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Housing - Renovation: What is the maximum thickness of Insulation above rafters?

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  1.  
    • CommentAuthorMike George
    • CommentTimeNov 5th 2008 edited
     
    Looking into this for a project. I'm interested in the maximum thickness of insulation that anyone knows of being used successfully on a true warm roof. [insulation on top of rafters]. Seems to me there are fixing limitations. Something like 50mm PUR being fine. What about 100mm PUR. Anyone done this?
    •  
      CommentAuthorTimber
    • CommentTimeNov 5th 2008
     
    I have seen 200+ mm thick tapered units on site before!

    You could use the plastic cups that are used to hold the board on, and change the screws for whatever length required.

    I am sure there must be a fixing solution that would allow shed loads of rigid to be fitted.

    Timber
  3.  
    Thanks Timber

    So the only limitation is dead loading then? No worries with wind loads etc?
    • CommentAuthorchuckey
    • CommentTimeNov 5th 2008
     
    This is a flat roof? I think the problem is with the compression of the foam when the top surface is loaded, because boarding bows when loaded and the foam compresses. The foam will only be supported on the rafters in your postulated case, so any roof loading will locally compress the foam over the rafters.
    So take a case of a roof of 10" thickness, bore a hole through the top board, relieve its edge, drop in your plastic cup and your 11" long "wood screw". Tightening up the screw will now compress, the foam and the boarding will bow down. How much do you tighten the screw? How do you measure the amount of foam compression to get the roof evenly flat.
    A better way would be to screw/nail down 3/4" boarding onto the joists first then glue down each succesive sheets on to it. I would think that you would have to order the sheets prepared to take the glue("stripped").
    Another point is that the "edges" of the insulation have to be "boxed" in so as to provide a facia board and perhaps something cobbled up to provide a soffit board.
    Frank
    • CommentAuthorCWatters
    • CommentTimeNov 5th 2008
     
    I believe the building regs require something like 150mm of Celotex these days to achieve the U-Values so must be possible or do they recommend some above and some between?
    •  
      CommentAuthorTimber
    • CommentTimeNov 5th 2008
     
    Well I don't think so but a chat with your structural engineer wouldn't go a miss.
    • CommentAuthorsinnerboy
    • CommentTimeNov 5th 2008
     
    Rafters imply a pitched roof ? Mike ?

    A little while back I spec'd 106 kingspan ( the one with 6m ply bonded to it - can't recall the actual product name now ) over rafters for a lead finished 10 degree pitched roof

    After much hair pulling during the build , settled for 66 (60+6) due to problems sourcing suitable fixings . Compensated by increasing insulation between rafters
    •  
      CommentAuthorTimber
    • CommentTimeNov 5th 2008
     
    Ah, i missed the rafter bit. My previous comments mayor may not be correct!

    Woops!!

    Timber
    • CommentAuthorMike George
    • CommentTimeNov 5th 2008 edited
     
    The job is a pitched roof farmhouse with reducing stone tiles [approx 30mm thick stone]
    • CommentAuthorMike George
    • CommentTimeNov 5th 2008 edited
     
    Just found this http://www.helifix.co.uk/

    Look under applications - warm roof battten fixing

    Thanks all. I think I will ring these people tommorow
    • CommentAuthorSaint
    • CommentTimeNov 5th 2008
     
    Mike,
    Helifix were the fixings recommended when using the Styrofoam T shaped board called Roofmate PR for warm pitched roof construction. Typically 30mm above the rafter and up to 80 or 90mm between (all part of the same 120mm thick XPS board). There is a BBA for this. Issues were the increased lever arm on the fixings as the tiling battens are not in intimate contact with the rafters and also a consideration for wind uplift. As fixing lengths increase so the chance of hitting the rafter decreases at least with someone like me in control of a hammer. There is a special driving tool available from Helifix. I seem to recall the other option was to lay thinner boards across the rafters but even then the maximum thickness was 50mm, the remainder of the insulation being added between the rafters. Roofmate PR did add racking strength to the roof construction. I guess now at least with XPS you would still need to insulate beneath the rafters.
    • CommentAuthorTheDoctor
    • CommentTimeNov 6th 2008
     
    i used a 165mm Celotex

    you can use 200mm

    there are specialist fixings for this. generally hammer driven helical fixings with a guide to make sure they go straight and actually hit the joist.
    • CommentAuthorpatrick
    • CommentTimeNov 6th 2008
     
    On a pitched roof, you neet to sandwich the celotex (or whatever) between the joists and sawn 2"X1" or you will have nothing to fix the tile battons to.
  14.  
    You use the tile battens (with the Helfixes) to fix the insulation to the rafters.

    J
    • CommentAuthorMatt
    • CommentTimeNov 6th 2008
     
    I have a just done an estimate for someone with 100mm hemp, 100mm of Pavatherm and100mm Pavatherm Plus on an old barn roof.....

    As you say, its fixings that become and issue, for this one we went with 100mm battens 'cross roof' on OSB (taped for airtightness) with hemp between,and then the Pavatherm on top. Effectively they are simply popping a 'new' roof over the existing structure that is sound and oversized/strong.

    We offer Ejot and Fischer fixings for 200mm depth roof insulation - common to do this kind of depth on the continent.

