Roof insulation advice please

[Feltwell]

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.

If you use solid foam insulation between rafters, then there should be an air gap above the insulation (can't remember how big - at least 30mm I think) in which the breathable roofing membrane (modern replacement for felt) is draped. The idea is that the membrane should not be stretched taught across the roof, it should be allowed to "sag" in between the rafters into this gap so that there is no possibility of water getting trapped behind the roofing battens.

But - if you use "counter battens" on top of your rafters before putting the main battens on, you can completely fill the rafter depth with insulation - as the counter battens provide enough gap to ensure that water cannot get trapped behind the main battens. Downside is it raises your roof by about an inch, which is why I couldn't do this (roof needs to tie in with neighbours as the house is semi-detached).

There are other ways of doing it as well, especially if you use sarking.

A picture speaks a thousand words, this guide is quite useful

http://www.proctorgroup.com/index.asp?lm=268

If you are insulating an existing roof which has non-breathable felt then my understanding is that you must always leave a ventilated gap between the insulation and the underside of the felt. If I was putting insulation in like this I'd put in some vent tiles to ensure ventilation to this void and stop any chance of condensation forming. There are lots of vent tiles available, but someone on here ages ago posted up one type that sat beneath existing tiles and was almost invisible from the outside? Can't remember what it was called!

 

[MdB]

http://www.lapvent.co.uk/

 

[skier-hughes]

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?

That's because the stuff between the joists is so old, flat, broken up and full of twigs, hay, straw, sawdust, birds and waps nests, it's being bagged up and I'm starting from new.

A problem I have if I'm going to remove this insulation though is, the roof line here extends for about 7 or 8 feet and I've tried pulling the insulation out, but it just breaks off and I can only lie down and reach in so far.

Any ideas on how I'm going to get this out?

 

[MdB]

???

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[Feltwell]

Hmm, tricky. How about a litter picker / grabber? (Please forgive the website, it was the first one google returned!)

http://www.betterlifehealthcare.com/products.php?catID=16&subID=54&gclid=CIOtkNPHkJ8CFVCZ2AodH2jGiw

 

[Feltwell]

http://www.lapvent.co.uk/

That was the one!

 

[DaveBrigg]

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

We have a very similar design of roof / ceiling, but without the insulation wedged down the gaps. The insulation on the middle, flat part is about 700mm. Towards the end of the last cold spell, as the snow melted, a horizontal black stripe appeared along the roof separating the snow which remained on the insulated top half and the overhang at the bottom.

 

[christianb]

If anyone is after Kingspan / Celotex type rigid foam boards, this firm is well worth a try.

Too right, Feltwell. I've used seconds and co and (without sounding like I'm doing their advertising) they save a fortune.

With reference to an earlier post, while 100mm sounds better than, say, 50mm, beware the law of diminishing returns. Twice as thick does not mean twice as good. Not by a long way.

And sorry if I'm repeating anything from earlier on, but there has to be a 2" (50mm) air gap between insulation boards and battens to comply with the regs.

 

[Feltwell]

And sorry if I'm repeating anything from earlier on, but there has to be a 2" (50mm) air gap between insulation boards and battens to comply with the regs.

Whilst that is true I got away with a smaller gap. My BCO (a jolly nice chap, complete with bow tie and a love of singing opera!) was happy to contact the manufacturers of the roofing membrane I used, who confirmed that a smaller gap was OK with their product. I can't remember what gap we had in the end, 25 or 30mm I think.

 

[philmac1944]

I am in the process of having to replace, from inside, the ceilings in one of our bedrooms which has vaulted ceilings.
Since I am also building an extension and carrying out extensive repairs, my local BCO is able to `give me his opinion`
on other matters. The Building Regs are interpreted differently depending on where you are. In my Borough they require,
under a felted roof:

50mm clear, ventilated air space which means creating a `flat` area under the ridge so air can circulate between
the two slopes and through the eaves ventilation.

75mm Celotex or similar between the rafters.

35mm Celotex or similar across the rafters, or battens to give 25mm gap and continuous layer of Triso Super 10 or similar.

25mm cross-batten and then plasterboard. I shall be using foil-backed plasterboard because despite all
this carry-on there is still a risk of condensation between the layers.

