Garden Room stud wall construction/ insulation and membranes

[I.likes.Armadillo's]

Hi all,

I'm planning to build a large garden Room and want to do it to a decent spec. Planning to use double glazed pvc windows/ doors and EPDM roofing. For the wall studs I'm probably going to use 4x2 C16. The outside will be glad in 38 mm thick double tongue and groove loglap. When it comes to insulation I am looking to use 50 mm rigid between studs and then the dilemma comes.

I really like the idea of fully wrapping the outside of the building in 25mm kingspan and then cladding over this. I would probably clad onto battens which are inline with the studs, so that the insulation is effectively sandwiched between a timber batten and the stud. I would use some sort of spacer the same thickness as the insulation to keep the battens straight.

I am assuming that when this outer wrap of foil board is in place it will also be a vapor barrier and leaves any breather membrane I choose to include a bit pointless?

This leads me to wonder if I am better off insulating the inside with the kingspan, over the studs with taped seams and then go for a breather membrane such as tyvek on the outside?

I may have kinda answered my own question here but hopefully it gives an indication of my current thoughts. I am also wondering if I would need an external cladding of osb on the outside of the studs.

So in summary:

1) Kingspan on outside or inside?
2) Do I need OSB?
3) Does anybody have any recommendations for the wall construction layers?
4) Does tyvek need and air gap on a particular side?

Any help much appreciated.

 

[I.likes.Armadillo's]

Having thought about this more my current train of thought is, from the outside in.....

38mm loglap double t +g cladding
Tyvek house wrap ( with battened air gap?)
11mm OSB
4" stud with 50mm insulation (polystyrene) between
25mm Kingspan all seams taped
Battened Air gap
Internal timber cladding of some kind
Any observations?

 

[tomfe]

I'd go for 6inch studwork with full fill rockwool.
But really is this all overkill?

 

[Notch7]

Im doing a garden cabin at moment.

I am doing 4x2 studs with 50mm celetex in between. Then 18mm osb covered with 50mm celetex, battens and featheredge.

The continuous layer of celetex on outside means no thermal bridging. My construction may have been better having osb on inside so it isnt fitted in between insulation.

I havent bothered with breather membrane as the insulation is foil faced.

Ive done a garden workshop with tyvek housewrap -its definitely worth the extra cost, dont use a cheap breather instead.

For the in between insulation, I cut it about 1mm small but with a tapered cut. I then foil taped it flush to the outside.

On the inside I went around all 4 edges of each piece with foam -use gun grade not hand held. Set the gun to a tiny flow, then go around so the foan pumps right into the gap. When set the insulation makes the studwork really solid -so it will help with lateral stability.

Cutting the insulation tight makes it easier to push in place before foaming. If you do it small it will work fine but itll need holding in place with trmporary screws or similar.

 

[John D v2.0]

I think a lot depends on how much you will use it. If you'll use it for odd days in spring summer and autumn as a work shop the answer will be very different to if you are working from home 5 days a week and having sleep overs on the end, using electric heating. In the former case you won't need much insulation, just cheap rock wool and decent draught proofing. In the latter case you'll want to insulate to at least building regs standards otherwise your electric bill will be expensive.

 

[^woody^]

Insulation wants to be on the inner face or in the frame, not external.

A timber frame needs timber sheathing board on one face, normally external, to make the frame rigid and stop it twisting.

25mm foil insulation is the just about worth it. 50mm polystyrene is less efficient than 25mm foil faced insulation. Insulation does not magically make a cold room warm.

 

[I.likes.Armadillo's]

Insulation wants to be on the inner face or in the frame, not external.

A timber frame needs timber sheathing board on one face, normally external, to make the frame rigid and stop it twisting.

25mm foil insulation is the just about worth it. 50mm polystyrene is less efficient than 25mm foil faced insulation. Insulation does not magically make a cold room warm.

That all makes sense thanks, I was thinking of putting the foilboard externally to give a "warm wall" effect in a similar way to a warm roof but I suppose keeping it breathable is the idea. So if I go studs > OSB > tyvek homewrap > air gap? > External cladding. Is the air gap required to enable the tyvek to work fully?

Also the base is going to be dpm > concrete > foilboard> floating t+g p5 chipboard > flooring. The concrete will sit in a blockwork perimeter which the wall frames will sit on. The dpm would go on the inside of the block work. I'm currently mulling over how to terminate the dpm and whether it needs to go under the frame to prevent damp rising through the blocks and whether it is beneficial to have the dpm lapping up the outside of the framing slightly (around an inch) obviously it would be behind the cladding.

Any thoughts anybody?

 

[I.likes.Armadillo's]

Im doing a garden cabin at moment.

I am doing 4x2 studs with 50mm celetex in between. Then 18mm osb covered with 50mm celetex, battens and featheredge.

The continuous layer of celetex on outside means no thermal bridging. My construction may have been better having osb on inside so it isnt fitted in between insulation.

I havent bothered with breather membrane as the insulation is foil faced.

Ive done a garden workshop with tyvek housewrap -its definitely worth the extra cost, dont use a cheap breather instead.

For the in between insulation, I cut it about 1mm small but with a tapered cut. I then foil taped it flush to the outside.

On the inside I went around all 4 edges of each piece with foam -use gun grade not hand held. Set the gun to a tiny flow, then go around so the foan pumps right into the gap. When set the insulation makes the studwork really solid -so it will help with lateral stability.

Cutting the insulation tight makes it easier to push in place before foaming. If you do it small it will work fine but itll need holding in place with trmporary screws or similar.

I'm thinking more that if I put the foilboard on the outside the walls will not be breathable so any trapped water may cause issues. I like the idea of the foam though.

 

[tomfe]

You need a ventilated cavity to allow your cladding to dry, you can counter baton if vertical cladding or just use batons if vertical.

You'll need a dpc under your timber frame either in your block work by lapping up the dpm or a separate dpc under your frame or in a course.