Garage workshop advice

[jack9595]

Evening all, hope your all enjoying the holidays.

Now down to business, I moved into my new house around July this year and now it's all renovated I'd like to finally make the garage into my workshop.

It's roughly 5 x 3m with the garage door at one end and a door leading to the laundry at the other. The wall to the right of the pictures is the external wall of the house. Hopefully the sketchup pictures will explain thing a little easier.

So, it's pretty cold in there at the moment, mostly due to a very poorly fitted up and over GRP garage door. I will replace this and draught-proof / insulate as much as possible.

The other cause for the cold (I think?) is the uninsulated roof. As shown in the pictures I plan to plasterboard and insulated between the joists with 100mm Rockwool.

Then I'd like to batten out the external wall, 25mm ridged foam insulation between the battens (flush to the face of the battens) and clad with 18mm ply. Then surface mount the electrics to this wall. This is to give me a wall I can easily hang tools / shelves. I've seen the youtuber "gosforth handyman" do this, linked below if I can.



That's the plan anyway, does anyone have any advice or see any problems I might have? I am wondering if I'll get condensation behind the foam insulation? Also do I need the put DPC behind the battens?

Any help would be appreciated.

 

[Sideways]

In winter, the bricks will get wet from the rain and if you put insulation up against them, there will be no air circulation internally to help dry them out.
That seems like a recipe for damp to me. Roof even more so. Roofs always leak in the end. How will you know if the leaks are simply soaking into your rockwool for a year or two, potentially encouraging rot in the roof timbers.
I have a very similar space and have left it uninsulated for these reasons. I have an 8x4 board with french cleats behind my workbench spaced off the wall on a 1" thick tanalised batten allowing free flow of air behind.
Basically, I reckon these old simple structures need to be left to breathe and the only real option is to suffer the cost of a decent fan heater providing dry heat when you're working in there. Fixing the garage door does sound like a good move.

 

[Jameshow]

How about using 2 1/2" CLS and 2" insulation thereby having a gap of 1/2" enough for any damp to evaporate. The wall will transmit a little heat which will be enough to evaporate any damp.

Cheers James

 

[clive griffiths]

If your roof is ok I would use an insulation board (celotex or similar) in the roof ,leaving 50mm air space between the ins and roof and get some ventilation in there. Possibly tank the walls before you start .

 

[mikej460]

Does the roof have a modern breathable membrane fitted? If so then make sure you leave a 25mm gap between it and the insulation. If your rafters are only 100mm then you have achieve this by first nailing 50 x 25 battens along each rafter. Fit the rockwool then fit a vapour barriers using staples. If you want belt and braces then you could then screw 50mm PIR insulated plasterboard to avoid thermal bridging through the rafters, otherwise screw in 9 or 12mm plasterboard.

For the walls you need to screw in 50 x25mm vertical battens then a membrane (house wrap) then a timber frame say 100mm x 50mm. Fit insulation in the frame then fit a vapour barrier followed by insulated or non insulated plasterboard or OSB - If you use OSB3 you don't need the vapour barrier.

Hope this helps, others will have their own take on this so do some research on each suggestion to make sure you are comfortable with it first.

 

[MarkAW]

Also helps if none of the soil around the garage is above the DPC like my neighbours have done to mine
My garage is pretty much the same as yours there. I intend doing something similar to you, in the same priority order before next winter: Door, ceiling, walls.

Is your garage wired for electric?. sockets and switches etc? Didn't see them in the drawings. They'll add a bit of fiddlyness

 

[MarkAW]

Looks like you have an apex roof rather than flat?
Gives storage and insulation options...

 

[jcassidy]

I will add, it's not DPM, but vapour barrier, which is much lighter. Always remember, the vapour barrier goes on the WARM side of the insulation layer, the air gap goes on the COLD side of the insulation layer.
I have taken the same approach only using high performance Kingspan insulation boards on the roof space.

 

[Jelly]

In winter, the bricks will get wet from the rain and if you put insulation up against them, there will be no air circulation internally to help dry them out.
That seems like a recipe for damp to me. Roof even more so. Roofs always leak in the end. How will you know if the leaks are simply soaking into your rockwool for a year or two, potentially encouraging rot in the roof timbers.
I have a very similar space and have left it uninsulated for these reasons.

...

Fixing the garage door does sound like a good move.

I am in a similar situation and couldn't accept an uninsulated space, so dealt with this by spacing the insulation off the external structure to give an unobstructed path for ventilation and adding the required ventilation by drilling holes of the appropriate size in the outer wall facing downward at 45° (literally did this today)...

