House Wrap - quick question

sbsquarepants

I'm currently building a den for the kids to play in (i.e. a timber shed!). It's basically a stud wall OSB box, wrapped in tyvek and overclad with ship lap timber. I'm going to insulate it with earthwool and then plasterboard for the internal walls.

My question is - I will have a lot of tyvek left over, so i was thinking of adding a second layer of it to the inside of the stud frame, between the insulation and the plasterboard. I'm aware i'm not going to gain a whole lot from this except maybe cut down on draughts, i'm only thinking of doing it because i'll have it left over, but i'm unsure whether doing so might cause any problems. Am i risking trapping condensation in the insulation or could it cause the plasterboard to get damp? Any ideas?

GreeBo

How is it going to stop draughts on the inside if it cant stop them on the outside? (Serious question btw, not being a dick!)

sbsquarepants

It's not really, I'm just going with the tesco principle, every little helps!

Calahonda52

is the osb and plaster board taped at the joints?
what grade is the OSB?

sbsquarepants

I was thinking OSB 3, should be fine behind the tyvek shouldn't it?
Plasterboard would be taped and skimmed.

Lumen

The membrane should go inside the insulation, to stop warm moist air hitting whatever is on the outside of the insulation (OSB?).

It would be useful if you could list the wall buildup, it's a bit confusing.

e.g. from inside to outside:

Plasterboard
Membrane
Studs/insulation
OSB
Membrane
Shiplap

Is that it?

sbsquarepants

Yea that's it Lumen.

I'm looking at the tyvek to protect the OSB from any rain that gets beyond the ship lap (which is more decorative than anything else). I didn't really think i'd need to be blocking any moisture from inside?

Lumen

Well I don't really understand the building physics of unheated sheds, but assuming that the inside of the shed is going to be warmer and moister than the outside (with kids breathing and playing in it), then if that moisture can find its way past the plasterboard and insulation it'll hit the cold OSB and condense there.

Since you have spare membrane, just put a layer against the inner face of the insulated stud wall and you're grand.

sbsquarepants

Cheers Lumen, Thanks!

Calahonda52

I have always viewed tyveck as a breathable, from the inside, wind rain barrier/ external house wrap.
When used against OSBI don't see much of its breathability functionality being used.
.
Then, proposing to use it inside, which surface with be out against the pb: eg will it be inside out?

Lumen

Calahonda52 wrote: »

I have always viewed tyveck as a breathable, from the inside, wind rain barrier/ external house wrap.
When used against OSBI don't see much of its breathability functionality being used.
.
Then, proposing to use it inside, which surface with be out against the pb: eg will it be inside out?

Ah good point, windproofing membranes are not normally air tight; the Dupont product for internal use is called AirGuard.

sbsquarepants

Calahonda52 wrote: »

I have always viewed tyveck as a breathable, from the inside, wind rain barrier/ external house wrap.
When used against OSBI don't see much of its breathability functionality being used.
.
Then, proposing to use it inside, which surface with be out against the pb: eg will it be inside out?

I always assumed it was arbitrary which side faces in or out?
Surely it's impossible for it to allow water vapour travel in one direction only? No?

Calahonda52

sbsquarepants wrote: »

I always assumed it was arbitrary which side faces in or out?
Surely it's impossible for it to allow water vapour travel in one direction only? No?

Research Intello Plus for intelligent membranes: IIRC the direction changes depending on humidity.
I don't know about tyvek but at the price its not just newsprint:D

Lumen

sbsquarepants wrote: »

I always assumed it was arbitrary which side faces in or out?
Surely it's impossible for it to allow water vapour travel in one direction only? No?

The intello stuff is smooth/shiny on the inside and furry on the outside. It's probably shiny so that you can tape it and furry so that any wetness stays spread out where it can evaporate through the membrane rather than dribbling down to collect somewhere.

Wartburg

Vapour control membrane to the inside please. Otherwise all the vapour (even breathing creates a good amount in cold days) will go through your insulation and condensate on the inside of your OSB. You won´t see this issue for a good while until you notice some discolouration and stains on your plasterboard. Plasterboard is might not the best idea for an unheated den as well, because she will get damp in the winter. If you´re in the position for doing an alteration on your structure, I´d suggest to put the OSB to the inside. She´s more tight than the Tyvek and will act as a vapour control layer. Give the OSB a coat of paint and you have created a more resistant internal finish in comparison to plasterboard.

sbsquarepants

All that's done so far is the base frame, so i could still make changes.
Do you mean OSB inside and out?

And when you say vapour barrier inside, do you mean in addition to outside? Surely wind driven rain is a much bigger concern than water vapour in breath?

Lumen

The OSB can perform several functions:

- Racking strength (stopping the rectangles of the studs from being pushed out to parallelograms).
- Air tightness (it's not very air tight, but it's OK).
- Holding insulation in place.

Since you were planning on having sheet material inside and out (OSB outside, plasterboard inside) you may as well just put OSB on both sides of the studs, it's plenty attractive enough to be a finished surface for this application. Were you planning on plastering the inside? The only downside with the OSB is that if you want to make it airtight-ish you need to tape the joints, and that'll detract from the appearance, but you could probably use beige-coloured Tek 7 instead.

Others have posted before about the practice of putting OSB on the cold side not being great for Ireland's high humidity, but then you have the Tyvek and that's what it's supposed to be used for.

You don't strictly need sheeting on both sides of the studs for structural performance, if you wanted to you could probably Tyvek over the studs and then use the shiplap, reserving the OSB for the inside only. Dunno about that, but it would save a bit of money that might be better spent on denser insulation (e.g. Metac).

Calahonda52

sbsquarepants wrote: »

All that's done so far is the base frame, so i could still make changes.
Do you mean OSB inside and out?

And when you say vapour barrier inside, do you mean in addition to outside? Surely wind driven rain is a much bigger concern than water vapour in breath?

You seem to like the word surely when asking for advice:D

My last input here.
PB
VB
OSB
Framing with insulation between members
Tyvek
2 by 2 pressure treated battens
wood lap, acting as a rain shield, with insect proof vents top and bottom.
.
but, lads, its only a shed surely!

sbsquarepants

Lumen wrote: »

You don't strictly need sheeting on both sides of the studs for structural performance, if you wanted to you could probably Tyvek over the studs and then use the shiplap, reserving the OSB for the inside only. Dunno about that, but it would save a bit of money that might be better spent on denser insulation (e.g. Metac).

I think i'd be nervous of just the tyvek over the studs, plus the reason i'm using the earthwool is because i happened to have it already.

Calahonda52 wrote: »

You seem to like the word surely when asking for advice:D

Ha ha, i'll have to watch out for that! :o

Thanks for the advice folks!