Shed Insulation - I know, done to death, use search etc ;-)

[chris_hebden]

Hi all,

I appreciate this has been covered before but there are conflicting views on all posts, so wondered what peoples thoughts were on whether I should install a vapor barrier (thanks Fitzroy) on the hot side of insulation?

From external to internal my constructions is:

Treated ship lap 15mm (no air gap), OSB3 18mm - 95mm CLS treated stud - OSB3 18mm (inner finnish)

I was thinking of putting a 50x600x2400 polystyrene insulation board between the studs, do I need a membrane on the inner face of the stud and then board over with the OSB3? I have read on here that the OSB will act as a break from any condensation transferring through?

The shed will be used as a workshop, I haven't finalized my eaves detail yet but I think I will leave some areas of natural ventilation here and the door wont be particularly well sealed around the bottom so should get good air movement. I may have a mobile gas heater with me when I work but doubt it, I am not expecting it to be toasty

Look forward to your thoughts

Chris

 

[Fitzroy]

Certainly not a breather membrane! If you want anything it is a vapour barrier, to which there is an argument that either the insulation (with joints taped) or the OSB will perform this job. Some will swear by the need for the barrier, others will say it is overkill. I doubt you will get consensus or agreement.

Regards

F.

PS. My wall construction is similar to yours and I am not doing a barrier, just going for good detailing on the internal OSB to prevent vapour paths. My logic is the space will spend more time out-of-use than in-use, so if there is a path for vapour and condensation the building has more time to dry-out than get wet. I'm much more concerned with rain penetration being in soggy Scotland.

 

[RobinBHM]

If it was me, I would do 15mm shiplap, 25mm tile batten, breather membrane, osb 18mm, 95mm stud with insulation in between, tape over joints with foil tape, 18mm osb

you dont need osb3 on both sides of the studwork although it is a very storng construction method.

The air gap behind the shiplap is very worthwhile, it will increase the lifespan of the cladding and increase the resistance to moisture penetration

if you use polystrene insulation rather than PIR celetex, then you the insulation wont have foil so you will need to fit a layer of vapour barrier, or rely on the osb3 which is quite vapour proof anyway as long as you are confident of having no gaps.

 

[RossJarvis]

On a recent job I did, the joins of the OSB were taped, don't know how much moisture gets through the joins but I think the OSB is good as a barrier. On this job the OSB was specced with 3mm gaps for expansion.

 

[chris_hebden]

Hi all, thanks for the speedy response and advice

Certainly not a breather membrane!

- appreciated Fitzroy, that's what i meant! That's good to know someone is doing something similar to good effect!

If it was me, I would do 15mm shiplap, 25mm tile batten, breather membrane, osb 18mm, 95mm stud with insulation in between, tape over joints with foil tape, 18mm osb
you dont need osb3 on both sides of the studwork although it is a very storng construction method.

thanks Robin, agreed but I had set my mind on the OSB and shiplap from the start as I wanted a strong, robust shed

On a recent job I did, the joins of the OSB were taped, don't know how much moisture gets through the joins but I think the OSB is good as a barrier. On this job the OSB was specced with 3mm gaps for expansion.

thanks Ross - i've but jointed my boards and jointed on a stud, studs are treated, so vapour transfer should be limited

With 50mm polystyrene i'll have an approx 45mm air gap between the insulation and the inner OSB, should I vent this gap do you think? Not sure how i could now, sides are up and roof rafters on?

Cheers Chris