling fixed to that. Visually I can do it without changing the appearance of my attic rooms too much, but it will give me a headroom problem in one area. Conservation wise it's not the best, but I can at least leave the original ceilings intact under the new insulation.

I'd have loved to use wool but there's just no way I could have created the depth required to get any decent level of insulation value.

 

[MdB]

The other bits of my roof space has some recycled plastic insulation, £5 for two rolls, buy 1 get one free and 15% off for spending over £50 sounded very green to me!!

I ruled this stuff out as being too flammable though I did think about using it. Celotex, Kingspan also burns but it goes out once it has formed a black layer.

 

[AMc]

50mm of Celotex would be too little for me. 100mm+ sounds a lot better

Only way forward for me is to underclad the sloping, "fixed to bottom of rafters" ceilings with more insulation and then create a new plasterboard ceiling fixed to that.

If you do this then look at Gyproc Thermaline Super and similar products - it's plasterboard bonded to PIR foam insulation. You fix it through to the rafters. Then tape and skim the ceiling and it creates a very efficient, easy to fit surface. We used 50mm of insulation bonded to 11mm plasterboard on our walls. The board (and all the other insulation) came from Sheffield Insulation - haggle for a discount off the list if you're buying a lot.

In the roof we put 70mm of Ecotherm between the rafters and 50mm over the top taping the edges with aluminium foil tape, battened then felted, then cross battened and retiled. It raised the roof - so work on the gable brick work, facia and guttering but it's very warm in that room now!

 

[Feltwell]

Thanks for that AMc. I may use the "ready bonded" product, but the problem I've got is access - I've got to carry these boards right through the house and up a steep and narrow set of attic stairs, I'm not even sure if it will be possible to get an 8' x 4' sheet up there! And of course I want as few cuts as possible, so seperate sheets of insulation and plasterboard may be easier to handle.

Having said that, I guess the plasterboard will be less prone to breaking if bonded to the insulation.....

If anyone is after Kingspan / Celotex type rigid foam boards, this firm is well worth a try. They can be a lot cheaper than buying A grade boards - the lot I had off them just had a wrinkle in the foil face of the board, that was the only reason they were "B" grade.

http://www.seconds.co.uk/index.htm

 

[Penners]

Perhaps it might be possible to put a false ceiling in and insulate between that and the lath and plaster? Does this sound viable?

I reckon that, if you want really effective insulation, that's what you're going to have to do. From the look of that purlin in your photo, if you run a new ceiling up underneath it, this would give you perhaps 150mm of insulation - much more of a proper job. And despite the green-ness of wool, I don't think the available thickness makes it an option - you need the super-insulation capabilities of one of the man-made foam materials.

 

[skier-hughes]

While we are on about insulation, should it be forced down the ends of the rafters like these pics?

 

[Penners]

While we are on about insulation, should it be forced down the ends of the rafters like these pics?

As I understand it, that depends on the ventilation arrangement of the roof space. If ventilation is provided via the eaves, it may not be a good idea.

 

[MdB]

My answer would be no, there should be an air gap under the felt.

In my loft, I've used some 50mm celotex bolted to the rafters to act as a solid surface with an air gap behind it and then filled the inside with rockwool which can be jammed up tight to the celotex.

 

[ljn]

I used "Actis super 10" multi-foil insulation. It gives a high performance and is under 5 mm thick.

http://www.actis-isolation.com/fonctionnement.php?p=3&l=3&rub=2&vert=1&blanc=2

 

[Feltwell]

While we are on about insulation, should it be forced down the ends of the rafters like these pics?

As I understand it, that depends on the ventilation arrangement of the roof space. If ventilation is provided via the eaves, it may not be a good idea.

Agreed - you can get ridged plastic sheets, which you push between the bottom of your roof felt and the insulation in the eaves, to allow an air flow past it.

Seems a bit odd though Graham that you've got insulation wedged in the eaves but not between the ceiling joists where it would usually be?

 

[Feltwell]

I used "Actis super 10" multi-foil insulation. It gives a high performance and is under 5 mm thick.

http://www.actis-isolation.com/fonctionnement.php?p=3&l=3&rub=2&vert=1&blanc=2

Lots and lots of old posts on this Ljn - there is a great deal of controversy as to if this stuff actually works anything like as well as the manufacturer's claim it does.

I think Biff posted up a "thin" insulation which seemed to have much more going for it - Aerogel or Spacetherm. Not cheap but very effective.

http://www.spacetherm.com/