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Housing - New Build: Warm loft on extension

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  1.  
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    Hi All - hope this is the right place to ask for some advice.

    We've just had an extension built, it's 2 storey cavity wall, and we specified a warm loft.

    The architect's drawing shows 160mm rockwool between the rafters with a membrane over it - this is translucent green, but I don't know what it actually is - the builder said it was breathable. I'm assuming that the builders have followed the drawing correctly.

    So far so confusing. However, on the first cold snap, the extension upstairs was noticably colder than the rest of the house. When I went up into the loft, there was a fair old draught blowing through. The builders have been back and taped all the joints, and this has reduced the air movement through the loft, but on a windy day there is still a cold draught, which I'm assuming is completely negating the insulation.

    My questions:
    Is this a normal way to build a warm loft, with rockwool and plastic sheeting? I've read through various other threads, and everyone else seems to use rigid insulation.

    How do we track down the draughts and seal them?

    Should we abandon the idea of a warm loft and just put the insulation back in the ceiling?

    A friend mentioned getting a Federation of Master Builders Survey to check the build work. Is this worth doing?

    Thanks in advance

    BFG
    • CommentAuthortony
    • CommentTimeNov 17th 2009
     
    The real situation is that you will have to put the insulation in at ceiling level -- 160mm sounds too little can you go for 300?

    The loft is always likely to be cold and draughty and at present your insulation is doing nothing -- cold air the wrong side of it. The green stuff sounds like a breather unless it is polythene.
  3.  
    A warm roof is not going to work very well without a rigid wind tight board on at least one side of the insulation & ideally on both sides. In addition, careful detailing is required at the eaves (behind the gutter) and on the gable to stop air whistling through the roof-wall junction. This is difficult, but not impossible to retro-fit.

    If there's no good reason to have a warm roof then you might be better with the insulation at ceiling level. But then you'll also need to make sure that any walls adjoining the roof space are sufficiently insulated.

    Why did the architect specify a warm roof? Is the space used for storage? Or was he seeking to avoid insulating an adjoining wall?

    David
  4.  
    Hi,
    Warm roof or Warm Loft ??
    Firstly is it polythene (probably to hold the rock wool) or not. I think if they came back to tape it to prevent draughts then it was not intended as a breathable layer as a part of some design, more likely they just thought it would hold the stuff in place .
    I cant see how you would have been able to get approval in the first place if its a loft its not the habitated space, so it probably should would have specified more rockwool type and at ceiling level, if it was approved as the habitated space you would need more than 160mm of rock wool - it would be a room in roof design. Warm loft is neither. Of course the criteria for ceiling and roof insulations are slightly different – but not enough to account for this.
    The builder followed the drawings so you need to ask the architect for the calculation.
    160mm of Rockwell on its own gives a U value of 0.3 (assume lambda = 0.050)

    Cheers
    Mike up North
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    Thanks to all. Some clarification probably in order.

    We specified a warm loft - it's not intended to be habitable, but we do need to store things that don't like the cold and damp, so we thought a warm loft would be the answer.

    The walls have cavity wall insulation, and on the drawings it's down as 100mm. Again reading the site, this is a bit low, but too late to change now. The detail on the architect's drawings for the roof/wall junction seem reasonable, as the house has dropped eaves and the wall insulation runs up the dropped eave and into the roof void. I've not crawled into the eaves to investigate whether this has been done, but I suspect I'm going to have to.

    The consensus seems to be that rockwool and green stuff is really not going to hit the spot.

    Would I be right in saying that the 160mm rockwool is just enough to pass building regs? We queried this with the building inspector, and he's agreed the 160 with the architect.

    Thanks to all for your input.

    Regards,

    BFG
    • CommentAuthortony
    • CommentTimeNov 17th 2009 edited
     
    I cant see how 160 could pass -- how long ago were the drawings passed?
  7.  
    Hi,

    pitched roof insulate between rafters requirement is U=0.2 and pitched insulate at ceiling is 0.16.
    Assuming the lamda value of about 0.050 for rockwool
    gives a thicknes of 250mm and 320mm resp.
    So assuming a ridged board type of PUR type had approx twice the R value ie half the lamda value so approx 0.025 (to cover all eventualities and give a rough order), you would just about do it.
    Are you sure they have not made a mistake and used/specified rockwool instead of solid PUR type insulation - ie simply wrong material.
    So with out picking over the potential thermal bridges and decimal places on specifications you have about half what you actually need, as you used something half as insulating as the depth of rafter allows.
    It may be the specification "forgot" or builder "forgot" to add an inner covering of ridged insulation (say 50mm) below the rafter level.
    All seems a bit odd.
    Cheers
    Mike up North
  8.  
    Assuming your rafters are 160x47mm at 600mm centres then Rockwool with a k value of 0.035 gives a roof U value of 0.3.

