I have a natural vented house with some short sloping ceilings on the first floor with 100mm pir between the rafters, then 50mm pir/ plasterboard and  skim coat over the top. Is it worth me sealing the joints between the rafters and the pir with foil tape before putting the 50mm/board layer on or is this a bit over the top.

It wouldn’t do any harm 

But not necessary 

I would fit a combined airtightness/vapour barrier on the warm side of the insulation, seal it to adjoining similar layers before plasterboard 

1 hour ago, tonyshouse said:

I would fit a combined airtightness/vapour barrier on the warm side of the insulation, seal it to adjoining similar layers before plasterboard 

Why?

No air will get through the tape and skim and the insulation should be cut tight against the joists 

Edited March 20, 20215 yr by nod

2 hours ago, nod said:

Why?

No air will get through the tape and skim and the insulation should be cut tight against the joists 

I wonder if this is why self-builds generally cost more to do, not because of cutting corners by non-selfbuilders (or self-builders that are in the trade like yourself @nod) but more that a self-builder will do stuff 'just in case' it's needed. so are very much more belt and braces as they don't have the build experience to know that what you say is correct so over-engineer solutions.

 

I know that when I do the insulation, VCL, taping, plasterboard etc I will definitely be using more material that is probably required just in case it needs it. it will cost me more but I'll know that it will definitely be enough!

2 hours ago, nod said:

Why?

No air will get through the tape and skim and the insulation should be cut tight against the joists 

Relying only on plasterboard and skim for air tightness is a very bad idea.  It is a "normal" occurance for me when I unscrew a light switch or a socket I am greetid with a howling gale of icy cold air.  Air tightness should be a planned well detailed layer before plasterboard and finishes go on.

 

And don't rely on solid insulation fitted between joists or rafters being a tight fit.  Experience from what I see suggests in a great many cases it is anything but well fitted.

 

This is where a self builder can make a difference, by ensuring the detail is done properly and carefully.

2 hours ago, nod said:

and the insulation should be cut tight against the joists 

 

8 minutes ago, ProDave said:

And don't rely on solid insulation fitted between joists or rafters being a tight fit.

I have done a lot of rigid insulation and it’s one area I always go OTT on even if the PIR look tight I still jam the foam gun into the joint between timber and insulation and give it a squirt every inch or so along the joint.... cut back any snots  and then I tape it carefully. I realise it’s a bit crazy but when it gets covered over I sleep easy knowing I did the best I could. The reason for this crazy detailing is that I live in a VERY exposed location where I have felt the wind wind blowing through less than 1mm gaps. 

The foaming of tight joints is totally  impractical on a commercial build but I am only dealing with 3 buildings all of which I never intend to sell... and I’m just playing around at the end of the day ! 

I undercut all around and foam in with inbetween fitting. Easier and better job IMO. 

 

I use foil tape if I have to do pir over top of joists/rafters. In my layman brain it completes the foil vapour barrier. 

Your probably better fixing a visqueen vapor barrier to the joists Then simply board over it 

Thanks for the replys. The prognosis is to seal it. I think this also means that I had better tape the joints on the loft side of  the foil backed plaster board on the horizontal first floor ceilings as well.

As others have said, when you tape stuff it’s belt and braces but you also spot the detail - @Cpd is spot on that you find the gap and shove the foam gun in it. You will get to know your house much better, and you’ll slowly remove all the potential issues. Foil tape isn’t expensive - get some 100mm rolls and you’ll be soon ensuring the whole place is airtight and you’ll benefit for years to come. 

Pir and foam between joists 

Then pir over all of that equally with foam where required . Then foil tape every joint .

That’s what I did - yet to feel a draft 

Edited March 20, 20215 yr by pocster

Good thread this. Can anyone recommend a good quality foil tape for taping insulation please? A link would be great thanks.

I'm using full fill 100mm Cavity Therm PIR insulation.

Edited May 15, 20215 yr by Johnny Jekyll

I have been using the 100mm aluminium foil ultratape, been getting it from toolstation or ebay.

I always wipe the surface with a damp methylated spirt cloth beforehand to get good adhesion.

Nothing will stick to dirt or dust.

On 04/06/2021 at 11:46, farm boy said:

Nothing will stick to dirt or dust.

I have a pair of trousers that stick to them both.

No idea what is special in them, maybe @pocster knows.

 

Edited June 5, 20215 yr by SteamyTea

On 15/05/2021 at 21:42, Johnny Jekyll said:

Good thread this. Can anyone recommend a good quality foil tape for taping insulation please? A link would be great thanks.

I'm using full fill 100mm Cavity Therm PIR insulation.

 

This tape with the scrim seems to be very strong and stick well!   https://www.ebay.co.uk/itm/302772723552?hash=item467ea91f60:g:ubMAAOSwo9VbI84~

Thanks very much for the replies. Much appreciated.

I am just doing it now - as said above use gun grade foam in any gaps then I am taping up with this stuff - Gocableties Aluminium Foil Adhesive Insulation Tape 50metres x 100mm (4") Silver Premium Metal Roll: Amazon.co.uk: DIY & Tools

 

Bit of a pain getting the backer off but no biggie.

1 hour ago, Faz said:

I am just doing it now - as said above use gun grade foam in any gaps then I am taping up with this stuff - Gocableties Aluminium Foil Adhesive Insulation Tape 50metres x 100mm (4") Silver Premium Metal Roll: Amazon.co.uk: DIY & Tools

 

Bit of a pain getting the backer off but no biggie.

 

Don't tear or cut through the paper and foil both at once.  Leave the paper longer than the foil.

It is pretty thin but worth a go.  Cheers.

Insulation installers use a belt larger than their waist and put the tape  through the belt so it’s always to hand and as Mr Punter says keep the backing longer when ripping the tape off.

 

rope or wire around the waist also works.

Thanks again for all the replies.