Home office wall construction

Dilly of a pickle

Will be starting walls on a home office at the end of garden in the coming days. Opinions on the following wall construction from outside in

Acrylic render
Base coat and mesh
100mm external insulation mechanically fixed
Breathable felt
18mm osb
4x2
Plaster board and skim.

A builder friend is in the middle of his house with this construction and just wondering the pros and cons before I order materials.

Thanks.

Lumen

18mm OSB seems like overkill, 11mm or 12mm should be fine, but tape the joints.

Not really sure what the "breathable felt" is supposed to be doing.

kieran.

I would drop the OSB and replace with a fibre cement board or similar 10/12mm probably thick enough. I would also add an airtight layer to the inner face of the studs.

Wartburg

You don´t need the breathable felt on the outside, but you can stick with the OSB, by using timber screws and separate plastic discs for fixing the EWI to the OSB. Put mastic or bonding foam between the boards and the OSB instead of mortar. On the inside you´ll be definitely in need of a vapour control layer underneath the plasterboard.

Dilly of a pickle

Thanks for the replies.

Felt is as much to keep the timber dry during construction between standing frames and putting on ewi. It's the stuff sold as ''breathable' but is the cheap version so don't know how great it is at that aspect.

Was thinking 18mm osb would be better structurally, also can get 18mm at almost same price as 12 so that doesn't worry me. Basic silicone for joint sealing or something else?
Interested to hear why cement board instead of osb?

Vapour control layer in what form?

Thanks again

BryanF

Silicone between studs & osb. Bonding foam between insulation and osb.

Ewi system supplier should be consulted if you want any hope of a system warranty or joint warranty with plasterer.

kieran.

I have use thed Knauf Marmorit System previously. It has the benefit of BRE testing and certification and there is a decent warranty with the product.

This system requires a Cement board; the sales rep at the time said it was because it wasn't susceptible to the same dimensional movements as OSB, it also has must better fire properties that OSB and I believe it is frost & rot proof.

https://www.redbooklive.com/download/pdf/Cert-159_13-Knauf-Marmorit.pdf

Wartburg

Are we still talking about a home office/ granny flat? The Knauf System is definitely very good and advisable but I think that´s a bit of an overkill for the original application. This system is made and certified for multi-storey appartment blocks.
On the other side, I´m always smiling about the objections regarding system warranties. The moment were you´ve read all the terms for such "promises" is the moment, were you notice that it´s more than unlikely, that manufacturers of building products are taking any responsibility for potential issues. Your best warranty is common sense and some knowledge about building physics.

BryanF

Wartburg wrote: »

Are we still talking about a home office/ granny flat? The Knauf System is definitely very good and advisable but I think that´s a bit of an overkill for the original application. This system is made and certified for multi-storey appartment blocks.
On the other side, I´m always smiling about the objections regarding system warranties. The moment were you´ve read all the terms for such "promises" is the moment, were you notice that it´s more than unlikely, that manufacturers of building products are taking any responsibility for potential issues. Your best warranty is common sense and some knowledge about building physics.

Where did the granny flat come from? We’re discussing insulating an office at the end of the OPS garden

If it’s a granny flat change the word warranty to structural engineers wind-load calculation