Log Cabin Insulation Advice

[djcla]

Hi im new on here so hope im in the right section

I have read a few threads but not quite sure they answer my questions 100% so here goes.

I am looking to but a log cabin 3m by 3m the cabin has 28mm log walls on the outside and then a 70mm gap and a second floating 28mm wall no studs. The company can supply wall insulation whih is foil backed Polyurethane 70mm thick to fit in the cavity ( no vapout control layer or breather membrane ). the cabin will be used all year round and probably have a small electric heater in winter but wont be a gym or anything like that so minimal moving around etc.

I was looking to use a better sound insulator as im not sure if Polyurethane is that good as a sound insulator . I was considering buying 70mm rockwool flexi slab to fill the cavity. So the questions i have are:

Do i need to put a Vapour control layer on the inside of the insulation and a breather membrane on the out or can i get away without it?

If i do need it will i have to have an air gap between the membrane and outside wall as this will be a pain to achive?

If i do need an air gap i was thinking of using 10mm battons and staple the membrane to that will this be enough of an air gap?

Im also not sure how to go about fixing a vapur control layer up can i just stick it to the insulation ?

The membrane and Vapour control layers i have seen seem to be in 1m widths which will obviously need lapping and taping.


Any help or advice would be approciated thanks

 

[Mrdjc]

My guess is that the outside won't be 100% wind/air tight and you might be affected by driving rain, but this depends entirely on the construction I would have thought.

PIR/PUR isn't breathable in its self, so if you fitted this material it would be pointless to double seal it as nothing will pass through it, all that you'd end up doing is creating a problem by "trapping" air in sections with no way out.

The breather membrane on the outside is so the insulation can naturally ventilate and react along with the humidity outside, but without the risk of driving rain penetrating the insulation, and any moisture that does end up inside can ventilate out naturally. (think of damp wood or something retaining moisture and slowly releasing it)

Vapour barrier really depends on what exactly you want or mean.

There's two or three different ideas.

One is foil, other is plastic. Both completely inhibit any movement of air.

Then there's Membrane, which just like Goretex stops water droplets and condensate moving through it, but the air passes through naturally and allows the insulation to breathe properly.
It just stops the warm damp loaded air from moving into the insulation, meeting the colder air on the cold side of the insulation and condensing.
It still allows the air to move freely across the insulation, just without high moisture levels.

My guess would be yes, you require some sort of gap for any moisture build-up to run down and out off material. If you stick it hard up against the outer wall you stand the risk of creating damp patches as there is no easy way for it to dissipate.

Grab some thin batons and use them as runners to keep the membrane off the wall.

I don't know how your floating wall works, but normally you tack the vapour check barrier onto the studs and then fix the plasterboard up against it. (at least, that's how I've seen it done on wooden buildings before)

 

[djcla]

thanks for the reply so are you saying that with the foil backed PIR boards i would not need a breather membrane between the extenall wall and insulation but it would require an air gap? If i go for the breathable insulation e.g. Rockwool then i need a membrane and air gap? with regards to the vapour control layer ( inside on insulation) i had in mind some foil back multi foil vapour barrer. The log cabins do bot have stud work and i think the inside wall is completley seperate to the outside wall. I believe the inside wall just locks together in the corners which does not leave me anything to attach the VPL too. I guess also im wondering if i can get away without the vapour control layer altogether?

Also any tips on a suitable breather membrane at low cost?

 

[djcla]

right ive decided to go with 10mm battens going left to right tacked to the outside wall evey 1/2 meter . I will then staple some light weight breather membrane to the batttens to leave a small airgap and keep the membrane off the wall. Then 60mm Rockwool flexi slabs and i will stick some foil vapour control to the insulation followed by the internal wall. Does this sound a good approch?

 

[Mrdjc]

Sounds fine, just remember to ventilate the inside of the cabin properly.

If you create any moisture it won't have anywhere to go, perhaps wise to add some closable trickle vents of sorts, or just opening a window from time to time, especially if its been very damp.

Just make sure that any damp that COULD end up between the outer wall and the membrane has a place to go.