    However, this is not a habitable space, so you need to look at the U value of the ceiling below. The adjoining walls are insulated, so this ceiling forms the boundary of the heated space & provides most of the heat lost through the roof. As the roof area is greater than the ceiling area by the ratio of the cosine of the roof pitch, the U value seen at the ceiling will be proportionately worse. Assuming a roof pitch of 45° gives a ceiling U value of 0.42.

    The building regulations limit depends upon a number of things, but as far as I know, the maximum permissible elemental U values are as follows:

    Pitched roofs - insulation between rafters - 0.20
    Pitched roofs - insulation between joists - 0.16

    You should be aiming to do better than this, 0.12 is a good target. Have you thought about putting additional insulation at ceiling level?

    David
    • CommentAuthorCWatters
    • CommentTimeNov 17th 2009
     
    Some of the previous posts have been mixing up terminology. You have a "Warm loft, Cold roof with vapour permeable membrane " design...

    Warm Loft - that was clearly your intention and the insulation has been put in the right place for that (eg at rafter level not joist level).

    Cold roof - You have a so called "cold roof" design because the insulation is between the rafters not above the rafters as on a warm roof design. Note this does not or should not mean the loft is cold! A cold roof design is fine when correctly implemented but I would specify a warm roof if buiding again as I reckon there is less chance of getting a poor quality end result).

    Permeable membrane - avoids the need for a ventilated void between the insulation and membrane. It only allows more insulation to be fitted between the rafters if counter battens are also used like this..
    http://guide.rockwool.co.uk/media/49113/flexi-007.pdf
    If counter battens are not used a small _unventilated_ void should be left between the insulation and membrane. This is to allow the membrane to drape and stop water pooling above roof battens,

    Regarding the draughts - there shouldn't really be any. Vapour permeable membranes (at least the ones I've used) don't really let them through, except at gaps/joints. Find out where any draughts are coming from using a smoke source or similar. It's possible they ventilated the soffets which isn't needed if you have a vapour permeable membrane. Is there a Vapour Control Layer on the inside of the loft?

    160mm of Rockwool is marginal. On a new house they recommend 270mm I think.

    It's also possible the loft feel cold because the amount of heat leaking through the plasterboard ceilng into the loft is small. Have you measured the temperature up there?
  10.  
    CWatters

    I've seen "warm roof" used where the insulation is above &/or between the rafters, or where the insulation is between the rafters & the space behind the insulation is unventilated. "Cold roof" is typically used where the insulation is below & between the rafters, or where the insulation is at joist level. I've never heard anyone talk about a "warm loft".

    160mm of Rockwool at rafter level in an unheated roof space gives a ceiling U value of 0.42. It's not clear to me whether the 0.16 or 0.20 limit applies in this case. I think the regulations assume builders will minimise the amount of insulation by putting it at the boundary of the habitable space. But either way the result is not marginal, it's out by a factor of 2!

    I had the same issue in a small loft space above a room in the roof with rafter level insulation. The builders confirmed my calculations, but claimed that they were not required to make any changes as "small roof spaces" were not covered by the regulations. I added insulation at ceiling level to bring the overall ceiling U value down to 0.12.

    David
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    Once again, thanks to all.

    tony - drawings were passed about 2 years ago.

    CWatters - I think you have the right of it, looking at the drawings. The architect has specified breathable felt under the roof tiles, then the rockwool between the roof trusses (are these the same as joists? :shamed:) and then the green membrane. I think there is probably a small gap between the rockwool and the breathable felt, as the rockwool is flush with the inside face of the truss, and then the green stuff is stapled to the truss.

    Any ways round, looks like we're in for a cold winter if we don't do something about it. I'll quiz the architect regarding the target U value and what calculations were shown to the building inspector. The building inspector initially thought it was wrong, and only agreed to sign off after talking to the architect and saying that the joints should be taped.

    In terms of thickness of insulation, shouldn't this kind of extension work count as new build, i.e. should have 270mm of insulation?

    Cheers,

    BFG
  12.  
    BFG

    The joists are horizontal and form the loft floor. The rafters are sloping and support the tiles/battens.

    David
  13.  
    The loft space is 'warm' but not habitable, therefore not necessarily anything to do with the BCO, unless compliance was gained by a combination of insulation at ceiling level (under the loft floor) and at roof plane level.

    Now the thing is as your using it for storage why are you so bothered that its a bit chillier than everywhere else...?

