Vapour Barrier

gsxr1

I am converting my attic.

Dormer trusses.
I have got a hold of 75mm cavity wall kingspan.

I have insulated tightly in between the trusses with the stuff. I also am using insulated slabs with another 50mm of kingspan stuff on the back.
over kill i know but,
Im hoping to keep the heat and the cold out.

I am at the vapour barrier stage.

My question is.

Should I tack the plastic right up the the ceiling and seal the room completely,
or should I leave an air gap along the length of the room.

I will have a 4" vent hole in the room.

But By sealing the room completely with plastic I am worried about condensation build up behind the vapour barrier plastic.

any suggestions on how to proceed

Mellor

You have to be very careful here. A vapour barrier completely resists vapour passage, what you need is a vapour control layer/vapour check. Getting the wrong product could cause problems.

Firstly, have you left a 50mm air gap between insulation and underside of felt?
This is important to ventilate the structure and prevent build up of condensation.

Secondly, you need to get a vapour control layer and fully seal the structure, this restricts the amount of vapour (and energy) loaded air escaping in to the structure.

The 4" vent is correct, this provides suitable ventilation.

gsxr1

Mellor wrote: »

You have to be very careful here. A vapour barrier completely resists vapour passage, what you need is a vapour control layer/vapour check. Getting the wrong product could cause problems.

Firstly, have you left a 50mm air gap between insulation and underside of felt?
This is important to ventilate the structure and prevent build up of condensation.

Secondly, you need to get a vapour control layer and fully seal the structure, this restricts the amount of vapour (and energy) loaded air escaping in to the structure.

The 4" vent is correct, this provides suitable ventilation.

Thanks for your reply. you have set my mind to ease as I have had to deal with condensation damage on builds before . It can be a real pain to get sorted.

The trusses are nine by two so there is seven inches of free air space behind the insulation. there should be a draft for the soffit vents on each side

I have seen builds in the past where there was a section of vapour control layer left out near the ridge. This seemed odd to me . Whats the point of this layer if there is a section opened.

sinnerboy

carefull here indeed .

you most likely have soffit vents located at 600c/c or you have a continuous vent strip 10mm wide . this is ok for the non converted attic , with fibreglass on flat ceiling . you most likely have no vents at apex

now you are insulating on the slope . you must have

1. soffit vents equal to at least 25mm continous strip
2. apex vents equal to at least 5mm continuous strip

you have nice deep rafters - use them . place 175mm between them . (75 you have + 100 more ) . as mellor says 50 vent space below felt is vital . foil tape the boards across the rafter bottoms .

you may think you are going over the top already but what I am proposing will only just bring you up to revised min standards . you will be glad you did it later

if you have not already purchased the insulated plasterboards - dont . get 50mm foilfaced pu boards and run them horizontally across the rafters . foil face the joints .

the two sets of foil taped joints will set up a good vapour resistant build up .

to really "go for it" on vapour resistance , now use foil backed plasterboard and skim .

dont undo all this good work with recessed lighting . make sure elec cables are located only in the vented cold spaces . if sandwiched between insulation layers - make sure you get good fire insurance

gsxr1

sinnerboy wrote: »

carefull here indeed .

you most likely have soffit vents located at 600c/c or you have a continuous vent strip 10mm wide . this is ok for the non converted attic , with fibreglass on flat ceiling . you most likely have no vents at apex

now you are insulating on the slope . you must have

1. soffit vents equal to at least 25mm continous strip
2. apex vents equal to at least 5mm continuous strip

you have nice deep rafters - use them . place 175mm between them . (75 you have + 100 more ) . as mellor says 50 vent space below felt is vital . foil tape the boards across the rafter bottoms .

you may think you are going over the top already but what I am proposing will only just bring you up to revised min standards . you will be glad you did it later

if you have not already purchased the insulated plasterboards - dont . get 50mm foilfaced pu boards and run them horizontally across the rafters . foil face the joints .

the two sets of foil taped joints will set up a good vapour resistant build up .

to really "go for it" on vapour resistance , now use foil backed plasterboard and skim .

dont undo all this good work with recessed lighting . make sure elec cables are located only in the vented cold spaces . if sandwiched between insulation layers - make sure you get good fire insurance

so in this method the plastic would not be used at all?

I have half the insulated slabs already . I will use them to insulate the jack walls instead of the ceiling now. (Terraced house).

thank you for your advice sir

Mellor

You have the luxery of nice deep rafters, i'd agree with SB that you should put the max insulation in here now. Hopefully this is possible

sinnerboy

I would add - use a strip of pvc , about 300 wide to transfer from sloped face of roof and turn down party walls - 150mm long roof slope , 150 down wall face behind dry lining . tape the pvc the the roof slope and wall face .

gsxr1

sinnerboy wrote: »

I would add - use a strip of pvc , about 300 wide to transfer from sloped face of roof and turn down party walls - 150mm long roof slope , 150 down wall face behind dry lining . tape the pvc the the roof slope and wall face .

I seen at that last home bond meeting at citywest there is a product especially made for that application. I will look into it

bauderline

SB,

How about sealed fire rated recessed lighting that is sealed around the rim when inserted into the ceiling ?

What are the alternatives ? I am not a big fan of the one lonesome light dangling out of the ceiling....

Thanks for the info BTW, very useful....

baud.

JohnBoy

lamps?

We rennovated an attic room recently where the ceiling was just shy of 7 feet (predates building regs) and rather than going with recessed lighting we installed switched lamp sockets instead.

kinda limits the room layout options in the future, but if you put in a few of em you'll have scope for change, our room was really only suited to one layout so the inflexibility wasnt a major concern.

sinnerboy

bauderline wrote: »

SB,

How about sealed fire rated recessed lighting that is sealed around the rim when inserted into the ceiling ?

What are the alternatives ? I am not a big fan of the one lonesome light dangling out of the ceiling....

Thanks for the info BTW, very useful....

baud.

the ones I've seen work by the use of intumescant materials which when heated expand to fill the very OPEN light housing . so they are still a source of air leakage .

where located they remove areas of expensive heat retaining insulation

wall lighting , free standing lamps or surface mounted ceiling track lights are the way to go