VCL across bottom chord of attic trusses

[MortarThePoint]

I plan to use sheet VCL (green poly) to make the VCL around the loft room. I have shown it in green in the image below. Battens across the rafters for under rafter insulation (all insulation is mineral wool) provide a surface to staple the VCL to (tape over). I'll have to tape around where the VCL crosses each stud of the dwarf wall which will be a bit tricky, but OK. What I'm less confident of is how I deal with the big transition to wall plate (masonry walls). This is where the green VCL is crossing the 222mm high bottom chord of the trusses. Does anyone know what is normally done here? Just cutting a slot and flaps out of the VCL and stapling/taping feels a bit fragile.

 

 

 

It feels like I should have something to support the VCL here. I can see two easy options with ~9mm OSB or 5mm ply. Shown below before and after adding insulation, though maybe the rafter insulation would carry on down before adding any sheet due to restricted access. In the top otption, the ply/OSB could be screwed in place and then sealed with Passive Purple meaning that the VCL itself wouldn't need to go down all the way, but may as well. Very time consuming though.

 

 

[MortarThePoint]

I suspect the top one (angled ply/osb) is easier if I install the insulation at rafter level first

[Dave Jones]

you have a cold roof, why bother ?

[MortarThePoint]

On 24/05/2022 at 14:39, Dave Jones said:

you have a cold roof, why bother ?

 

Not sure I follow. The insulation at ceiling level chiefly acoustic.

[MortarThePoint]

Is it a viable solution to create two separate airtight 'cells'? One above the joists and one below? The air in the height of the joist would not be sealed. That shouldn't be a problem from a thermal perspective as there is insulation at the ends and the two 'cells' would be airtight so not transferring moist air to this space.

 

This is the easiest approach, but is it valid?

 

[MortarThePoint]

The detail below is a solution except I am using resilient bar to separate the ceiling plasterboard from the joists so I need to bridge that gap. The noggin would normally do this but that world short circuit my resilient bar.

 

I'm not really sure why in that detail it's a double noggin.

 

[MortarThePoint]

On 18/06/2022 at 10:03, MortarThePoint said:

The detail below is a solution except I am using resilient bar to separate the ceiling plasterboard from the joists so I need to bridge that gap. The noggin would normally do this but that world short circuit my resilient bar.

 

I'm not really sure why in that detail it's a double noggin.

 

I wonder about using expanding foam tape under the noggin, so between the noggin and the plasterboard. That wouldn't sort circuit the resilient channel so much

 

[Note the minimum R=1.2m2K/W can be achieved by 53mm of 0.044W/mK Loft roll, so 150mm is plenty to meet that).

[MortarThePoint]

Here's an option that uses the existing noggins at the base of the dwarf walls. Before putting the ceiling up I can apply the lower VCL. Before putting the floor down above the joist, I can apply the upper VCL, lapping the joint. This creates a much larger volume outside the airtightness though. Not a problem if the joist is filled with insulation there.

 

 

[CH_18]

Hi mate, sorry to resurrect this thread from the dead but did you come to a solution for this? I have the exact same issue of maintaining a continuous VCL through the dwarf wall timbers and truss chords.

Thanks mate

[MortarThePoint]

1 hour ago, CH_18 said:

Hi mate, sorry to resurrect this thread from the dead but did you come to a solution for this? I have the exact same issue of maintaining a continuous VCL through the dwarf wall timbers and truss chords.

Thanks mate

 

I used OSB and battens to make horizontal and vertical surfaces:

1. Stick (50mm double sided tape) VCL to top surface of timber wall plate draping down wall

2. Add battens to follow with OSB

3. Horizontal OSB from top of wall plate

4. Vertical OSB up from free end of horizontal OSB up between joists

 Battens along OSB joint.

5. Apply double sided tape to OSB edges

6. Pull VCL onto surface of horizontal OSB

7. Cut VCL to allow sections to pass up vertical OSB

8. Wrap where VCL touches joist with polythene tape

9. Apply sealant

 

It required carefully measuring the joist gaps and thicknesses and pre cutting the VCL before attaching to wall plate.

 

I'll try to dig out some photos 

 

I applied sealant the the OSB box before attaching the VCL to it but that's me

Edited February 3 by MortarThePoint

[MortarThePoint]

Some pictures below. I don't seem to have one that shows how the horizontal OSB laps onto the wall plate by about 50mm and is screwed down onto the VLC that is double sided tape taped to the top of the wall plate. That leaves a stable joint that can then have sealant applied.

 

It's a slow process, but one a became reasonably proficient at and should be good and airtight. Having sealed the OSB box itself, if there is a leak through the VCL joints then there is a second line of defence. The VCL will be continued along the rafters later. There is enough VCL for it to extend about 1ft beyond the top of the vertical OSB I think.

 

 

 

 

 

 

 

[MortarThePoint]

2-ply trusses are a difficulty as they have a thin gap between the plys. I dribbled so PVA glue into such gaps as well as smearing sealant along the grove at the top and bottom of the truss bottom chord. It won't be perfect, but should help. Perhaps the ideal would be to apply sealant all the way along top and bottom, but I didn't.