pitched ceiling, plasterboard and vapour barrier

[DJM]

We have unfortunately had a small outbreak of dry rot which I have atributed to leak which took me a while to fix and failing to do the remedial work when I should. This has forced me to take down the lathe and plaster ceiling ready for the dry rot specialists to weave their spells.

Part of the bedroom ceiling is pitched with 4" rafters and the rest is stadard ceiling into loft area. As there was no insulation before, the obvious answer is to add some prior to restoring the ceilng. I have got 50mm Kingspan for the pitched part (to leave the min 50mm air gap to the roof tiles), and will get some 100mm mineral wool (minimising costs) for between the beams on the flat part to loft space.

My question is, do I need a vapour barrier?

And if I do, polythene or foil backed plasterboard?

 

[noseall]

If you are going to this just once and avoid any condensation issues and subsequent rot then you need to do it adequately.

We have recently fitted insulation to an exact same scenario to yours. We fitted 100mm celotex between the rafters and a further 40mm across. We had to nail thickening battens to the rafters to achieve the air gap above.

You will need to fit 300mm of quilt insulation to the loft ceiling. 150mm between the joists and 150mm across. The regs state a minimum of 270mm but it is simpler to use the one size.

Incidentally, you will need to vent the air-gap if you are using non-breathable roofing membrane unless there is no membrane at all?

All the above was to satisfy current building regulations.

 

[noseall]

Forgot to mention the v.b.

If you buy foil covered insulation and foil tape all the joints then you will have your v.b in place. For belt and braces approach i would use foil back plasterboard in any case. This will be relevant to the non-raked ceiling part.

 

[DJM]

noesall,

putting building regs requirements aside for a second, and out of curiosity, are you suggesting that by using less insulation there is a larger chance of condensation, and where are you suggesting this condensation will occur?
- on tiles in the air gap between tiles and insulation
- On insulation In the air gap between tiles and insulation
- between insulation and plasterboard
- internal face of plasterboard

The roof tiles are open to air with no sarking felt or membrane. Plenty of ventilation at present ( howling gale even in the calmest days) so based on the building reg recommendation I think 50mm will be sufficient between the insulation and the tiles. Comments against this (or for it) appreciated.

I had considered putting celotex across the rafters, but was conserned about movement and screw length. I had thouight about insulated plasterboard as well, but the cost is very high and seems to have similar issues for the sloped ceiling.

The loft area isn't an issue because I can build the insulation up over rafters.

So your saying that I should put a VB in place irrelevant of the type or thickness of insulation used.

 

[DJM]

Just another thing, is that the pitched section does not directly meet the flat section. There is a beam which traverses the room. If I were at home I'd take a photo, but I'll try and explain.
In section running right to left.
Flat standard ceiling joists sit on top of structural beam.
Left to right: pitched roof travels over top of structural beam.
So the beam has to be boxed in.

How will I keep the integrity of a VB across that unless I use polythene?

 

[noseall]

Condensation will occur where warm moist air hits a cold surface. In your case this will be the underside of the tiles and the exposed rafter or tile battens.

If you have no roofing membrane then the chances are you will have sufficient ventillation to deal with the most air.

Having insulation across the rafters (as well as between) prevents cold bridging thus preventing 'rafter shadow'.

Having thicker insulation will increase your chances of condensation prevention. Being thorough with v.b. controls, i.e. taping joints or applying mastic to the joints will all help. I would insist on using foil covered insulation though.

If after taking all v.b. precautions and any sneaky moisture laden air finds its way past the insulation, then you need to have adequate ventillation to deal with this. I guess by having no membrane you are pretty much there. Unless your tiles are all mossed up of course..?

 

[DJM]

Thanks
I have knocked up a quick sketch in lieu of photos.

How can I ensure a VB around the beam? And how do I keep the barrier integral at the walls?
I will be paying a spreader to skim it all once the plasterboard is up. Unlike the quick cheap as possible fix that Richard C gave advice for on a thread a while back when I was helping SWMBO's friend out, this is in my own house and I want to do it right.

 

[DJM]

Thought I'd try and show what I was proposing with a VB and see if you guys had any comments. I was intending using foil tape for any joints (including bewtween the poythene and the celotex. I assume this will work

I also assume it's OK to put the celotex directly onto the beam

 

[noseall]

Looks ok to me...

 

[mellie_man]

I know this is an old thread but has my sceanario exactly.

What I want to know is what would be the real risks of not completing the VB round the purling. i.e VB across ceiling and up the slope to the back of the cross beam, but not round it, as I don't want to plaster over it, but leave it exposed!

I have just put 75mm of foiled kingspan under the rafters and filled and taped gaps so far.