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Insulating a cold, unvented roof - possible?

Discussion in 'Roofing and Guttering' started by seneca, 7 Feb 2019.

  1. seneca

    seneca

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    Hello everyone. I've got a cold flat roof which is closed on all four sides. It's on a garage/storeroom and is used as a non habitable room. Last year it was leaking, so I had it re-felted. It has a number of skylights, so a new warm roof wasn't possible. The question is - now I'm ready to re-board the ceiling I was wondering if I could insulated between the joists. The roof has loads of full height noggins, so is really a number of small unvented rectangular spaces. The depth of joists varies and is between 160-180mm. My plan is to board the ceiling with 50mm insulated plasterboard (with foil barrier included on it) and leave the roof space empty.

    I was wondering - Could I push up a load of mineral wool to completely fill all of the voids, so there is no air space and then board over?
    Ideally I'd like the room to last 5-10 years max, as I will get an extension once my finances allow. Until then, it's a useful storage room and has a fridge and freezer in it. Just want to get the chill off it. Mineral wool is cheap and seems something that might help. Any opinions or should I just leave the space empty and overboard, as originally planned?

    Thanks
     
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  3. foxhole

    foxhole

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    Insulation won’t take the chill off, it provides no heat .
     
  4. seneca

    seneca

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    I suppose I meant make it appear less cold, in the way that when you go into a well insulated room vs a poor one, you notice the cold is a little less (not by any means warm) and the same goes for the summer - less hot that uninsulated. But I might be wrong. Anyway, I don't expect a heater to be on in the room, as it's just a storeroom and most of the time we just pop in to grab something from the freezer. Just a thought I had before putting the insulated boards up - should I fill the voids? You reckon more harm than good? and no difference to leaving empty?
     
  5. Mr Chibs

    Mr Chibs

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    I would think you would need to ventilate it really, have a search in this forum, there will be others who've done such a thing.

    Just for ref; Celotex is twice the insulation of rockwool so 50mm celo, is worth 100mm in rockwool, you are right rockwool is cheap, you won't be able to get it air tight though, and I would think you'd need to do this if you didn't leave an air gap.

    I recently saw a builder stapling string from one joist to another in a zig zag fashion, he used this to hold the rockwool to the ceiling before boarding.

    How big is the room?
     
  6. seneca

    seneca

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    2.5m x 6m. As I mentioned I had planned to leave it empty and just put insulated plasterboard underneath. Just staring at the space made me think I should ask the question! The closed spaces left by the noggins and doubled beams, and the overall drop in roof height for rain to run off, means there are no spaces that have regular heights and depths. Celotex would need to be eg. 70mm-1.2mx 400-450 wide and go from 200mm to 170mm in a series of uneven drops. Rockwook is compressible so could cope with the unevenness, but may leave spaces.
     
  7. Mr Chibs

    Mr Chibs

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    Compressing rockwool type stuff, reduces it's thermal properties, so ramming it in might not give you what you want.
    If you used celotex, could you not introduce some vent holes or soffit ventilation?

    As the goal is to stop condensation/moisture, I'm wondering If you filled these sections with rockwool, then put 25/50mm celotex afterwards (inside) and taped the joints, then boarded, this would stop vapour from the inside reaching the cold roof, the theory would be you wouldn't need an air gap.

    I'd be interested to here other folks thoughts.
     
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  8. seneca

    seneca

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    Haven't bought the insulated plasterboard yet, so could just get a 50mm celotex or similar foil PIR board and cut to fit the inside of the joists as you suggest. However, how is that different to my original idea of Rockwool with the insulated plasterboard underneath the joists? Other than I wouldn't be able to tape the joints, as I think the foil is sandwiched between the PIR and plasterboard. Still, I would welcome any ideas anyone has, even if I don't end up with filling the roof void, I too would be interested in any theories or experiences. This thing has got me intrigued!
     
  9. Mr Chibs

    Mr Chibs

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    I don't think that insulated plasterboard has the silver foil layer (vapour barrier). This is why separate insulation and boards are preferable.

    When redoing my bathroom, the building inspector had said you could use heavy gauge polythene sheeting before the plasterboards go on the ceiling, to stop moisture transferring the the loft are.
     
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  11. ^woody^

    ^woody^

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    That plan is ok.

    Just ensure that you leave no gaps in the void, and properly seal it with a vapour check barrier - foil board or polythene.
     
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  12. seneca

    seneca

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    Thanks for the input Woody. So Rockwool (or similar) compressed to fill all the space, then a vapour barrier and then foil board has your approval. Will an insulated board make any difference to plain old foil backed board?

    What is the difference between vapour check barrier and vapour control barrier? same thing? Could I staple a Polythene damp-proof membrane sheet (1000 gauge) to the ceiling joists to do the job? I have one just the right size. That way it would hold the rockwool insulation in place before I board it. Otherwise, it will keep falling down.
     
  13. datarebal

    datarebal

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    Compressing the rock-wool will be counter productive as another has previously mentioned.
     
  14. ^woody^

    ^woody^

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    In this situation, compressing the quilt is adventitious as then it reduces the air pockets, and thus the condensation risk. The effect on insulation value is insignificant.

    Putting some celotex insulation across the joist underside and taping the joints would give best results and act as the vapour check/barrier.

    Foil plasterboard has no insulation value on its own. Is just a vapour check. Yes 1000g polythene can be used if that's what you have.
     
  15. datarebal

    datarebal

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    compress it to half its original thickness then you reduce its insulting properties by half. so not really insignificant.
    but a plus point, better for sound proofing...
     
  16. ^woody^

    ^woody^

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    No it doesn't calculate like that. But anyway, if the OP uses 200 mm quilt in the 200-170 gap, then yes it's insignificant.
     
  17. datarebal

    datarebal

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    it does actually. But anyway , I prefer to advise good practice.
    Based on the fact that we have stripped a fair few roofs that had filled rafter voids (by others ) transmitting moisture onto ceilings.
     
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