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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorEsphino
    • CommentTimeNov 12th 2008
    Hi All,

    I am renovating a house in Mid Wales and need some help regarding the way I install the Kingspan/Celotex insulation. As it stands I plan to install 120mm of kingspan/celotex in the following way 40mm between the rafters and then 80mm over the rafters with plasterboard over that.

    My problem is that I am unsure of how I should attach the 80mm boards to the rafters, I could use 100mm screws but then I am faced with the issue of how I would then attach the plasterboard to the insulation as this would need 110mm dry wall screws at least to get through the insulation into the rafters.

    Can the pir and plasterboard be bonded together and if so what would be the best product to use for this?

    Any help on this would be much appreciated
    • CommentAuthortony
    • CommentTimeNov 12th 2008
    Easiest is fix battens over the insulation then fix plasterboard to the battens.
    • CommentAuthorsinnerboy
    • CommentTimeNov 12th 2008 edited
    You could use composite insulated plasterboards ( CPB ) -where the plasterboard is bonded to the insulation at manufacture . Sizes include 38 ( 25 ins + 13 pb )
    50 ( 37 + 13 ) 63 ( 50 +13 )

    When you say insulation over the rafters - do you mean along the underside ? So really BELOW rafters ? I am assuming yes

    I am also assuming that your roofing felt is the older bituminous type which is not breathable - so you must maintain a 50mm gap between the underside of the felt and the top of the insulation placed between the rafters

    So - if your rafters are 125mm - use 75mm between the rafters and 63 CPB below ( total insulation 125mm - 75 +50 )

    Don't forget - that 50mm vent space - you must introduce a continuous 25mm vet strip at your roof overhangs AND a continuous vent strip of 5mm at the apex .

    Be carefull not to sandwich electrical cables in between insulation layers - they will overheat - run them in the 50mm vent space

    Don't undo all this good work with recessed lighting - cutting away that lovely insulation - use wall lighting and / or track lighting - low energy ( of course )
    • CommentAuthorEsphino
    • CommentTimeNov 12th 2008
    Thanks for the quick responses, as sinnerboy has mentioned I do mean below the rafters for the 80mm insulation,

    I have a breathable membrane VP400 under the slates and was planning on leaving a 50mm gap between the slates and the insulation boards as the insulation will not continue for the full height of the roof just up to the ceiling and then continue above the flat ceiling rather than going all the way up to the ridge.

    There is plenty of ventilation above and below the membrane to remove any potential moisture issues, and I am aiming to create an insulated box upstairs.
    Due to the depth of the rafters and the need to keep the 50mm gap I only have room for 40mm of board between the rafters and would then have to have the 80mm under them with the plasterboard over the top. I have already purchased the PIR so would not be able to use the board bonded to platerboard option. :o(

    In order to attach battens over the 80mm boards I would need 120mm screws which I have not been able to find?

    But the batten idea would mean that any cabling could be run in the void between the insulation and the plasterboard.

    • CommentAuthortony
    • CommentTimeNov 12th 2008
    I got a load of em if you want them
    • CommentAuthorsinnerboy
    • CommentTimeNov 12th 2008
    Screw fix the 80 pir with long screws and then plasterboard over with slightly longer screws ( from tony :-) )

    Foil tape the pir joints first to maximise the vapour barrier / check

    If you do use battens between the pir and plasterboard - yes place cable here . Although I do think a battens cavity here is a lot a hassle for little benefit
    • CommentAuthortony
    • CommentTimeNov 12th 2008
    how many times have you missed the joists trying to fix a sheet through the insulation then? and dont say never :smile:

    battens make the job easy, I use them to hold a vcl/air barrier in place and form a service void too.
    • CommentAuthorEsphino
    • CommentTimeNov 12th 2008
    Would I need to put washers on the screws holding the PIR board in?
    • CommentAuthorsinnerboy
    • CommentTimeNov 12th 2008
    not using tony's method - the batten would act like a washer . Warming to it tony

