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    Having just moved into a property with no insulation, I am going to get the loft insulated. I was looking at using environmentally friendly materials... then I saw the cost of sheeps wool! Also I don't really want warmcel as its loose fill. This pretty much leaves glass wool (which I'm told uses recycled glass, so better than rockwool I believe).

    I have two questions:
    1) how much insulation? Part L says a u-value of 0.16, the Approved Document L1B says 250mm of mineral wool will do that. I want to exceed this though, but my research and calcs seem to show that it'll take 15-20 years to payback an extra 100mm of insulation at the minimum... is this everyone else's findings? Will extra insulation make that much of a difference?

    2) Secondly, the roof sits very low on the house, so internally at first floor round the perimeter is a sloping plasterboard piece (see very bad photo attached!). The gap in the roof space from the top side of this plasterboard to the roofing felt is 4inches. I'll need to keep 25mm for ventilation to the roof - so I only have 75mm for insulation around this part. What do you suggest I use? I was thinking some insulating board with better R-value than mineral wool. Any ideas?

    • CommentAuthorCWatters
    • CommentTimeJan 11th 2008
    It will be difficult to control the ventilation gap if you use a loose fill or non-rigid insulation under the eaves. I would look at something like 75mm thick foil covered foam insulation (Celotex, Kingspan). I don't think 75mm is enough to achieve 0.17 but it's probably best you can do at that point. Most insulation manufacturers have a U-Value calculator on their web site.
    • CommentAuthorCWatters
    • CommentTimeJan 11th 2008
    PS I think I saw a document on the Sheffield Insulation web site that compared U-Values for the different boards they sell.
    • CommentAuthorskywalker
    • CommentTimeJan 11th 2008 edited
    Welcome mrwonderful

    Have a look at this thread:


    and have a search on 'skeilings'.

    In your case I would go for foam backed plasterboard internally on that small a skeiling - as thick as you dare!

    I allways feel payback is a bit of a crazy concept - do your car/telly or kettle pay back?

    That said with the way energy prices are going you may even find the 950mm of insulation (recomended by the manufaturers!) regarded as an 'environmental maximum' becomes economic.

    Go as thick as you can/can afford & don't forget to leave a ventilation gap at the eaves (above your skeiling).

    Thanks for all the help... skeiling - so thats what its called! Insulating plasterboard internally looks a good option as we're redecorating anyway. Calculating it all out roughly (http://www.vesma.com/tutorial/uvalue01/uvalue01.htm) leaves me to think any less than 100mm insulating plasterboard will need some insulation behind the skeiling also which (from the links - useful thanks!) looks hard to put in.
    • CommentAuthorskywalker
    • CommentTimeJan 11th 2008
    Not hard but messy and on an old house you have to cut each section to fit as the rafters wander somewhat. However given you only have a foot or so to insulate I would think twice before doing it the way I'm doing mine. If your skeiling is plasterboard it should be easy enough to fit cut sections of foam board down from above (you will need to swear a bit of course). Just cut them 2mm or so too big (between rafters) and run them in by clamping battens to your roof rafters (the depth of the insulation above the back of the plasterboard) and banging in with a bit of wood over the end. Just make sure they stay nice and tight to the back of the plasterboard and are a real tight fit between rafters. You could even cut the foam board in long enough sections so that it forms the edge of your loft insulation thus giving good air spaces to ventilate the roof structure and a draught proof/insulating box around whatever you put in the loft.

    One benefit of just fitting it internally is that you will get the full value of the insulation (rather than the value with rafters in the equation) and it will be a lot less messing about.

    • CommentAuthorCWatters
    • CommentTimeJan 12th 2008
    Generally If fitting insulation internally don't forget this moves the cold point. Might need to move the vapour barrier to stop vapour reaching said cold point. Although I think the foil on the insulation can act as the vapour barrier.
    • CommentAuthorcaliwag
    • CommentTimeJan 12th 2008
    Called a coom in Scotland
    How about that Ecowool as a alternative to glass/rock wool ?

    BQ are doing a 2 for one, I think

    350mm (150mm+200mm laid the otherway) of glasswool should be about £6-7 m2 ( £300 for a average house )

    I believe Phenolic board (kingspan) is one of the best insulator for thickness
    • CommentAuthorskywalker
    • CommentTimeJan 13th 2008
    My B&Q are virtualy giving away ecowool & that bagged up knauf stuff(polythene on side metalised film on the other) wish I could use it somwhere but I have space restrictions on every surface.

    Bring on the cheap aerogel - amazing stuff.

    • CommentAuthorjudy
    • CommentTimeMay 6th 2008
    skeiling....coom....eh what?
    If it helps Kingspan is approximately twice the thermal performance for the same thickness as mineral wool or glass quilt.
    I insulated my roof with kingspan but I could only fit 100mm between the rafters, still allowing an airgap under the felt for ventilation.
    This is where my room is open to the pitch. Where we have a conventional loft we boarded over the rafters with kingspan so there is a large ventilated space behind. This keeps the loft warm as we use it for storage.
    • CommentAuthorEd Davies
    • CommentTimeAug 22nd 2008
    Posted By: Joinerbird
    skeiling....coom....eh what?

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