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General: Best method to create more insulation space in cut roof

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  1.  
    Hi all,

    We have a timber cut roof made of 6"x4" timbers but I want to create additional space for more insulation, planning on using the Icynene expanding foam insulation. Ideally want to add another 6"x4" timber to the inside of the current rafters but wondering the best/most effective solution. There is another 2"x2" timber to be added after this to create a service void so bit worried about stability.

    Have the following questions:
    a) Should I run the new rafters parallel with the old ones or run them across the old ones
    b) Would it be best to simply screw right through the new rafter to the old with some chucky screws or is there a better solution, maybe some kind of joist hanger or bracket on the sides
    c) Would some kind of wood I-beam be better than using 6"4" timbers, no idea how they compare in price?

    Any advice would be great.
  2.  
    I'd say option C , using osb as the web (say a 200mm square every 600mm ?) and fixed to 2"/2"
    Like a larsons truss.,
    you could then run your 2"/2" (or 2/1) for the service void at right angles to stiffen it up a bit

    There was an article on similar a few years back in Green building mag , I think GBP Keith wrote it about his place
    • CommentAuthorEd Davies
    • CommentTimeJul 2nd 2012
     
    “Like a larsons truss.,”

    That's spelt “Larsen truss” for anybody who wants to do a search.
  4.  
    thanks spell check wizard:smile:
    • CommentAuthorTimber
    • CommentTimeJul 2nd 2012
     
    Yeah, I would OSB onto the side of the existing, and then then 2x2 glued and fixed to the OSB to create a composite beam. If you use a D4 grade adhesive on the OSB where it meets the existing and the batten, it will create a hell of a strong bond. Then battens perpendicular to the OSB beams.

    Alternatively, use 50x50 battens and create a weave of battens below the existing. This will help to reduce the thermal bridge and help hold the insulation in place should it decide to move or shrink or whatever (not saying that it would).
    • CommentAuthortony
    • CommentTimeJul 3rd 2012
     
    counter batten the opposite way the rafters run and fill the whole lot with quilt, possibly with a ventilation gap above

    ie build roof, next day insulate from below, vb, line insulate from above, felt then go home come back to lay tile etc
    • CommentAuthorborpin
    • CommentTimeJul 3rd 2012
     
    If I understand correctly, this is to increase the depth before putting in a service void and plasterboard?

    If so, the I would 'counter' batten across the existing with 6x4 and fill the resulting void with Icynene. This gives you the depth and reduces thermal bridging. However, is there a limit on the depth this insulation can comfortably do or will they do it as 2 passes?

    What is the rest of the roof construction? I am not a believer in the 'always needs a ventilation' mantra, but if this is going hard against sarking, then the other side of the sarking needs a breathing membrane. If hard against felt or the tiles themselves, I would think it to be OK but do check.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 3rd 2012
     
    I put insulation hard against the sarking and it was wet within hours, redid it with a ventilated gap and not had a problem since.
  9.  
    Posted By: SteamyTeaI put insulation hard against the sarking and it was wet within hours, redid it with a ventilated gap and not had a problem since.
    This sounds like warm moist air coming into direct contact with cold surface. Was the air barrier in place, any loft hatches closed & all ceiling penentrations sealed?

    Ventilating below the sarking is the old school solution for those not planning a well sealed air barrier.

    If you have a continuous air barrier with a reasonably high vapour resistance then you can fully fill the rafter void, put a breather membrane over sarking boards & ventilate over the top of the breather membrane. This maximises insulation performance by minimising thermal bypass & wind washing effects.

    David
  10.  
    http://i119.photobucket.com/albums/o156/do-more/temporary-19.jpg

    Here is what my roof looks like, I'd imagine that you could do similar with 3x2 fixed with 3x2 blocks at 600 centres or so and it would help with reducing the cold bridge.

    I have this insulation system at the eaves
    http://www.isover.se/produkter/produktvisning?id=20763

    Then I have 2x1 tacked under the sarking with hardboard over to create and extend the ventilation gap up to the ridge.

    VB will be tacked on to the inner edge of the trusses and counter battened with 4x2 to retain the 475mm of loose mineral wool insulation and create a service void.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 3rd 2012
     
    Posted By: davidfreeboroughWas the air barrier in place, any loft hatches closed & all ceiling penentrations sealed?

    No it wasn't, though sometimes get confused about what is the difference between 'air barrier', 'vapour control layer' and 'weather tight layer'.
    On the homemade MVHR thread you can see me poking my loft insulation. And I think on the iButton thread there is some data about it.
    Works and cost next to nothing.
  12.  
    We already have 70mm PIR board on top of the rafters so no issue with thermal bridge through rafters, with breathable membrane on top and then battens and counter battened and tiles. I know it would have been better if the PIR had been something breathable but its where we are, we are installing airtight VCL on inside of service void and then fermacel boarding. Looking at the videos I think the Icynene can easily fill 200/300mm in one go.