    I have done this sinnerboy's way several times. As usual Tony has put a sprog in the works by coming up with an alternative which is probably better. I suppose it depends if there is a height restriction.
    Today I've been fixing 75mm under rafters , using 12mm ply cut into 50mm strip as battons screw in with 120mm screws
    I'll then stapple my VCL to this and screw the plasterboard to these strips with short plasterboard screws
    similiar to tonys way , perhaps a little less sturdy
    • CommentAuthorEsphino
    • CommentTimeNov 14th 2008
    Thanks for all the help guys I think I am going to go for the Tony approach :bigsmile:
    • CommentAuthorjohnt
    • CommentTimeNov 15th 2008
    Just to stir things up a little and turn the problem around... why not put the 80mm Celotex between the rafters, leaving 10/20mm gap for a little ventilation and for running your wiring. You do have breathable felt so should not be a problem with condensation and no worry about building regs. Then fix the 40mm over rafters, foil face into room, tape all joints with foil tape as belt and braces vapour check and plasterboard over that. More headroom, quicker job, same level of insulation.
    • CommentAuthortony
    • CommentTimeNov 15th 2008
    less insulation/more bridging -- insufficient air space between felt and insulation -- tape is not best either.
    • CommentAuthorJeff B
    • CommentTimeNov 16th 2008
    Tony - I want to do a similar thing to Esphino except that I want to put 2 inches of Celotex over the existing plasterboard ceiling/skeilings (dormer bungalow) and then cover with plasterboard. There is already 2 inches of Celotex between the rafters and I don’t want to take the existing plasterboard ceiling down!

    Do I need a vapour barrier with this scenario or is the Celotex itself (with tape over the joints) sufficient? I’m not sure whether to add battens to attach the plasterboard to, or simply screw through both the new plasterboard and Celotex into the rafters. Locating the latter may be an interesting problem!

    Jeff B
    • CommentAuthortony
    • CommentTimeNov 16th 2008
    Screw or nail through but fully locate all rafters and joists first. No vb needed if you use tape but a sheet of polythene is cheaper and better as I see it.

    Beware of outdoor air blowing arround under the floor. You will almost certainly need to attend to this problem and its not easy so do it first in case you have to go through a wall.
    • CommentAuthorJeff B
    • CommentTimeNov 16th 2008
    Tony - thanks for your comments. I have addressed the problem of outdoor air blowing through the "channels" between the joists under the bedroom floors by stuffing a rolled up piece of fibreglass insulation at each end. During this winter I intend to lift the floorboards and lay fibreglass fully between all the joists, as we hardly use the upstairs. I don't see the point of allowing any heat to get up there through the downstairs ceilings at the expense of cooling our living spaces/downstairs bedrooms. Probably an overkill but the insulation was on a buy-one, get-one free basis at the time!
    • CommentAuthorLegolas
    • CommentTimeNov 16th 2008
    Ahem, my first post so here goes.
    Hope you don't mind my butting in to this conversation, but as you're on this topic, I have a question of my own.

    I'm doing a similar job to this myself at the moment on my loft conversion. The plan is Celotex between rafters, then Thinsulex below rafters, then foil backed plasterboard sandwiching it all together.

    My question is this: We've got the 50mm airgap above the Celotex, but only half the roof (the newly built bit) has breathable felt, the other half is the old non-breathable bituminous stuff. I can add soffit vents to let air in at the bottom, but the builder hasn't fitted any kind of vent in the small triangular ridge void at the top. Am I right in saying that the breathable felt will be sufficient to provide this exhaust ventilation? The tiles are large, wavy concrete ones that I believe are naturally breathable. They interlock but they don't sit tightly together like slate tiles and must be draughty as anything.

    Also, I have some Celotex left over. At the moment the small triangular void above the ceiling just has regular loft insulation running over the top of the joists. Is it worth me using up the last of the celotex inbetween those ceiling joists, leaving the wool type insulation above, or would that add another level of condensation complexity?
    • CommentAuthorjamesingram
    • CommentTimeNov 16th 2008 edited
    thinsulex is one of those multifoil insulations, so you might want to have a look here as there is some debate whether they live up to their claims

    I believe with the right breathable felt ,no gap is required for ventaliation , though generally its prudent to leave 50mm anyway
    If you add soffit vents all round, as you suggested, I'd have thought your reasoning sounds fine .Though its below the bitumen felt that
    is the important area , not the tile above

    Yes use the celotex above as well , its roughly twice as good insulator per mm than glassfibre etc.
    as suggested above by Tony use a cheap plastic sheeting as a vapour control layer under all the insulation , paying attention to details and joints before you plasterboard
    it works out cheap than the foil backed

    care needs to be taken with the fitting of the PUR/PIR board to avoid any gaps , I personally cut the celotex 10mm short all round then use expanding foam to
    create a tight fit

    cheers Jim
    • CommentAuthorLegolas
    • CommentTimeNov 16th 2008
    Thanks for your thoughts Jim.

    I know there's been some debate over foil insulations and their merits or otherwise. It's what the architect specc'd and he, the builder and everyone else I've spoken to say it's great, will work brilliantly and will give no condensation problems.

    Condensation is my big worry at the moment. I keep getting images of the Panorama Special in the year 2020 when they investigate the scandal in the noughties when people were told to fit all this stuff and then their roofs rotted out within 15 years. A bit like asbestos which people were told wasn't remotely harmful when they were sawing it up without so much as a mask in the forties. Now we know better.