This all takes space away from the room. If you do any alterations of this nature, be sure to use treated timber.

The claims made by the producers of the new `thin` insulations have not yet all been substantiated so this is why
there remains a difference of opinion. The makers of Celotex / Kingspan etc are contesting the claims because
they will suffer significant cut-back in their business.

None of these products are eco-friendly in their production ( and looking into the future, their disposal ) even though they save
energy waste in the short term. Hope this info is helpful.

 

[Moo]

This viciously cold weather has been a real test of our roof insulation, and it has passed with flying colours.
For reasons I won't bore you with, the whole front pitch of our roof had to come off and be replaced. Both of these rooms are open right up to the point, with all the major woodwork exposed, so I imagine that any insulation must have been very thin indeed. Each room has a small dormer with a single-glazed and rather rattly sash window, and is heated with one of those reclaimed school-type cast iron radiators. But no-one here has felt the need of an electric blanket, or even a winter-weight duvet.

I don't know the details of how the roof was done, but somewhere there must be the specifications from which the builders worked. It was a dozen years ago, so I don't know whether the same products are available, but if anybody would like to know, I'll be happy to do a bit of scrabbling through files and boxes on their behalf.

 

[lou]

Well I'd love to know as that sounds exactly what we have or will have, when the roof's finished, the ceilings and dormers replaced and some nice old radiators in place. see pic.

But don't spend hours if it doesn't come to light quickly. I hope my builder will be up to the job himself. It's a great relief to know your rooms are warm. I'm a bit of a freezer.

 

[headgear]

A few thoughts.
Multi foil insulation; using internationaly agreed standard methods for testing thermal properties the BRE found Actis product to have a measured U value equivalent about half the claimed performance in the makers own test method. They also demonstrated that the required air gaps either side of the foil are crucial to it working, and that an improvised panel made of cardboard covered in tinfoil with sealed air gaps performed better in the same tests.
Actis refused to release to me copies of their test specifications or reports which I requested as part of my MSc studies. There is a working party within the ce standards authority examining methods for testing multi-foil insulation which ought to be reporting sometime soon.

Wool - wouldn't it be great to create a new market for wool; the farmers get SFA and only shear the sheep for welfare reasons these days. The issues are the treatments required for it's use as a building product; insect repellant and fire retardant; just washing it uses huge energy; and the weight of the final product. For the same insulation value as warmcell or rockwool wool is much heavier and in older buildings with shallower rafters this is non trivial. In new build it's easier to deal with as you can factor these in from start.

The last loft conversion I did involved cutting off part of the height. At the level of the second purlin I fitted beams between the walls and ceiling joists (2x4) suspended below these with 300mm of rockwool between. Ventilation from eaves was maintained behind foam and EPS on the slopes and the ridge was vented. When fitting any rigid insulation between rafters there is a risk of leaving small voids at contact edges - cut them slack and fill in with squirty foam.

Hope this helps.

 

[dave]

Seriously considering wool roof insulation. Just a couple of queries.

Does anyone think there is much difference between the different brands of wool insulation now available ? There is a fair bit of variation in price and it isn't particularly clear that there is much, if any difference, between the various products. In particular, are the premium products worth the extra expense ?

Also has there been any more reports of this moth problem with wool insulation and is it common for the wool to treated for this (or to deter rodents).

It's obviously not the cheapest insulation option so hoping to spend very limited funds as wisely as possible.

Any advice, or experience, much appreciated

 

[skier-hughes]

I've got the thermafleece in one part of my loft. Mostly wool. Treated for infestation. No problems in the time I've had it.
There is black mountain wool and I believe they claim slightly higher wool contect than thermafleece.
Then there is the likes of earthwool which has no wool at all

 

[MdB]

Then there is the likes of earthwool which has no wool at all

Yes, this was my thought. Some insulation products labelled wool have no involvement with sheep so be careful. I would also check the U values which should be specified. Lower is better.

 

[dave]

Definitely sheep wool that we are after.

Noticed on quite a lot of these EPC certificates that lots of people do seem to be putting down 250 or even 300mm of lost insulation down.

