Insulation and vapour barriers

[Tricky Dicky]

I am about to start dividing off my extended garage to create a man cave. This involves a studded dividing wall which along with the other three outer walls need insulating. Incidentally the garage is a timber framed structure with CLS roughly 4x2 frames with 18mm shiplap over a breathable membrane and lined behind with plywood. The plan is to fill up the spaces in the framework with either Celotex or Kingspan foam panels.

So the question is, can the foil coating do as a vapour barrier using sealing tape across the framework as I did when insulating the kitchen floor or do I need to add a polythene sheet over before boarding the inner wall?

I have been watching some YouTube of people doing similar things and have been left confused. The following video is a compilation of a number of videos of a professional build of a garden room.

The guy insulates the bays between frames with foil coated foam panels before adding a rock wool bat sound deadening layer and then a vapour barrier before plaster boarding but then cuts holes in the PB for sockets destroying the vapour barrier behind?

Got to admit some of the work looks rough and some to my mind over engineered. See what you think the relative insulation is towards the end of the video or you can look at the daily video record of 15 daily videos.

Richard

[John Rumm]

The foil will act as a vapour barrier if any joints are taped up with foil tape.

ok not had a chance to look at the videos.

However, the general rule for vapour barriers is that they need to be in a position where they can prevent warm moist air reaching a cold surfaces - especially if those are in voided off parts of a building (aka interstitial spaces).

So in the case of the rigid Kingspan et al boards (aka Polyisocyanurate foam (PIR)) with a foil cover, there is a barrier on the warm side of the insulation. With something like rockwool there is no "built in" barrier, and the insulation itself is also air permeable - so there is some logic in preventing the warm wet air reaching the warm side of the insulation since that (and its moisture) can permeate the insulation and migrate toward the cold surface.

Where you have rockwool over PIR boards, its probably less critical since the warm side of the PIR boards will likely be above the dew point anyway.

[The Natural Philosopher]

yes it can

First of all making a you tube video is no guarantee of competence. Secondly vapour barriers are not about perfection as much as a 90% reduction in humidity etc

I have foil backed plasterboard and rockwool insulation. Vapour barrier is not perfect, but its *good enough*

[Tim Lamb]

How much moisture are you planning to put in a *man cave* anyway? I suppose it might convert to a micro-brewery:-)

Small point on foil taping (in my case Celotex) there is a release agent on the foil surface and I found tape peeling off after appearing to stick securely. Meths. worked well.

[John Rumm]

So long as you refrain from exhaling you should be fine :-)

Interesting - not seen that before, but then again I have not often used genuine Celotex. (and for that matter have usually ended up covering up the taped areas fairly shortly after anyway)

[Tricky Dicky]

Thanks for all the advice, put my mind at rest. Now just a few jobs to complete and it is on with the mancave.

Richard

[Tim Lamb]

In message [e-mail], Tricky Dicky [e-mail] writes

Breaking news!

Post lunch international call enquiring after our loft insulation. Nicely spoken English voice.

After a few opening gambits, it became clear she was hoping to arrange some sort of follow up to the *green deal*? loft insulation we had fitted to the farmhouse some years back.

She confided that both Rockwool and Glass fibre absorb moisture leading to degradation and less effective insulation due to compaction!

At this stage I decided to be merciful and said we were no longer at the farmhouse but had retained our landline number. Undeterred she moved on to the loft insulation at the cottage. However, became totally flummoxed at the idea a modern construction would have a vapour barrier....