The frame is all built from treated carcassing with the floor plate secured with 10mm concrete screws, and the verticals all secured with 5x30mm steel banding screwed to the wall at half and full height.

So it's not insurmountable, but is a lot of effort and about £90 worth of drill and core bits if you don't have an SDS already.

Agree fixing the garage door should be priority №1.

 

[jack9595]

Thanks for all the input so far everyone. This is the style of garage I have (*Not my house , just a random house from google images).

I'll take a couple real pictures inside tomorrow.

I didn't mention but this garage is a recent addition to the house, built around 2008ish. Hipped / tiled roof as the image above.

The laundry at the back is actually by far the warmest room in the house since the previous owners moved the boiler into there.

MarkAW - Currently I have 2 double sockets and a single fluorescent strip light. After the holiday I'll have a sparky take a look see if I can get any more sockets / lights.

Sideways - You are right, I will need to allow some access up there as the pipes from the boiler run from the laundry into the house in the space above the joists but I'm still keen to insulate if possible.

Jameshow - I think we are on the same lines. If I use 3x2 (63 x 38mm) with the 25mm insulation I will achieve that 13mm air gap. Shown in the image below.

I mean I could forego the insulation behind the ply altogether as I believe that is a solid 2 leaf brick wall (No cavity). I doubt the 25mm insulation would help massively anyway. Thoughts?

The image below shows roughly what is looks like above the joists.

 

[Jelly]

I mean I could forego the insulation behind the ply altogether as I believe that is a solid 2 leaf brick wall (No cavity). I doubt the 25mm insulation would help massively anyway. Thoughts?

Doing some back of a fag packet calculations your brick wall has a U value of 2.92 W/m²K, add 25mm of celotex PIR, a 25mm air gap and 9mm plasterboard and it drops to 0.389W/m²K

Based on an assumption of a 2H×2.5W×6.3L garage with one of the long and short walls warm (17.6m²), which you keep at 15°C, that means that you'd need 6730kWh of energy to keep it warm uninsulated, vs. 895.7kWh with the insulation, which assuming an electricity price of 13p/kWh, works out at a difference of about £754.48/year...

Effectively that 25mm board will make it economically feasible to heat the workspace to a comfortable working temperature.

However, if you're not wedded to PIR board, then for the same price as each PIR board you could get a pack of 100mm rockwool RWA45, covering the same area, or 50mm covering three times the area, which would give respective U values of 0.233w/m²K (saving of £805.20/year) for the same outlay, or 0.349w/m²k (saving of £770.38 per year) for a third of the cost.

Depends what you want to do, but for my needs I found that made a compelling argument that insulation made sense, and that mineral wool was the most cost effective way to do that.

 

[MarkAW]

Yeah I have just added a few more sockets (and filled by them already )
I guess I'll need to remount them when I insulate.
You may also want to consider if your single strip light is enough. Many on this forum have used have LED panels. It's something I'll probably do as the ends of the garage are quite dark. Just saying, if you're getting a sparky in, may be cheaper to do all in one go.

which assuming an electricity price of 13p/kWh, works out at a difference of about £754.48/year...

Interesting, but heating for how many hours a day? I imagine the average usage is 8-16 hours a week

 

[Jelly]

Interesting, but heating for how many hours a day? I imagine the average usage is 8-16 hours a week

The kWh price was just a way to monatise the energy loss (which is conceptually hard to grasp otherwise).

In practice if you just used a fan heater when you were in there, and didn't heat it to maintain a set temperature you would have much lower energy costs, but also not get any of the benefits of maintaining the contents of the garage above the dew-point temperature during the winter months.

 

[Cabinetman]

Exactly what Jelly said, it’s amazing the difference on your attitude to wanting to go into the garage and do things if everything you pick up its freezing cold! If it’s well insulated it needn’t cost much to take the chill off in the winter months and you avoid the dreaded rust.
Whilst you have the electrician there, it’s well worth getting him to put a 16amp supply in – that’s the round blue sockets, you might not need one now but it won’t cost very much at all to put in and guaranteed you will want one in the future. Ian

 

[Jelly]

Whilst you have the electrician there, it’s well worth getting him to put a 16amp supply in – that’s the round blue sockets, you might not need one now but it won’t cost very much at all to put in and guaranteed you will want one in the future. Ian

If it was me I'd seriously consider asking him to install that 16a plug on a 32a radial circuit too, for the small amount that using larger CSA wiring would cost.

That effectively future proofs for any sane woodworking machine you might acquire (including 3ph machines on convertors) because you can switch out the 16A socket for a 32A socket (or add a second 16A) yourself, but you can't/shouldn't go fiddling with the wiring at the consumer unit.