Advice wanted - Placing and insulating a garden office

[OvineLoving]

Hi all, long time lurker, first time poster.

I'm in the process of building my my wife a studio in the garden (link below) and I have a couple of questions that internet research has been unable to answer so I'd like to seek opinions here if that's ok.

The office we've ordered - Billy Oh Devon, 4.5x2.5 in 35mm wood with double glazing

I intend to erect the studio and then batten and insulate myself with Celotex/Kiungspan foil backed board. The pdf file linked below shows how I'm planning on doing this.

Insulation diagram as a pdf

[1] To what extent should I be concerned about condensation between the exterior wall and the insulation. My current answer is to drill small holes in the floor close to the wall so that they add airflow to the cavity. Same at the top of the walls. Is this likely to be sufficient?

[2] To what extent does my overall plan for battening, insulating, and plasterboarding the inside of the studio seem like a good idea? Can anyone (perhaps who's done this before) recommend any improvements? Should I worry a lot about later expansion and contraction in the wooden structure due to heat/cold/moisture?

I'm pretty concerned about water getting under the studio and rotting the treated floor bearers over time. I am laying the concrete slab today (100mm onto DPM onto sand blinding) and have sized the slab @ 100mm on each side larger than the treated bearers or the floor so that there's not a large amount of space for water to collect.

[3] What else can I do to limit the chance of the floor bearers rotting over time? Place the bearers on lines of DPC rather than direct to the concrete? Bevel/groove the edge of the concrete to give any standing water and exit?

Many thanks in advance.

 

[fencer35]

if I were you i would make my base slightly smaller than the shed so any water can run off. Also bear in mind that these slot together shed move quite a bit (expansion and contraction) so if ur going to batten the inside you will want to make groves for the screws or else when the timber settles you'll have gaps and the boards will separate
Also personally I would use moisture resistant mdf as lining because its more durable and is very easy to finish. Also make sure you get good quality redwood, douglas fir or larch pressure treated bearers and not spruce.

Good luck

 

[OvineLoving]

Many thanks for the advice.

I'm constrained on the size of the base [1] because I've already done it! but [2] because the building is an exact fit to the bearers and, as its my first time, I wasn't that confident in my concrete-laying skills. Once the concrete has cured I'm going to angle grind some drainage gooves around the edge for the run off.

Good point on grooving the screws - I'd not come up with an answer for the expansion/contraction I was happy with.

I'll take a look at the moisture resistant MDF. Someone else has recommended using T&G cladding inside as this would also deal reasonably well with the expansion and contraction.

And lastly the pressure treated bearers come with the cabin but I'd not considered replacing them - clearly I should. I was also concerned about their height (38mm) as ideally I'd have more space for unsulation and airflow. Replacing them would cover this and any potential quality issues.

Cheers