I built a garage and I don’t want it to rot away

In the middle of building a house I realised I was using the house to store all my tools and work materials and other junk so I’ve built a garage to move it all into. I’ve built using the same foundation technique as I did for the house: type 1, sand, DPC, reinforcement mesh, all that good stuff – but to save time and money the garage itself is timber frame with Stirling board exterior walls that I’ve wrapped in the same membrane as I did the house. It’s about to have timber cladding on top of that to finish off the exterior.

I’m not sure how to do the inside walls though. The house has cavity wall blockwork on the outside of the timber frame, but the garage is just: exterior > cladding > breather membrane > Stirling board > interior.

I want to insulate and board over on the inside with plasterboard/Stirling board/etc. but not sure how to do this without creating a condensation trap. Am I good to just stick some rockwool between the studs and board over or is this A Bad Idea™? What’s the preferred way of doing this if so?

As its a garage I’d use standard mineral wool full thickness in the kit and then line it with a vapour barrier and 12mm plywood.

Plasterboard limits you for fixing things to the walls and is more susceptible to moisture over time.

I built a garage and I don’t want it to rot away

Build a car port over it?

Is it going to be heated?

Are you using a “rain screen” and creating a vented air gap between the building wrap and the wood cladding?

have you got a gap to stop driving rain getting to the inside layers? if not I suspect you’ll have some degree of damp

i did from outside in on both the walls and the roof – Roofing tin  – Batons – Building paper – 12mm ply – 4×2 (6 x 2 on the roof)  timber – rockwool in the gaps- Fibreglass mesh – 12mm ply.  all sat on a ring of breezeblocks on a raft foundation.

Have not had any damp inside in the past 4 years – including when the roofing tin was removed from part of the roof during Arwen.

Cladding is going to be pinned to 1″ straps so yes there’ll be an air gap between it and the vapour membrane.

As its a garage I’d use standard mineral wool full thickness in the kit and then line it with a vapour barrier and 12mm plywood.

So as long as I’ve got the air gap between cladding and outer wall I’m good to just insulate between the studs, vapour barrier and board over? No air gaps needed inside?

At the moment there’s no plan to heat it but once the house is done and the volume of junk is much-reduced it’s not impossible that I might add a little workshop heater of some kind.

The membrane on the outside should be vapor open, but weather resistant.

On the interior, id avoid poly vapor “barrier” where possible and there are varying degrees of smart vapor “retarders” out there that allow drying to the inside as well as permeance to the outside to allow for exterior drying through the wrb and rain screen behind the siding.

being unheated changes the “need” for a vapor retarder, but an air barrier is still important,  but if you do plan to heat at some point I’d be looking at something more vapor open in high humidity conditions, closed in low humidity.

There are expensive products that do this, intello, siga etc, but the cheapest way is an asphaltic coated kraft paper, these products exist in the wood flooring underlayment space.

How breathable is sterling board.

1-12 perms at 7/16″ assuming it’s just OSB.

A 6mil poly vapor “barrier” is <0.1perms

Breathable and vapor permeable are two different things, OSB with taped seams is air tight, so not breathable.

Breathable and vapor permeable are two different things, OSB with taped seams is air tight, so not breathable.

Oh I know. But in an OSB sandwich it seems pointless adding it on the inside .

Here is how you do it, these are the layers.

From inside to outside:

1) Interior finish (whatever you think looks nice/is practical)

2) Vapour barrier (stops warm moist air such as your sweat or shower steam going inside the wall build up)

3) Studwork (50*150mm?, depends how big your garage is) with insulation full in between, same depth as studwork

4) Breather membrane (this stops water getting in from outside but allows any moisture that gets into insulation to get out)

5) Vertical battens that are fixed to studs over the breather membrane (usually about 25mm deep). This is what gives you an air gap between main studwork/insulation and cladding, it is important that there’s an airgap, air flow here will mean any moisture getting into the wall can dry out = it needs vent gaps at top and base

6) Horizontal cladding (whatever you think looks nice)

(if you want vetical cladding then you need an extra layer of horizontal battens in between layet 5 and 6)

To try make it simple and clear with the moisture and proper wall build up: Living in a warm heated space will create warm moist air from stuff like washing the dishes, having a shower, breathing, that air will make its way towards the cold outside and when it gets cold enough it will condensate and become your unwanted moisture. The easiest way to stop this is to put in a vapour barrier on the inside of the insulation line then it is important to ventilate the room so that the warm moist air can escape.

So if you have future plans to heat the space then I would just put in the vapour barrier now. it is important to tape all the joins and seams and any holes you make in the vapour barrier.

remember membrane is not vermin proof – so do something to address that if you do the above.

I’d still be wanting a layer of ply or OSB between 3 and 4 to add some stability in shear and stop the garage being a wobbly mess.

I’ve got OSB between 3 & 4 but the rest of that looks spot on. Thanks AnyExcuse ??

I’ve got OSB between 3 & 4 but the rest of that looks spot on

Will that allow the insulation to breathe (layer 4 ^^)?

Ive recently built a workshop of  similar spec to those outlined above, with profiled steel cladding externally, battened in front of breathable membrane wrapped over 9mm OSB, rockwool batts between studs (for sound proofing and fire resistance), and with a 6mm cement board lining behind the 12mm interior ply, to ensure the walls have increased fire resistance, as they are within a metre of both my house and my neighbour’s fence, as per most council requirements for garden buildings. I also used a 1mm stainless mesh top and bottom of the cladding to keep out unwanted visitors. Epdm covered, insulated warm roof keeps the top waterproof, and  so far all looks like lasting a good while…

Yes, no harm in having your OSB layer between 3 and 4, the build up will still breather OK.

As others have mentioned you can/should put an insect barrier at the top and bottom of the cavity opening (layer 5) to stop mice getting in and nibbling your insulation or bees building nests in there.