    Matt
  16.  
    I am going for 100mm Celotex between 100mm rafters and 65mm over the top fixed to rafters with counterbattens, then breather membrane and tile battens and tiles. Should be less than 0.14 U-value.
    •  
      CommentAuthorPaulT
    • CommentTimeNov 6th 2008
     
    twistfix also do a 156mm insualtion depth helical screw fitting.

    Our foreign SIP supplier has just brough out a 300mm deep system with just an interior board:
    from inside:
    plasterboard
    300mm EPS with splines
    Foil taped joints
    Vertical battens (25 x 75) screwed (Spax) and nailed through to internal supports.
    They are using some impressive screws and nails

    SPAX seem to have the best range, but hard to get hold of specialist sizes.

    - Note their structural engineer told us that the shear strength (rather than pull out strength) is nor important than you would expect as roofs are continualy tring to spread outwards - Thier nail are not round, but almost like 4 small nails mouled together and very thick.
    • CommentAuthorchuckey
    • CommentTimeNov 6th 2008
     
    Ah, a stone pitched roof! Same problem but even worse, due to the very high weight of the large bottom stones loading the first batten, then going up the roof the loading will get lower, so the foam will compress less.
    If you have say, 10 " of foam on top of the rafters, what are you going to fix your top batten on to, because there will be no wood at right angle to your top batten fixing, only the edge of the other foam covered pitch?
    Frank
  19.  
    Have received a written spec from the company linked above. No problem with 50mm or 75mm PUR - The thicker the board, the greater the length and number of fixings.

    Good point about the ridge Frank, but with only 75mm PUR insulation the vertical battenms will easilly give enough fixing for the counter [horizontal battens]

    The weight of stone is not a big issue apparently, subject of course to the rafters being designed to take the loading.

    There are several types of board with sufficient compressive strength- basically it is the same insulation used under concrete floor slabs. Both Insulation and Fixings carry BBA certification as well. Am going to run with it
    • CommentAuthorhowdytom
    • CommentTimeNov 7th 2008
     
    how much thermal bridging will take place with the stainless steel fixings ?
    tom
    • CommentAuthorMike George
    • CommentTimeNov 7th 2008 edited
     
    Minimal, only their diameter, which is much much less than the bridging when using inslation between rafters. They are also fixed into timber, which is a good insulator compared to masonry, where most are concerned about tie bridging.
    • CommentAuthorEd Davies
    • CommentTimeNov 8th 2008
     
    The roof in the test image I used on this thread (near the bottom of the page, second to last image):

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=281&page=2

    is 2 x 60 mm Pavatherm and 1 x 20 mm (I think it is) Isolair making 140 mm. Each vertical counter-batten has three pairs of screws through to the joists. The roof's at 45°. Each screw is at about 30° to the normal to the roof surface. In each pair one screw is from below the normal so nearly horizontal (pointing slightly down from about 15° above the horizontal) and the other is nearly vertical (pointing steeply down from 85° above the horizontal). More screws hit the joists than not :bigsmile:, particularly after the first four counter-battens were in place.
    • CommentAuthorndon
    • CommentTimeNov 9th 2008
     
    I have found that Wurth do fantastic fittings, we have used their coach screws (very similar to spax, but seem to cam out less often, and cheaper) upto 300mm, and they fly home with a good impact driver behind them
  24.  
    Any chance of a link to the screws ndon? I've looked at the Wurth site and cannot find Coach Screws
    • CommentAuthorMatt
    • CommentTimeNov 10th 2008
     
    <blockquote><cite>Posted By: howdytom</cite><div class="CommentHeader"><ul><li><a href="http://www.greenbuildingforum.co.uk/newforum/account.php?u=843">owdytom</a></li><li><span>CommentTime</span>2 days ago</li></ul><span><a onmousedown="quote(37510, 843, 'Posted By: howdytom');">quote</a></span></div><div id="CommentBody_37510" class="CommentBody">how much thermal bridging will take place with the stainless steel fixings ?
    tom</div></blockquote>

    Ours are 'countersunk' wide plate heads, with little plastic 'nubbin' to insulate the fixing....
    • CommentAuthorhowdytom
    • CommentTimeNov 11th 2008
     
    more detail please Matt
    tom
    • CommentAuthorSir Harry
    • CommentTimeNov 26th 2008
     
    Ask not how deep is the maximum above rafter insulation. Ask instead how best to calculate the type and density of fixings required to withstand windloads, sliding-loads, compressive loads etc. in a manner approved by the BRE.
    Visit www.heliCalc.co.uk for the answers.
  28.  
    Been there- done that- and they did give me the answers I needed- thanks
    • CommentAuthorMatt
    • CommentTimeNov 26th 2008 edited
     
    Posted By: howdytommore detail please Matt
    tom


    http://www.ejot.co.uk/build_htk-tkr.htm

    http://apps.fischer.de/poc/default.aspx?page=layer&sprache=EN&kat=$MART-HK-$MKAT-HK-$MPG-DAEMMST&ekat=$EKAT-HK-DE&path=$MART-HK-$MKAT-HK-$MPG-G7&path=$MART-HK-$MKAT-HK-$MPG-DAEMMST
    • CommentAuthorhowdytom
    • CommentTimeNov 26th 2008
     
    Thanks Matt

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