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Loft insulation + VCL?


Radian
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I'm sorry if this proves to be a bit of an "old chestnut" but I've spent hours trying to find a definitive answer so I'm asking one more time...

 

If putting in fresh mineral wool insultation between (and then a second layer crossing) over rafters in a ventilated (cold) loft space, what kind of, if any, Vapour Control Layer should I be putting under the insulation first?

 

Some sources seem to suggest a complete vapour barrier (e.g. polythene) draped over the rafters and resting on the plasterboard to prevent inside moisture reaching the insulation. However I never like the idea of wrapping things in plastic: If extreme cold temperatures should ever bring the 'warm side' of the insulation down below the dew point (or if extreme loft temperature/humidity in the summer exceeds indoor conditions) then condensation may form in sensitive places that have insufficient drying-out capability.

 

Other sources suggest using a Breather Membrane defined as a material having a limited degree of vapour resistance thus letting some moisture to pass in either direction.

 

The latter sounds more sensible to me but just doing nothing sounds very similar given the finite permeability of gypsum etc. After all, in all the lofts I've seen, pink fluffy stuff just goes straight in! (not that  that makes it the right thing to do though)

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The vapour barrier would go on before the plasterboard , stapled to timbers and joints taped , lapped etc

 

failing that 2 coats of emulsion is deemed to be a vapour barrier, so crack on with the insulation

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46 minutes ago, Radian said:

I'm sorry if this proves to be a bit of an "old chestnut" but I've spent hours trying to find a definitive answer so I'm asking one more time...

 

If putting in fresh mineral wool insultation between (and then a second layer crossing) over rafters in a ventilated (cold) loft space, what kind of, if any, Vapour Control Layer should I be putting under the insulation first?

 

Some sources seem to suggest a complete vapour barrier (e.g. polythene) draped over the rafters and resting on the plasterboard to prevent inside moisture reaching the insulation. However I never like the idea of wrapping things in plastic: If extreme cold temperatures should ever bring the 'warm side' of the insulation down below the dew point (or if extreme loft temperature/humidity in the summer exceeds indoor conditions) then condensation may form in sensitive places that have insufficient drying-out capability.

 

Other sources suggest using a Breather Membrane defined as a material having a limited degree of vapour resistance thus letting some moisture to pass in either direction.

 

The latter sounds more sensible to me but just doing nothing sounds very similar given the finite permeability of gypsum etc. After all, in all the lofts I've seen, pink fluffy stuff just goes straight in! (not that  that makes it the right thing to do though)

Radian.

 

To get the best response can you do a sketch, take a photo and upload? Give as much info as you can.. if in doubt about how much info to provide just chuck it all in!

 

 

 

 

 

 

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Mass air movement into the loft also needs to be considered. Seal any gaps in the ceilings - around pipe and cable penetrations, light fittings etc.

 

Not sure that emulsion paint will create much of a vcl as its moisture vapour resistivity is only about 0.5 MNs/g (polythene is 250 MNs/g or more)

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I'm adding extra insulation to an existing loft with only 200mm max. up there at the moment. I don't mind taking the old stuff up to put down a VCL but I wanted to check to see if it's really necessary. The only signs of condensation are on the vertical stack exiting the roof but I think I've nailed that. So the absence of a discrete VCL has proved OK for over 20 years in this house.

 

However, if adding more insulation (aiming for 300mm everywhere) changes the vapour behaviour, I'm not sure what might happen. I don't know about new-builds but it never used to be a consideration in vented roof spaces. There again, they rarely had much in the way of insulation.  Unvented spaces would certainly need additional vapour control - I get that.

 

The only issue I can forsee is in the region I have boarded for access and storage. This is currently just T&G chipboard laid over the bottom truss chords which trap the insulation underneath. My plan is to attach 150mm x 50mm joists to the webbing, running parallel to the chords, to support the flooring  250mm further up to allow for 50mm airflow above the insulation. Alternatively I could bring it down to the sit on top of the insulation saving 50mm height so long as I have a vapour barrier under the insulation. But 

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If you have decent ventilation above the insulation I wouldn't worry about it. 

 

Your instinct is correct not to wrap timber in plastic. 

 

Almost all moisture gets into a structure via air leakage not diffusion so like @ADLIan says seal all ceiling penetrations and you'll be fine. 

 

A consideration might be the thermal bypass. wind will simply blow through the mineral wool. No risk of timber decay but your insulation won't perform well on a windy day. I've heard suggestions of covering the mineral wool in building paper as a possible solution but I can't find the reference. 

 

 

 

 

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12 minutes ago, Iceverge said:

A consideration might be the thermal bypass. wind will simply blow through the mineral wool. No risk of timber decay but your insulation won't perform well on a windy day. I've heard suggestions of covering the mineral wool in building paper as a possible solution but I can't find the reference.

 

I've seen that mentioned as well "windwash" is the term I believe. Makes sense. I was wondering about using a breathable foil "space blanket" style sheet on top to prevent windwash and also to create a sensible surface that can be vacuumed - the current fiberglass has a layer of a couple of million dead flies and can't be hoovered.

 

57263.thumb.jpg.df0f8c26d0eab5e9e7dc51f2a880c958.jpg

This YBS BreatherQuilt is described as "Low vapour resistant (good at allowing moisture to pass)" but does 0.15 MNs/g sound OK?

 

breatherquilt-technical-data.thumb.png.da6a42e0fc82c6b2c6472f38b8b2b059.png

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'Wind wash' is not normally an issue with loft insulation as the wind speed over the insulation is insignificant. From memory BBA and/or BRE have done work on this

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1 hour ago, ADLIan said:

'Wind wash' is not normally an issue with loft insulation as the wind speed over the insulation is insignificant. From memory BBA and/or BRE have done work on this

 

They didn't try it in my loft ? ? 

 

Seriously though, it can get very drafty up there with around 480,000 mm2 venting along the eaves perfectly aligned with the prevailing South Westerlies.

I'm still thinking it would be worth it almost to get a maintainable surface. Cluster flies are an annual occurrence.

 

IMG_20220104_110954580.jpg.b543ae4d05867f0d71875cd86c230cb1.jpg

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Do you have any more pictures of the structure of the attic. 

 

It almost sounds like you might be better off raising the floor, insulating underneath and installing some flooring.

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