Correct way of upgrading insulation for existing (1980s) timber frame section of house

[Jdwood38]

Hi all,

Final step in our renovations is to sort out the insulation in a timber frame 'overhang' section of my daughters bedroom. The rest of the house is brick and overall this section is relatively small.

The make up appears to be the timber frame with c.50mm of fibreglass insulation between that has generally just been done badly, dropped in places etc. Inside there appears to be a VPL between the timber frame and plasterboard, externally it appears to be timber cladding fixed to the battens with a small vented space (you can see the vents at the bottom).

So the question is, what is the correct method for insulating this area to modern standard? There is absolute zero appetite to change anything internally, so all needs to be done from the outside.

1. Remove timber cladding and fibreglass insulation, fit PIR between timbers (foaming joints etc), breathable paper and then new cladding. This option to me feels like the timbers would remain a cold bridge. Would this option require a vented space on the external side?
2. As per 1, but overlay something like 25mm PIR on the outside of the timbers before cladding. I can see how this would reduce the timbers being a cold bridge but I always thought the overlay needed to be on the inside (something to do with dew point?). Would this need vented space again?
3. Something else?

Thanks in advance for any suggestions.

 

[freddiemercurystwin]

You seem to have a good grasp of things, fully fill the stud depth with PIR, fit 25mm continuous PIR on the outside, then a breathable membrane, then a ventilated void, then cladding.

 

[^woody^]

Is there sheathing on the frame?

 

[Jdwood38]

You seem to have a good grasp of things, fully fill the stud depth with PIR, fit 25mm continuous PIR on the outside, then a breathable membrane, then a ventilated void, then cladding.

Thank you, everything I've learnt has been through a DIY journey with so much coming from here. Can I just check if the PIR should be foil taped as if doing it internally and would I batten through the 25mm PIR to create the ventilated void?

 

[Jdwood38]

Is there sheathing on the frame?

Thanks woody, could you please elaborate? Do you mean VPL type sheathing (which there is between frame and internal plasterboard or timber sheathing (ply, OSB or something?) on the outside before the cladding? I'm not sure on the answer to question 2, but pretty sure I haven't seen any in the limited probing I did

 

[^woody^]

A timber frame should have sheathing board on the outer face to stiffen the frame.

Fitting insulation on the outer face presents several issues

- The membrane requires air to allow it to perform and moisture to evaporate.
As you don't know what protection the frame has internally, and what level of moisture is getting in to the frame, dealing with it is important.

- PIR face fitted, will prevent the frame breathing, and the membrane working. Consideration will be needed for fire stopping too.

You may be able to use a Rockwool insulation such as Rocksilk by Knauf, but then you may have problems with the cladding fixing and extra width of the finished wall.

So it's not a simple thing.