Timber frame garden office, insulation and vapour barrier advice required

[llama]

Three or four years ago I built myself a timber outbuilding with the aim of one day, insulating it and using it as a small garden office. Tbh, it’s one of those things that I thought I might never get around to but I was offered some leftover 80mm and 100mm PIR and I accepted thinking it might give me the impetus to crack on.

I’ve half an idea of how to go about this but I am concerned about air gaps and moisture control. I’m hoping that someone with the necessary knowledge can give me some guidance.

Summary of the construction:

Size: 2.4m x 3m
Floor: Timber frame, suspended with 150mm PIR between joists and over boarded with 18mm ply.
Roof: Pitched, 125mm joist depth, top boarded with 11mm OSB and finished with bitumen roof shingles. Full-length ridge vent and full-length soffit vents on each side.
Walls: Timber frame (95x45mm). Outer leaf boarded with 11mm OSB, wrapped with Tyvek, battened and timber clad.

Rough plans:

Roof: Put the 100mm PIR between the joists, leaving a 25mm air gap and board the underside with plasterboard.
Is this too small or likely to be ok over such a small area and given the full-length vents?

Walls: 80mm PIR between studs – PIR pushed back against OSB leaving 15mm void for electric cable. Overboard studs with 27mm insulated plasterboard.
Should there be a vapour barrier and if so whereabouts in the layer does it go? Or should I just foil tape all the PIR joints and studs.

Floor: Likely to be laminate with an underlay. Should there be a vapour barrier or would a foil-faced rubber underlay be ok?

Advice on suitability of the PIR I’ve been offered and if/where vapour barriers should go would be appreciated.

 

[noseall]

Floor: Likely to be laminate with an underlay. Should there be a vapour barrier or would a foil-faced rubber underlay be ok?

What would the foil do that the rubber isn't already doing?

A vapour barrier (if needed) is fitted on the warm side. Any air gaps (if needed) are on the cold side.

 

[robinbanks]

Is this too small or likely to be ok over such a small area and given the full-length vents?

I'd push it up against the underside and seal

Should there be a vapour barrier and if so whereabouts in the layer does it go? Or should I just foil tape all the PIR joints and studs.

They go on the warm, moist side to prevent moisture entering the fabric of the wall and condensing to liquid moisture somewhere along the temperature gradient, if the wall fabric is affected by it

Sure you could tape the joints/studs

 

[noseall]

Three or four years ago I built myself a timber outbuilding with the aim of one day, insulating it and using it as a small garden office. Tbh, it’s one of those things that I thought I might never get around to but I was offered some leftover 80mm and 100mm PIR and I accepted thinking it might give me the impetus to crack on.

I’ve half an idea of how to go about this but I am concerned about air gaps and moisture control. I’m hoping that someone with the necessary knowledge can give me some guidance.

Summary of the construction:

Size: 2.4m x 3m
Floor: Timber frame, suspended with 150mm PIR between joists and over boarded with 18mm ply.
Roof: Pitched, 125mm joist depth, top boarded with 11mm OSB and finished with bitumen roof shingles. Full-length ridge vent and full-length soffit vents on each side.
Walls: Timber frame (95x45mm). Outer leaf boarded with 11mm OSB, wrapped with Tyvek, battened and timber clad.

Rough plans:

Roof: Put the 100mm PIR between the joists, leaving a 25mm air gap and board the underside with plasterboard.
Is this too small or likely to be ok over such a small area and given the full-length vents?

Walls: 80mm PIR between studs – PIR pushed back against OSB leaving 15mm void for electric cable. Overboard studs with 27mm insulated plasterboard.
Should there be a vapour barrier and if so whereabouts in the layer does it go? Or should I just foil tape all the PIR joints and studs.

Floor: Likely to be laminate with an underlay. Should there be a vapour barrier or would a foil-faced rubber underlay be ok?

Advice on suitability of the PIR I’ve been offered and if/where vapour barriers should go would be appreciated.

You can achieve a decent VB by foil taping the b'jeeezus out of any joints you see. If you fit another layer of insulation boards across the studs - tape that too. Combine that with foil back (duplex) plasterboards.