    Unless of course its not really for storage....?

    J
  14.  
    See Page 21 of the attached document. I assume it falls into the category "Insulation at rafter level - flat ceiling". BRE state that the thermal resistance of the insulated roof structure should be multiplied by the cosine of the roof pitch. I'd like to know how the architect persuaded the building control officer otherwise.

    David
  15.  
    Posted By: davidfreeboroughI assume it falls into the category


    ...assumptions can be dangerous... :wink:

    I don't think there'd be any problem persuading the BCO.

    J
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    James - we're not bothered if the loft is a bit chillier - the problem is that the bedrooms beneath it are cold. It's too small to be a habitable space anyway.

    David - Thanks for the info. I've contacted the architect and asked them to run me through their calculations for this. The roof pitch looks to be about 38 degrees rather than 45, but even so using the cosine method this comes out as 0.38 at the ceiling, when I think we should be aiming at 0.2 or better.

    I suspect in the end we'll have to put the insulation at the joists level, i.e. directly above the bedroom ceilings. Trying the get the rafters level stuff airtight sounds like a job we can't afford to do. If we do go down this route, should we just leave the stuff that the builders have put between the rafters, or take it out?

    Thanks,

    BFG
  17.  
    Posted By: BFGthe problem is that the bedrooms beneath it are cold. It's too small to be a habitable space anyway.


    mmmhh....

    Suggesting no insulation between the ceiling joists... so my earlier suggestion couldn't be true...

    ...might be worth going back to architect/builder after all...

    Can you get the 'loft' boards up easily...?

    J
    • CommentAuthortony
    • CommentTimeNov 17th 2009
     
    Use it to insulate the ceiling with. then add some more.
    • CommentAuthorCWatters
    • CommentTimeNov 17th 2009
     
    Rockwool say 150mm achieves 0.3W/m^2 with 400mm rafter spacing here..

    http://www.rockwool.co.uk/thermal/thermal+faq's#f984

    Presumably that reduces to approx 0.3 x Cos(45) = 0.21 W/m^2 for a 45 degree pitch. Perhaps if you have 160mm rather than 150mm it just scrapes in? Do they make a 160mm?
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    Ah - I may have gotten this the wrong way up then. I thought from davidfreeborough that it was the inverse of the cosine, which seemed to me to make sense as the roof area is larger than the ceiling area.

    Using this, 0.3 * 1/cos 38 = 0.38 - I think our roof pitch is around 38 degrees looking at the drawings

    Should it have been 0.3 * cos 38 = 0.24 ? In which case, they're in the right ball park to get to 0.2 using 160mm rockwool.

    Thanks all for the advice.

    Regards,

    BFG
    • CommentAuthorBFG
    • CommentTimeNov 17th 2009
     
    Hi All, just heard from the architect. Apparently the BCO accepted the 160mm rockwool as "Part L does allow for differing values providing the values do not fall below a limitting value of 0.7 for walls and 0.35 for roofs."

    Seems like we'll either have to eliminate all draughts, or put the insulation in at the ceiling level.

    Thanks to all.

    Regards,

    Rob
  22.  
    Posted By: BFGHi All, just heard from the architect. Apparently the BCO accepted the 160mm rockwool as "Part L does allow for differing values providing the values do not fall below a limitting value of 0.7 for walls and 0.35 for roofs."


    Only if you do an area weighted U value calculation, ie you have better U values elsewhere or windows reduced below 25% floor area...

    ...what do the drawings say... (presumably the architect sent you these....) ?

    J
  23.  
    CWatters & BFG

    The thermal resistance of the roof is multiplied by the cosine of the roof pitch to work out the effective thermal resistance at ceiling level. Cos 38 = 0.788 which is less than one. So this has the effect of reducing the effective thermal resistance at ceiling level or increasing the U value of the ceiling. So 0.3/0.788=0.38. This is even beyond the limiting roof value quoted by the architect.

    David
    • CommentAuthorCWatters
    • CommentTimeNov 18th 2009 edited
     
    That seems wrong because it suggests that removing the plasterboard ceiling would improve the overall U-Value of the roof insulation..

    Edit: Ah I see. The area also changes so removing the plasterboard would overall result in increased heat losses.
    • CommentAuthorBFG
    • CommentTimeNov 18th 2009
     
    Hi All,

    Just measured the roof pitch - it's 33 degrees with a protractor off the drawing. That gives a U value at ceiling level of 0.353 - rounding gives 0.35, so it's right on the border.

    Thanks to all for helping me understand this. It puts me in a much better position to decide what to do with the insulation.

    Regards,

    BFG

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