loft condensation/insulation

[yeman]

Hi guys, we get condensation in our loft, we have boarded the floor out with t&g chipboard with 100mm loft insulation between the joists, the roof is felted i assume it is warm air hitting the cold roof in winter that caused the condensation, it isn't really bad but it does make items stored wet/damp, i was going to fill the rafters with 100mm insulation then staple a plastic membrane over then plasterboard so my kids can use the space for toys etc.. would this work? or can you sugest another way?

thanks

 

[big-all]

have you got propper airflow through the eaves from one side to the other!!!!!!

the problem may only happen when you open the trapdoor as in you keep checking dos'nt let it dry out
try isulating around the trap door incase warm air is leaking past!!!

 

[yeman]

hi big-all

thanks for your reply, i do have a gap around the hatch as we put a new loft ladder/hatch and havn't done the landing ceiling yet! also the hatch is right outside the bathroom so this won't help.
I am on to finishing that soon, as for venting, we are due to replace the gutters etc later this year i was going to add vents in the soffit boards when i do this.

my main concern is filling inbetween the rafters with insulation, covering in plastic and then plasterboarding over this, would it cause a problem in doing this? i don't want the condesation to form behind the insulation? but i can't see this happening as moisture won't be able to get in.

Regards

yeman

 

[Deluks]

i don't want the condesation to form behind the insulation? but i can't see this happening as moisture won't be able to get in.

That it correct, you must tape all joints though and make the vapour barrier truly airtight. You could also use foil backed plasterboard and do away with the plastic.

You will also need a clear 50mm gap on the cold side of the insulation, so you may need to add extra timber to the rafters to ensure you get the required depth of insulation once you take the 50mm into account.

 

[yeman]

do i need a 50mm gap? the roof joists are 40mm x 100 mm i was going to staple 100mm insulation in the gap, the dynamic of the job changes if i need a 50 mm gap, as it means a lot of woodwork.

if i just put up plastic and plaster board will this stop the condensation?
i am using plastic as i have a good supply of standard plasterboards.

 

[Deluks]

Get a roll of vapour barrier, about £15 from Wickes/B&Q etc.
You could use less insulation between the rafters, you already have some in the floor, it shouldnt matter if you only have 50mm in the roof if only using the room for storage. Plus it will be a bit cheaper. 50mm celotex/kingspan/xtratherm will do, or you could get the tri-iso stuff, but this is more expensive and will need counter-battens on the rafters before fixing the pb.[/url]

 

[masona]

the roof joists are 40mm x 100 mm.

Are they gang nail trusses?

The only way to get rid of condensation is more ventilation

 

[yeman]

They are 100 x 40 wooden joists at approx 18" centers, then felt and tiled. we only get the problem is only in winter, assuming that warm air hitting the cold tiled roof is the cause. it is not really bad but it does make items stored damp / wet.
I beleive that if i insulate and plaster boad it it will solve the problem, but i don't want to make a bad job of it and cause other problems. hence the questions.

i appreciate all the comments i can get so i can get the info i need.

regards

yeman

 

[masona]

I beleive that if i insulate and plaster boad it it will solve the problem, but i don't want to make a bad job of it and cause other problems. hence the questions.

It could go the other way creating more condensation because you're making the loft warmer although if you put a top roof ridge vents to allow air circulation from soffit to top should help. If you have gang nail trusses you shouldn't use the ceiling joist for playing area although there are way round it.