Maintaining airtightness/VCL around trusses

[CH_18]

I have (maybe stupidly) decided to insulate my roof all the way down to the eaves to maintain a good thermal envelope with the wall insulation. All good. However, I now need to find a clever way to maintain the VCL/airtightness layer from the top all the way down to the walls whilst cutting around the dwarf wall timbers and floor chords of the trusses? Any ideas/tips how to maintain a good seal through and around the timbers and to make this as unfiddly as possible.

Thanks

[Iceverge]

5 minutes ago, CH_18 said:

I have (maybe stupidly) decided to insulate my roof all the way down to the eaves to maintain a good thermal envelope with the wall insulation. All good. However, I now need to find a clever way to maintain the VCL/airtightness layer from the top all the way down to the walls whilst cutting around the dwarf wall timbers and floor chords of the trusses? Any ideas/tips how to maintain a good seal through and around the timbers and to make this as unfiddly as possible.

Thanks

 

 

Warm attic is a top plan in my opinion. 

 

Do you have a wider shot showing the who house section, is it a cut or trussed roof? What kind of performance are you hoping for? A few cans of expanding foam doesn't fill me with confidence really regarding airtightness. 

 

 

 

[CH_18]

Yeah sorry I added that pic just as an example. It's a roof with attic trusses so need to work around the timber uprights of the dwarf wall etc 

[Iceverge]

Ok here's an option for a cut roof. 

 

The red line is airtightness membrane/board

In essence it separates the roof and wall and internal bracing is added afterwards.  The fasteners would be taking a fair strain in this case so best run it by an SE. 

 

[Russell griffiths]

With a pre formed attic truss there’s no easy solution. 

put the radio on and get your scissors out, every one you tape around is one less you have to do tomorrow. 
 

head down arse up, get it done. 
 

I personally would cut 600x600 squares of vcl and cut these in around the uprights, then when you come along with the big roll you can cut it very rough and tape it on to the neat squares you have already sealed. 

[CH_18]

1 hour ago, Russell griffiths said:

With a pre formed attic truss there’s no easy solution. 

put the radio on and get your scissors out, every one you tape around is one less you have to do tomorrow. 
 

head down arse up, get it done. 
 

I personally would cut 600x600 squares of vcl and cut these in around the uprights, then when you come along with the big roll you can cut it very rough and tape it on to the neat squares you have already sealed. 

Yeah good call with the 600 x 600 squares, might be a bit easier. Not looking forward to it put it that way 🤦

 

Any more gems to make the junction at the wall plate a bit easier? 

Thanks

[Iceverge]

Yup. 

 

Install 18mm OSB racking on top of the trusses and use this as your A/T layer. Taped at all seams. Then wrap an airtight membrane around the ends of the trusses returned to the internal face of the wall. You'll need trusses without rafter tails for this. 

 

Then install 100mm mineral wool between the trusses and 100mm PIR over the top like a hybrid roof for a u value of 0.14. 

 

You'll need to put extra timbers on top of the OSB for overhangs and rafter tails. 

 

From the top. 

 

Slates

Slate battens

Counter battens

Breather membrane

100mm PIR (rafter tails installed here too)

18mm OSB taped for airtighess 

100mm trusses with mineral wool infill. 

Edited February 4 by Iceverge