    Could using 6x4 be a weight problem in terms of the engineering of the roof, its like hanging another roof under the current one, this was never discussed with the engineer? I am favouring your suggestion Timber, seems like a good one, avoids the weight issue I assume would be cheaper as well than 6x4s.
  13.  
    yep, larsen truss system is best way to build out (or down/up ) using the least amount of materials to achieve your goal ie. the least thermal bridging and maximising space left for insulation
    • CommentAuthorborpin
    • CommentTimeJul 3rd 2012
     
    Posted By: Phil.Chaddah-DukeWe already have 70mm PIR board on top of the rafters so no issue with thermal bridge through rafters, with breathable membrane on top and then battens and counter battened and tiles. I know it would have been better if the PIR had been something breathable but its where we are, we are installing airtight VCL on inside of service void and then fermacel boarding. Looking at the videos I think the Icynene can easily fill 200/300mm in one go.
    I'd be a bit wary of effectively sealing the timber between 2 vapour control layers. Leave the inner one off. As long as you have more insulation value outside the rafters than in you should be OK. The danger is the inside of the PIR is too cold and any moist air that gets to it will condense (this may happen already). The Icynene will provide the airtightness better than any additional membrane and any moisture that does form will then be able to escape inwards. However, this is unlikely as the Icynene will prevent the bulk air movement required.
  15.  
    I was not really happy about relying on just the Icynene for airtightness, has anyone used it and had a blower test done and how long has the product been around, is there a chance that in the long term it may become somehow less airtight?

    Also, its my understanding that it will not provide any VCL properties so moisture will pass straight through it but I guess you are right at least it could then dry back out in to the house again.
  16.  
    Anyone else have a view on air tightness of Icynene, is a good as a membrane system. Was searching the web today to find some quoted figures on air tightness but very little info on this aspect of it! Question is can I really do away with my internal VCL? Would avoid a lot of work and expense so will be happy to go that way if possible.
  17.  
    Posted By: Phil.Chaddah-DukeAnyone else have a view on air tightness of Icynene, is it really as good as a membrane system. Was searching the web today to find some quoted figures on air tightness but very little info on this aspect of it! Question is can I really do away with my internal VCL? Would avoid a lot of work and expense so will be happy to go that way if possible.
    • CommentAuthorTimber
    • CommentTimeJul 5th 2012
     
    Yeah, what about gaps between the foam and the timber rafters? How well will the foam stick? How will you deal with air tightness detailing at junctions? How will you control moisture vapour moving through the structure?

    etc, etc, etc.

    If you are adding deep beams for insulation, why not go mineral wool. Has the same lambda as icynene, so will give the same U-Value. It is breathable. It is cheap. It won't seal the timbers in. It will not make things worse if there is a roof leak. You can fit it yourself. You can achieve air tightness with a membrane outside and in. You can lap a VCL with other elements.

    If you can't add an air tightness barrier on the outside of the mineral wool, consider some rigid foam between rafters. Sort of in the middle when it comes to advanges and disadvantages. Will need less for a set U-Value, or you can get a better U-Value for a given depth. Doesn't suffer from wind washing like mineral wool.

    etc, etc, etc.
    • CommentAuthorborpin
    • CommentTimeJul 5th 2012
     
    I am pretty much going to rely on it for air tightness and it is exactly its expanding and 'sticky' properties that I am using to help ensure a good seal in the Light Steel frame. The outer PIR will be taped as well though!

    Posted By: TimberYeah, what about gaps between the foam and the timber rafters? How well will the foam stick? How will you deal with air tightness detailing at junctions? How will you control moisture vapour moving through the structure?
    Well as it expands a great deal I'm not expecting any gaps. Very sticky! It is exactly the problems at junctions I'm hoping the Icynene will solve!

    I'll let you know how I get on. As it is going into a warm roof I will be able to monitor it very carefully!!
  20.  
    Borpin, are you planning to have an air test done?

    Timber, as we have PIR board on top of rafters already then as Borpin highlighted I would be effectively sealing any insulation between two VCL which is a worry. Is the thinking correct that if I can create the airtightness on the inside without a VCL then any moisture can pass in and backout again with no problems? I do have concerns about details like double rafters where they are tight against each other so too small a gap for the foam but certainly a gap in terms of air, those would need to be taped.

    It seems it has been used to achieve high levels of air tightness such as:
    http://news.archiexpo.com/press/icynene/icynene-selected-by-dragonboard-to-insulate-its-future-homes-project-59333-158536.html
  21.  
    Posted By: Phil.Chaddah-DukeTimber, as we have PIR board on top of rafters already then as Borpin highlighted I would be effectively sealing any insulation between two VCL which is a worry. Is the thinking correct that if I can create the airtightness on the inside without a VCL then any moisture can pass in and backout again with no problems?
    The water vapour would tend to be driven outwards in winter & inwards during the summer. So it can work in principle, but there's a risk that condensation will occur during winter & that this will only be dried out when summer arrives.

    Pro Clima Intello is a variable diffusion air barrier which is designed to manage exactly this risk. It works by being vapour tight in winter (when the internal relative humidity is low) & vapour open in the summer (when the internal relative humidity is high).

    David
  22.  
    I am will be using Icynene in my build. The only problem I am aware of, is that it doesn't stick very well to surfaces that are cold. A friend had their studio insulated with Icynene and it didn't stick to the OSB in places causing voids. They did use LDR50 which I was told is less sticky than the classic foam.
  23.  
    Thanks for all the info guys.

    Ideally then I would reduce the amount of moisture entering the insulation layer to avoid risk of condensation so it seems the use of a membrane like Pro Clima Intello would be good choice to provide a VCL on inside which will let any moisture back out again in Summer.

    One other question, we have continuous runs of pre-insulated corrugated stainless steel solar pipes that run down the said roof. I would like to bury them in the Icynene to maximize the insulation of them, can anyone see any potential issues with this, there are no joins etc so I see no need to ever access them?

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