Vapour Barrier between ceiling and loft insluation.

[dhutch]

I have seen it mentioned on here about the possibility of laying a vapour barrier onto of the plasterboard before putting down the loft insulation, but although ive found a number of threads mentioning it it (including disagreement as to if its necessary) have yet to find many reasons for and against. So I'm open to thoughts and suggests for and against before tackling my loft.

Presumably the intention is to prevent/reduce the transmission of moisture from the living space, through the ceiling, into the insulation and loft space, where it may condense on the underside of the colder floorboards (boarded loft) and or felt/rafters causing dampness. Which doesn't sound like a bad idea to me, particularly over the (frequently used) bathroom room due to the shower. All other rooms are bedrooms, with no decenable landing.

Cant really see any downsides with it, bar the cost and time to fit. Which given I am removing all the current insulation (there's not much) , extending joists from 4inch to 10inch, re-insulating, and re-boarding, is negligible.

Obviously it would be better if it where below the joists, and there being enclosed on three sides but still open on the underside is the only downside i can see in terms of the possibility of them spending time being cold and damp. But the only way to achieve that would be to removed the ceiling, which isnt happening! A compromise would be to cut the vapour barrier into strips and just lay it in between the joists against the plasterboard only.


Daniel

 

[dhutch]

Right, all the old insulations out and its due to go back in a week today (there doing on a gov grant, alongside injecting the cavity wall) so i need to make a decision.

Im very much in favor of putting something down, while its all clean and hoovered out (hurry for industrial vacs) but its just a case of what (breathable or not) and if i put it over the joists or just cut strips to lay agsinst the PB between the joists.

Somone say somthing!


Daniel

 

[freddiemercurystwin]

Stick a vapour barrier in if you want or don't bother. some may argue its necessary however I would argue that historically one has never been fitted in this position and there do not appear to be any quantity (well no one to be precise) of people complaining their insulation is saturated with moisture. So fit it if you like but I don't believe there is anything to be gained by it.

 

[dhutch]

Stick a vapour barrier in if you want or don't bother. some may argue its necessary however I would argue that historically one has never been fitted in this position and there do not appear to be any quantity (well no one to be precise) of people complaining their insulation is saturated with moisture. So fit it if you like but I don't believe there is anything to be gained by it.

I see your point.

There is clearly some signed of mould formation, mainly over the bathroom. Although you are proberbly right that its nothing to get hung up about, or singnifcant.

The main reason for concern, although i admit even i am not that concerned, is that i am planning to fully board the lost with 18mm chipboard, as aposed to the existing boarding with floorboards to only around 60% coverage, hence a largely inventiated space containing the insulation, anda cold floor for any damp to condense on.


Daniel

 

[big-all]

you shouldn't get any condensation as the attic is cold draughty place with the eaves open and or tile vents

if theres not enough air flow your in trouble

 

[dhutch]

Having been up there all weekend empting it, taking up the old floor and making it good for extending the joists and re- insulating I can certainly confirm there is a through flow of air!

Having read a few articles over the weekend its also appears that the majority of vapour transfer occurs through leakage rather than transfer through the plasterboard anyway. Im also planning to paint the bathroom with a cost of oil-based undercoat due to the leaching of cigarette odour anyway and there have been suggestions of doing just that as a reduction for vapour transmission as well. Its just


Daniel