Foil taping between celotex and rafter

[RussTaylor]

Hello all,

Ok I'm trying to insulate my lean to, and currently having a go at the ceiling.

I had 3" rafters, but wanted 100mm insulation so I've extended them down by about 30mm. The felt seems ok but I'll probably refelt it with something more modern (it has bitumen stuff at present) Between the original rafters I've wedged the 100mm celotex.

I believe I'm supposed to make it air tight, should I foil tape between the celotex and joists before I plasterboard it?

The joist extensions leave a slight gap in some places by the celotex. Should I try and get some expanding foam in there? Sounds bloody messy if I can get it to stay up.

I'm considering an insulated plaster board (35mm backing I think), or maybe OSB first then normal plasterboard for the ceiling and the walls, worth doing?

Thanks for your help,

Russ

 

[KenGMac]

Russ, Hi.

I assume that you are trying to make the space or "void" above the new Insulation as air tight as is possible, air tight that is from the "warm room and an atmosphere that has a high level of moisture suspended in it"

It appears that you are concerned about "intesticial condensation"? where moisture suspended in the warm air that will percolate through the Plasterboard and Insulant will condense in the "cold" void above the Insulation?

Rather than use filler and foam why not use sheet polythene? this will form an impenetrable barrier to moisture vapour.

Many years ago, Scottish Local Authorities during construction fitted polythene sheet material between the Plasterboard and the ceiling joists, it was not clear as to why this was fitted, but it did stop condensation forming inside the loft space. One reason for the polythene could have been to protect the PLASTERBOARD [yes Plasterboard] "Sarking" [ Sarking - a Scottish technical term for generally timber or plywood boards fixed to the upper surfaces of the rafters, there after the slate or battens and counter battens are fixed.

At the time of the above to cut costs, the Local Authorities began to use Plasterboard "Sarking" but [and it is a big BUT] the Plasterboard would not last unless the condensation problem was addressed, this was achieved by placing a layer of polythene file as described above.

Russ the object is to fix a layer of polythene or a layer of CONTINIOUS film between the upper side of the plasterboard and the Insulation. The polythene will stop moisture vapour getting into the loft space and thus no condensation. the polythene can be simple clear "vapour barrier" or indeed depending on the area you could use the blue damp proof membrane used below concrete ground floor slabs, the important thing is to if possible fit the polythene vapour barrier in one continuous piece.

The process is akin to a very large single piece of wall paper, fixing can be achieved by a staple gun, take care at the junction of the ceiling and wall, as that junction is the only [very small] area where vapour could get into the roof void.

Hope this assists?

Ken

 

[RussTaylor]

Ken, good idea thank you!

I already have a load of polythene for drylining the walls and the dpm of the floor so I'll do the ceiling as well.

Is it worth putting polythene under the felt on top of the rafters as well?

Thanks again

 

[KenGMac]

Russ, hi again.

The prompt reply indicates just how good the TV is tonight?

Bluntly it is not worth placing the barrier on the underside of the felt, simply because the felt is in so many words impervious to the passage of water vapour.

The entire "condensation" problems are based around "Dew Point" and the resistivity of the myriad of building materials, all of which have different resistivity values to the passage of water vapour [that's the technical bit]

It is worthy of consideration that you consider installing some sort of ventilation above the insulation to ensure that there is a flow of air above the Insulation, you did mention in your post that you were considering replacing the felt?.

Cheers.

Ken

Bottom line is that this is a cheap and cheerful fix

 

[RussTaylor]

Great thanks, hadn't planned on any ventilation for the felt apart from where it tucks under the lead at the top I hadn't planned to seal it down.

Should I use this technique on my walls, both sides of the timber frame as well then?

Mrs. has the TV watching some pretend greek drama

 

[freddiemercurystwin]

Ventilation at the eaves and top is essential to avoid condensation problems. It shouldn't be a consideration it is essential. Hope you left a good gap between the insulation and felt .....

 

[RussTaylor]

Thanks Freddy!

About an inch between insulation and felt I'd guess, and I'll see about a gap at the bottom when I'm up there. I was going to get some felt support trays to run along the bottom like these

http://www.screwfix.com/p/felt-support-tray-1-5m-pack-of-5/36622

they must go under the felt and hook over the fascia board. When I'm up there I'll see if I can find a way to leave a gap between the trays and the fascia to let some air flow.

Thanks again

 

[RussTaylor]

Ah hah, I need something like this I guess ...

http://www.screwfix.com/p/over-fascia-ventilator-10mm-x-1m-pack-of-10/65515#

and that'll go somewhere between the tray and the rafter. The house has never had any of this stuff and the roof is 80 years old so far ... it'll outlast me even without all this stuff I bet

 

[Perry525]

In a typical home water vapour is created by cooking, washing, drying, breathing, sweating.
All this heads for the nearest cold surface to condense.
All you need is a water vapour proof plastic membrane between your water vapour and the cold roof.
As the molecules of water vapour are much smaller than those of air, you will note that keeping them away from the roof is not easy. They can get through the smallest gap/crack. However, as long as the room side of the plastic sheet is kept above "dew point" the water vapour will stay out.
Backing the plastic sheet with 4 inches of foam insulation will do this.
Cover the plastic sheet with plasterboard, the plasterboard is full of water, it provides a 30 minute fire break.

 

[Alastairreid]

the plasterboard is full of water, it provides a 30 minute fire break.

Hope not.

You need a minimum 50mm between insulation and roof felt! unless you are changing the felt to breathable membrane then 25mm will do.


You also need high and low level ventilation, the over fascia vents will do for low level...you need to decide what you are going to do at high level.

Use 30-35mm insulation for an underdraw, tape the joints then a vcl or duplex boards.