    As for vapour barriers, the Thinsulex instructions say that it works well as a vapour barrier in itself (provided you seal the joints). I plan to tape the Celotex right along each rafter so that's sealed, then seal up the Thisulex completely then fit the foiled p/board. (I've already bought the board and it only came to about £50 more than standard so wasn't too bad). Hopefully that should do it.
    • CommentAuthorwellburn
    • CommentTimeNov 20th 2008
    I fixed my 70mm kingspan direct to rafters with 100mm screws just to 'tack' it in place, then I over fixed my 12.5mm plasterboard over, - through the kingspan to secure.
    I marked the purlins with chalk so i didn't miss the rafters (often)
    I found that 4 screws would hold the kingspan in place without washers, and the plasterboard sandwich did the job.
    main issue concerns dealing with the edges and making moisture tight.
    • CommentAuthormikeajill
    • CommentTimeNov 26th 2008
    I am trying to work out how best to insulate an attic room, slate roof with bitumous non breathable under-felt on 70mm rafter. All the guidance I have seen recommends a 50mm gap under the felt to the insulation. Because of the roof construction and velux windows it is difficult to ensure effective cross rafter ventilation. My archetect has specified completly filling in the rafter voids with foam board, no air gap, then under rafter foam board with a continuous vapour membrane, sealing all joints. The logic is that by preventing moist air entering the roof there is no risk of condensation. Any comments appreciated.
    • CommentAuthortony
    • CommentTimeNov 26th 2008
    He wont get away with it -- but it might work. Condensation can and will form in the air spaces in the new insulated roof but only on rare occasions. The warm air in the spaces contains more water vapour than it does once cooled and on the underside of the felt it will condense. --- dont take the chance. You dont need any air from the room to assist this process but it unlikely that you will be able to stop that even in the short term.
    • CommentAuthormark_s
    • CommentTimeNov 27th 2008 edited
    Similar question, If I fit under rafter insulation now, then in a couple of years redo the roof and fit between and over rafter insulation & breathable felt am I introducing any condensation risk? To get the proper U values all the way across my attic roof with under rafter insulation I am going to bring the room height down too much. I'd rather do as much as I can now with the option to top up when I get the roof redone (out of budget at the moment)

    Oh, and where do I get these 120mm screws for fixing through the plasterboard and 80mm Celotex? I see the batten option but I'd rather have 80mm celotex than 50 mm and a batten.
    • CommentAuthortony
    • CommentTimeNov 27th 2008
    I can give you the screws, May be best wait til you redo the roof I think and do a very nice job then.
    • CommentAuthormark_s
    • CommentTimeNov 27th 2008
    Very Kind offer Tony. If you were nearer I'd take you up on that.

    I've sourced some 120mm screws and washers. Although I take your point about waiting to redo the roof properly, I dont think thats going to be an option. Or at least not now the plasterboard is down.....

    I've ordered what may be an unreasonable amount of celotex. I see a week or two of work. (I know it shouldnt take that long but I have small kids and that limits my available time.)
    • CommentAuthorabomb1969
    • CommentTimeNov 27th 2008
    A week or two? It takes bloody ages! I have just done 150mm Kinspan bewteen rafters with a 40mm air gap. 40mm on top of the joists to stop coldbridging. My house is 120m2 and it has been very tedious. My rooms are in the roof = skeilings. Put the foam in place and hold it with nails, then use expanding foam to seal up and hold in place, remove nails. Infill between with the 40mm and foil tape over the top, vcl stapled over. Seconds and co have saved me a fortune.

    Buying two layers of insulation was going to be too expensive and this does the same job, got the 8x2 sheets of Kingspan for £15 each, bargain!
    • CommentAuthorwellburn
    • CommentTimeDec 3rd 2008
    After experimenting with 120mm screws perpendicular to foam board into rafter we have found it mich easier using 150mm screws horizontally through kingspan into rafter. Kingspan might not weigh much, - but still rides down the perpendicular screw, - so use a longer one horizontally.
    boxes of 100 in 120mm cost me 7p + vat, in 150mm cost 9p + vat from local screw & fixings specialist.
    Brand on box is: TIMco 'Classic' multipurpose 6.0 x 150mm S/C NO TIM-77346
    Fixing at an angle through foam board is very positive, - takes longer finding joists and marking with chalk than fitting up boards. sealing edges and vapour proofing is time consuming too.
    Warm wishes!
    • CommentAuthorjemhayward
    • CommentTimeApr 29th 2010
    Does anyone know of a source of screws that are over 160mm - I'm putting up 155mm and would like to screw at an angle, so really need about 180mm - I've been using hammer in fixings, but screws would be much easier.
    • CommentAuthortony
    • CommentTimeApr 29th 2010
    Ironmongery shop or store or try on line.
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