Workshop advice needed.

[Kellie]

Hi- I’m hoping I can ask for some advice from some knowledgeable people here! - especially as google is giving me such alternating advice!!!

I have recently had a 20x10ft summerhouse/workshop built for my sewing business- a way to get everything out the house and away from the kids!!!

I’d like this to be useable all year round and so am looking to insulate with 50mm Celotex/Kingspan/similar and then osb board over the top.

I’ve been reading lots of info on having breather material such as Tyvek fitted between the shed wall and the insulation. The problem is that the joists are already in place as it came in ready made panels so we would have to attach the membrane to the side and around the joists- so the membrane would be in contact with the insulation.

I really want to make sure that we do this properly as I’m planning on using this as my workshop for the foreseeable future and don’t want to have rotting wood or other issues down the line.

I’m planning on having a small fan heater just to warm the space up when needed- and never unattended.

Advise would be really appreciated as to fitting of a breather membrane and whether it is necessary. And if so whether we can fit it around the joists (not sure that’s the correct term for them!)

Many thanks

 

[MikeG.]

Hi Kellie.

Basically, you can't do it properly now, it's too late. Anything you do will be a compromise.

The fundamental you are trying to achieve is a ventilated airgap behind the cladding. This protects the structure of the building from moisture ingress from outside, and, more importantly, from condensation within the structure caused by heating the air internally. As you can no longer achieve this, the best you can do is to have an airgap between the studs (the framework), and to put a good vapour barrier internally to prevent as much moisture as possible getting from the interior of the shed into the walls. A breather membrane on the outside of the insulation would act only to hold the insulation up, so you can dispense with it. Any attempt to put it between, or wrap it over, the studs would be doomed to failure, and would actually be fruitless even if done well.....unless you were planning on a soft insulation like mineral wool/ fibreglass, which it would help hold in place.

So, whatever the depth of your frame is, subtract an inch from that and fit that much Celotex. Thus if the frame is 75mm deep, fit 50mm of Celotex, finishing flush with the inside of the frame. I would then fit another 25mm over the whole lot, frame and Celotex, giving you a total of 75mm. Vary that according to your frame depth. Then, over that, internally, your choice is going to depend on what you want the inside of the walls to look like/ be made of. If you want plastered walls, then you could use foil back plasterboard, or staple up a polythene vapour barrier and use ordinary plasterboard, before plastering and decorating. If you are planning on boarding the inside, then OSB would act as a vapour barrier itself, ply would benefit from having polythene behind it, and softwood t&g boarding would definitely require polythene to be fitted first.

I wish that shed suppliers built their buildings properly. As they don't, customers like you are going to just have to do the best you can. I hope to goodness they've raised the base up off the ground on a plinth. otherwise they've taken decades off its life.

 

[Dibs-h]

So, whatever the depth of your frame is, subtract an inch from that and fit that much Celotex. Thus if the frame is 75mm deep, fit 50mm of Celotex, finishing flush with the inside of the frame.

Mike - wouldn't that 25mm space between the back of the insulation and the shed "outer" covering need (ideally or otherwise) venting?

Cheers

Dibs

 

[MikeG.]

Ideally, but there are likely in practice to be enough gaps between the boards to give a little air movement. To do a good job, insert some big screened vents (circular soffit vents work well) between each pair of studs under the eaves. To do a first rate job, in addition to the above find a way of venting behind the bottom board.

 

[Dibs-h]

Ideally, but there are likely in practice to be enough gaps between the boards to give a little air movement. To do a good job, insert some big screened vents (circular soffit vents work well) between each pair of studs under the eaves. To do a first rate job, in addition to the above find a way of venting behind the bottom board.

Cool - thanks for that. Thought it should (ideally) be vented but wasn't 100% sure.