Advice on moisture for external office

[DustyB]

Hi All,

Enthusiastic amateur here - I've fitted a couple of bathrooms, a kitchen, false walls etc, but I'm no expert. I have a desk job, so I enjoy doing something practical in my spare time. Not sure if you can help with a question about moisture, but thought I'd ask...

I'm planning on building an outside office to the side of my house. It will be used year-round, so will need to be dry and well insulated. I'm planning on having a concrete slab laid, 3m x 2m, then building a timber structure on top.

A UVPC door and window(s) will be salvaged, hopefully, from Gumtree or similar. A simple pitched roof on top, most likely with an EPDM rubber coating over OSB board.

The plot is immediately adjacent to our house (standard brick outer wall), and I've not decided yet whether to have 3 walls as a lean-to and use the house wall as the back wall, or build 4 shed walls and butt it up to the house.

My main concern is moisture/mould, particularly with the house wall. Does anyone have any thoughts on what measures I'd need to factor in with regard to the floor and walls? Also is it better to use the house brick wall or push up the structure with regard to mould/moisture?

Any thoughts would be appreciated - I'll be planning this for a little while before making a start.

Thanks in advance

 

[John D v2.0]

I think thermal bridging would be your biggest problem, where do you plan to put the insulation and how will you address thermal bridging?

 

[ivixor]

To prevent humidity, First of all it needs to be completely watertight of course. Roof yes epdm, but the walls need a breathable house wrap before the cladding.

Don't try and put internal flooring straight onto concrete - that's asking for damp. Creating an insulated concrete base with DPM and getting the detailing at the wall/base junction right is complicated for a DIYer. Instead create a suspended insulated timber floor with ventilation underneath. Ie, like a shed.

Finally, you need to prevent warm air from inside from touching anything cold. To do that, create an airtight seal on the warm side of the insulation, with plastic sheets. These go between your insulation and the plasterboard.

Also, watch this youtube series:

 

[DustyB]

Thanks for replying both.

Insulation - I'd initially thought of using regular glass mineral wool insulation on walls and ceiling. I'd read about the various layers, so it would be as follows, from inside to out:
foil backed plasterboard > insulation > ply/osb > builders paper > air gap/batons > cladding

I'd not come across the term 'thermal bridging' before, and have just had a read about it. I'm not sure if there's a way around that unless I use SIPS is there?

Floor - I'd considered a suspended floor, either on the full concrete base or on small 'stilts' or piles into the ground. The idea of rodents/litter/leaves collecting underneath had put me off having some kind of open ventilation underneath. I'll look into that some more, especially if building straight onto a concrete base may create damp issues. I'll also watch the video, thanks.

Do either of you have any thoughts on 4 walls vs 3 walls against the house wall?

 

[Notch7]

Do either of you have any thoughts on 4 walls vs 3 walls against the house wall?

If joined to the house, its a house extension, so you should check permitted development rules......

I am just building a garden office / cabin. Ive built with 4 x 2 for walls, 50mm insulation inbetween, then 18mm OSB, then 50mm insulation, then 25mm battens for air flow cavity then timber cladding.

Roof is 120mm insulation -which is the same as a flat roof on a house extension.

If you build with the correct detailing of vapour barrier on the inside and moisture barrier on the outside, you wont get damp problems.

If you go for a concrete slab, then you could lay 50mm or 70mm insulation and T&G flooring as a floating floor

 

[ivixor]

I wouldn't use mineral wool in the walls. It'll slump and won't be nearly as effective as PIR.

I insulated my garden room floor walls and ceiling with 100mm PIR and it's very cosy. Most days the heat from my PC alone is enough to keep it warm.