Celotext between or over the rafters?

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hi all, I'm thinking of insulating the underside of my roof, and am not sure if I should go over or between the rafters with celotex. The gaps between the rafters are all uneven due to the age of the house, so each section will have to be measured and cut. I've got enough space in the rafters to use 25-30mm of celotex and still leave a decent breathing space. My other option to stick sheets over the rafters, will probably have to use some packing due to uneveness so that it's all smooth when viewed from inside.

thoughts? option 2 will probably be a lot less time consuming, but not sure if at some point in the future I want to plasterboard over the rafters. it's not a space to be used as a bedroom

hope the attached image helps explain - btw I'm talking about the sloped sides mainly

cheers all
 
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What sort of membrane do you have under the tiles? Breather membrane is fine but if it's solid felt membrane you need to be very careful not to create condensation issues.
 
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What sort of membrane do you have under the tiles? Breather membrane is fine but if it's solid felt membrane you need to be very careful not to create condensation issues.

Thanks for pointing that out, hadn't considered it. There's no membrane, just felt and then tiles.

So with that in mind, should I apply a membrane to the felt, of the back of the celotex?
 
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Sorry, when I say membrane I mean the under-tile membrane - i.e. the under-tile felt. You mentioned leaving a good cavity above the insulation but that cavity must also be ventilated to avoid condensation. You might already have suitable ventilation at the eaves. If so you just need to worry about the top. Maybe half a dozen tile vents in the loft part would do it.

My advice then is to fit whatever you can get between the rafters allowing a 50mm clear cavity above and then a layer over. Problem is, if you want to get up to current regs, that will be a thick layer - probably 100mm or more.

Another option is what we often do in loft conversions - add 50mm battens to the underside of the rafters to make them 150mm deep. That would allow 100mm between and 50mm over.
 
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But if you want the ventilation to work then you need a vent between each rafter or a continuous ridge vent.
 
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Thanks both for the replies - very useful information.

The loft is well vented along the eaves, but we don't have any tile vents.

The roof is very steep, so any work on the tiles will require scaffolding for access, especially if a vent per rafter gap is recommended. No point in doing things wrong, so I think this project might have to wait a little longer :)

Thanks again, really appreciate your knowledge!
 
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But if you want the ventilation to work then you need a vent between each rafter or a continuous ridge vent.
The rigid insulation doesn't go up to the ridge - it stops at the loft space. So as long as the whole loft area is vented sufficiently with vent tiles (you'd need to work that out based on the volume) there should be air flow behind the insulation. If that was, say, 6 or 8 vents I would have thought they could be fitted off a roof ladder plus suitable precautions.
 
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Thanks Jeds, good point. I wasn't planning on fixing rigid board right to the top of the pitch, just to where the top horizontal collars (?) are, so might be able to get some vents in wwithout too much trouble.
 
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