Bonjour Catherine,
I thought I’d give you my two-penneth – not to specify what you need, because lots of factors come into play – but to identify a few basics regarding roofing, in the hope that you spend your money wisely and create something worthwhile.
My approach to anything is to investigate what is required, and why – at a fundamental level – and only then to investigate how this can be provided at least cost to give greatest peace of mind. All too often people have no idea what they need and get bogged down in recommendations from suppliers, contractors or from people who have chosen one thing or another and are proud of that choice – but whose agendas are not necessarily yours.
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Firstly, roofs have a lot to do. Off the top of my head here’s a non-exhaustive list...
- Keep moisture out
- Allow moisture out
- Keep heat in
- Keep heat out
- Keep wind out
- Keep pests out
- Stop house walls from blowing over
- Survive snow and ice
- Survive wind
- Survive building movements
- Survive maintenance access
- Survive moisture changes
- Survive insect infestation
- Survive animal infestation
- Survive mould growth
- Survive plant growth
- Survive solar radiation
- Survive impact from wind-blown debris
- Not hinder fire escape
- Not contribute to fire
- Not permit fire spread
- Satisfy regulations
- Be easily repairable
- Look pleasing/ be ‘in keeping’
- Not cost the earth.
Of course if you have a turf roof, you can strike the last. ;-)
You said you’re interested in ‘the best insulation’, but haven’t actually said what type of roof situation you’ve got. Since roofs only really need to be insulated if the space immediately beneath them is to be habitable (and you mentioned ‘between the rafters’) I assume you have a loft-type conversion situation?
In any event, all of the above should be considered by anybody building a new roof, buying a house that already has one, or rendering habitable the space immediately beneath the roof.
I’m afraid I haven’t the time to elaborate further on this whole subject, but on the subject of Tri-Iso multi-foil type insulation and felting my input is this...
1 Felting (i.e. underlay)
I mention this first because keeping out rain is of course a roof’s fundamental job.
Traditional European dwelling roof covering is tiling – slate, clay, concrete, felt or wooden shingles, etc. I assume you’ll be using one of these? None of them will keep 100% of rain out, no matter how well they’re installed. The reason for this (notwithstanding tiles breaking or becoming dislodged) is wind-blown rain. It is to combat this that tiled roofs are recommended to be ‘underfelted’ – though the word ‘felt’ is to tiled roofing what the word ‘Hoover’ is to vacuum cleaners.
Felt refers to a sheet material of (typically) bitumen-coated hessian or fibreglass mesh – used as a tile underlay since relatively recently (in the great scheme of things). Hundreds of thousands of houses in the UK still have tiled/slated roofs without underlay – driving rain combatted by mortar fillets on the underside of tiling battens. If you’ve ever owned a 30’s/40’s house you’ll know all about the eerie sound of mortar clods dropping onto the ceiling as the house goes about shaking off winter or summer.
Modern underlay is known as a ‘breathable membrane’ and is far superior to traditional roofing felt. It’s so called because it keeps water out but also allows moisture from the building below to escape. If this isn’t allowed the rising moisture will condense in the roof space (temperatures permitting) – i.e. condense on the (cold) roof timbers – and thus wet them. Not very good when you’ve gone to the bother of stopping rain doing exactly that. And before you think that the house ceilings will surely stop that moisture rising from, say, a compartmentalised kitchen or bathroom? – they won’t, unless they’ve had a vapour barrier inserted when installed. And then of course there are doors everywhere...
The use of a breathable membrane is thus an absolute necessity. Once in place your roof is essentially rain-proof – as attested by the thousands of rain-lashed ‘works in progress’ across France. Like everything, however, it has to be installed properly. Look at FAQs on suppliers’ websites.
2 Multi-foil insulation vs conventional
Firstly, MF doesn’t have a traditional ‘U’ value like conventional insulants (mineral wool, glass wool, foam, loose fill products etc) because it works in a different way (using trapped air as an insulant, using the surface resistance of multiple layers to reduce overall heat transfer, and minimising heat loss by radiation by virtue of its opacity). It is thus difficult to compare with conventional homogenous products from a purely manufacturer’s ‘material performance’ point of view. I’ve used it only once – for a ‘warm roof’ loft conversion I did for someone in the UK – and it worked (and is still working) fine.
[NB Multi-foil insulation is robust enough to keep the rain out during build, but it is NOT underlay. The generally accepted lay-up is: MF laid across rafters; counter-battens then fixed along the rafters (holding the MF in place); breathable membrane then fixed across the counter battens (as though the counter-battens are rafters) ; then tiling battens fixed to the counter battens.]
3 Insulation position
The location of insulation (i.e. the roof ‘type’) is also very important. There are essentially two roof types created when insulating a roof; cold and warm. There are, however, three typical ways of insulating a roof.
A) Fitting insulation beneath the rafters. This creates a ‘cold’ roof; the rafters being on the cold side of the insulation – i.e. outside the habitable space. This is the more usual configuration because one can apply the insulation without having to remove anything; assuming that a breathable membrane is already in place and that tiling battens are of treated timber. It does, of course, sacrifice headroom, hide the rafters, require finessing where rafters aren’t straight, and leave you with a huge surface to, er...do something with.
B) Fitting insulation on top of the rafters. This creates a ‘warm’ roof; the rafters being on the warm side of the insulation – i.e. within the habitable space. This obviously requires removing not only the roof tiles, but their battens too – which is a much more demanding job. It doesn’t waste any headroom and leaves the rafters visible – if that’s what you want?
C) Fitting insulation between the rafters. This is still a ‘cold’ roof, since the rafters are still exposed to external temperatures – and unless your rafters are very deep (or you can afford NASA grade insulation) is unlikely to give you what you’re looking for thermally. Moreover, this arrangement creates cold bridging at each rafter position – since the thermal conductivity of wood is (generally) greater than that of the insulation used. It involves a lot of work – not least figuring out a way to support the insulation and to accommodate seasonal timber movement.
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Okay, there’s a ton of other stuff pertaining to fixings, cross battening, ventilation of roof timbers, etc., which you’d be wise to investigate, but I hope that I’ve given you food for thought – that getting your roof right needs quite a bit of research.
Best regards,
Mick