Presumably not sheep wool as this would be prohibitively expensive.

Most of the sheep wool rolls seem to have depth of 50mm, 75mm or 100mm.

Do you save a significant amount of heat going for 100mm rather than 50mm ?

Considering the cost and our very tight budget is there point is there any point even considering putting down more than 100mm ?

 

[MdB]

270mm is the current standard I think.

Rockwool and sheep's wool perform roughly the same so to match the performance of 270mm of rockwool you would need 270mm of sheep's wool. Not cheap...

Doubling the thickness of the insulation will roughly halve the heat loss. Due to edge effects the performance isn't quite as good as this. I would guess (without looking up the figures) that 270mm would allow a quarter of the heat to escape compared to 50mm.

B&Q prices - rockwool 300mm thick is £3 per m^2. Sheep wool 300mm thick is £9.50 per m^2

As for whether it is justifiable of not, you need to do some rough sums. Our neighbours completely uninsulated 3 bed semi cost £2k per year to heat and was always cold inside. In an uninsulated house, roughly 25% is lost through the roof (according to B&Q) which in nextdoors case is £500 per year. I don't have all the figures to do a calc but I would guess that 270mm would drop this heat loss to £100 or so. Rockwool would easily pay for itself in a year, sheep's wool might take a couple of years.

 

[dave]

From what I can tell the only products you can really call sheep wool are Black Mountain and Thermafleece original but for that depth it would cost circa £1000 pounds !

That would pay for a lot of extra sets or thermals / wooly jumpers !

 

[AMc]

Definitely sheep wool that we are after.

Noticed on quite a lot of these EPC certificates that lots of people do seem to be putting down 250 or even 300mm of lost insulation down.

Presumably not sheep wool as this would be prohibitively expensive.

Most of the sheep wool rolls seem to have depth of 50mm, 75mm or 100mm.

Do you save a significant amount of heat going for 100mm rather than 50mm ?

Considering the cost and our very tight budget is there point is there any point even considering putting down more than 100mm ?

Current building regulations want you to put 270mm of rockwool into a standard loft or the equivalent U value in another material. This is not an arbitrary number and is the result of a complex calculation between the cost of insulation and the amount of energy loss it prevents. As you get thicker you lose less and less heat but beyond 270mm you will be spending more money on cheap insulation than it will save in a 5 year simple payback calculation.

The idea behind the different thickness is to suit the installation. Typically you are encouraged to lay a single layer up to the top of the rafters in line with them, then lay a second layer at right angles across the rafters. This minimises the edges and joins where warm air can rise up beside the insulation carrying heat from your house into your cold loft - losing you money and encouraging condensation.

We have bats and can't use our loft so it's stuffed full of fibreglass. If you intend to use your loft as storage space or you need to access water tanks etc. you may want to look at laying a 'wool' between the rafters and laying insulation backed boards fixed to the rafters for access.

It's a good idea to consider the ecological credentials of the material you are installing, but the environmental impact of skimping on your loft insulation is IMHO more important - comfort aside. If you waste twice as much energy heating your house that extra energy must be created somewhere - usually by burning fossil fuels. There is more embodied energy in the manufacture & transport of rockwool etc. than sheepswool (should have) but that will be "paid back" in energy saving from installing it. Once its there it lasts a long time. If it's not there then you are literally throwing money away by heating your loft.

If you have NO insulation in the loft at all there are subsidies available to professional installers or your energy provider will arrange a complete installation for sensible sum e.g. http://www.britishgas.co.uk/energy-efficiency/products/home-insulation.html

According to British Gas sums it will cost £250 to install the recommended amount of rockwool and save you £150 a year. So that will have paid for itself before the second winter is over. Even if you went for the more expensive sheepswool and DIYed you would be looking at 7 winters before you were quids in ASSUMING that fuel doesn't get more expensive. Having gone from 30p-55p a litre in oil in 3 years you could find that an investment of £1000 now will have paid for itself in less than 5.

If you budget is too tight for 270mm of sheeps wool I would bite you eco credentials and lay 300mm of rockwool. I wouldn't install either 50mm or 100mm of anything, it's simply not enough to justify the aggravation of doing the job yourself.