Basic modification requirements to garage to all for all-year-round workshop

[diynovice99]

Hi all,
We are looking at doing some basic modifications to a single skinned, brick built detached garage to turn it into a workshop that I could use most if not all of the year and would appreciate your feedback on the most economical way (ideally in the low triple digits), in terms of materials (happy to do the work myself, so there is no labour cost) - to achieve this. Some details:

-It is 6.5m x 2.5m in size.
-There is a up-and-over aluminium door at the front.
-The floor is concrete.
-The roof has 2x12(ish) joists that were using to raise the ceiling height to about 2.3m by the previous owner. The top surface is some sort of gravel coated felt/rubber.
-There is a 2mx1.2m west facing window.
-The 2 side walls open out into enclosed side passages of ours and our neighbours house, so don't suffer the brunt of the ambient outside temperature or the winds.

Initial thoughts were to put a stud wall (3x2 timers with 12mm plasterboard on both sides) up about 2m behind the door to insulate the east-facing door end of the garage, yet to retain an operational door - for in-case we revert the garage back - and a bit of space for garden tools. Not sure whether to put insulation of some sort in or not, or whether given the nature of the rest of the garage, it would make little difference?

I was then thinking of sealing up any major drafts with gap fillter and/or silicon, followed by cutting and wedging 50mm polystyrene board insulation (or some other economical product?) into the roofspace. I am aware of the need for ventilation above this, so would be spacing it at least 50mm away from the root boards and putting vents at each end of the 8 joist spaces.

Finally, I was going to cover the floor with some of the 10mm rubber tiles and to put a couple of 2km electric convector heaters in there to inject some heat.

I would very much value your input on what would be the most feasible economical options - if there are any!. Please don't hesitate to rubbish my ideas, as I am sure they stem from a very naive understanding of this area.

IN the future, we may consider spending more to do things more effectively, but we'd like to give things a test run, as it were, before ploughing thousands into it.

Thanks in advance!

 

[motorbiking]

I wouldn't bother with a stud wall, you can get insulation backed plasterboard which can be mechanically fixed to the wall or dot n dabbed. For the roof celotex or rock wool and then plasterboard. The garage door can be insulated with celotex attached via expanding foam and then brushes to cover the gaps. Be realistic about the U-value you hope to achieve. It will cost very little to heat a 'room' of this size even if you only achieve 0.5 or even .75 U-value. Anything under 1 is realistic and it would probably take 20 years to cover the extra cost of fuel vs the higher insulation.

 

[diynovice99]

Hi motorbiking,
Many thanks for the reply! Very useful!

Re the garage door, I guess it's around 4sqm in area. My understanding of the cost of celotex - or a most cost-effective equivalent? - would make this quite a bit more expensive that a basic stud wall. Do I understand correctly, and If so, would the u-value of the celotex be significantly better than the stud wall?

Equally, there is about 15sqm of roof space, which I would imagine would really stretch my budget if I went the celotex/equivalent route? I like the idea of some sort of 'board insulation' as it means I can just cut it and push it in place (easier than the juggling of plasterboard) and I get the feeling of more space as a result of the still-open joist space, but am concerned about the materials cost.

It makes sense that since this is a small space, that if I spent too much I would never recoup the investment cost in saved energy bills. However, I'd rather not have heaters going all the time and creating a very stuffy dry environment. DO you think it would be possible to obtain a 'passable' u-value - whatever that is - that would mean the structure would retain a steady, say, 15 degrees for an hour or so?

kind regards

 

[Tigercubrider]

If buying celotex type insulation, look on eBay
Companies sell part load returns, damaged sheets etc a lot cheaper

 

[LondonLad21]

Plaster backed board is expensive too, the walls with suffer water ingress through wind riven rain, the adhesive will fail.

You need ventilation in the void that you create.

Get some roof battens, apply to wall, to be super careful, put some dpm beind the batts.
Line the concrete floor with dpm, pull up behind batts
Apply cellotex to floor, and walls, going through batts, then a simple floating floor
Then some sterling board to walls through celotex and batts so you get some good fixings for tools

Upshot, it may not be pretty, but your tools won't turn orange, you drop a tool and it will be ok, and you will be warm. I did this to an old workshop and it was transformed, it warmed up very quickly, and a simple greenhouse heater kept edge off when really cold. The only issue is in the summer, it was warm....though that's hard to deal with unless you get a lot of insulation, and a thermal mass in a cold concrete floor out of the sun.

The key thing is that the walls have an air gap, and there is some way for the floor to weep away any moisture.

 

[motorbiking]

There is no need for an air gap if the building is water tight. If you want to do it on the cheap then you can use mineral wool in the stud, but its significantly less efficient than Celotex and the like. It does however have the benefit of being fire retardant.

For more info on bonding celotex see the video.

 

[LondonLad21]

A single skin garage isn't watertight, especially with a flat roof, as there's no overhang from the roof, so in wet weather the single skin gets wet, draws in moisture, hence so many people having issues with their garages.

That means that eventually the frost and damp will pop the bonds on the inside, and, being a workshop, fixing to the wall will mean going straight through to single skin outer wall.

This is a useful link, a little old, but relevant.

https://www.diynot.com/diy/threads/garage-conversion-inside-wall-construction.233260/

 

[diynovice99]

Hi all,
Many thanks for all the input and the links. All very insightful!

I'm leaning towards the battened approach mentioned by LondonLad21. If I went this approach, but did it in phases based on prioritising heat retention and reducing moisture, am I right in thinking my schedule would be:
1. seal drafts (inc door)
2a. insulate door
2b. insulate roof
3. batten walls, apply celotex over them
4. apply dpm to floor
5. insulate floor
6. apply osb over celotex

Also, I was thinking 50mm of celotex in the roof and walls. Does this sound reasonable? I gather it will give me a u-value of somewhere around 0.44 in the ceiling and walls (although slightly better in the walls, and I guess slightly worse in the roof).

And, if I insulate the floor - which I gather currently has a u-value of around 0.9 - I guess I can get away with 25mm celotex/polystyrene?

Oh, and one last thing: the walls have what I believe would be called piers ever couple of metres or so. I assume these are just treated as 3 mini walls?when it comes to putting up the board and battens?

Thanks again!

kind regards

 

[LondonLad21]

Don't obsess with U values, they are a statistic based on a lab, with the sum total of efforts getting to what you want, e.g., seals, door/windows/floor etc etc, but your priority is a working space, hence I would

1. Sort floor first, pull everything out whilst we have good weather, 50m Celo' is fine, or even polystyrene, it will make a huge difference
2. Sort you roof too, then you can start on walls.
3. Then once stuff in there, look at doors, seals windows.

Ref piers, yes, treat as mini walls, batten edges and cut through only one side of celo whilst preserving foil on other side, and fold over. That way the foil on one side (inside) acts as hinge and continuous seal.

Spray foam on any gaps after.

 

[diynovice99]

Once again, many thanks LondonLad21! Your help is invaluable and hugely appreciated!

Understood about the artificial nature of u-values - I guess they're more of a rough guide.

Could I please just ask whether you think my insulation levels in each of the below areas sound reasonable (for a relatively low heat-input space of ~6.5x2.5), or whether I'm falsely economising:
1. 50mm celotex in-between the roof joists?
2. 25mm (or should I go 50...) celotex on top of 50mm deep battens on the walls
3. 50mm polystyrene on the floor

I'm trying to get my head around the priorities of which areas need the best level of insulation - for both this project and my general understanding. I realise that the roof is essentially naked at the moment, so that's the biggest potential heat loser. And also, it's the highest point, so I guess that means it's going to be subjected to more of the hotter air. Therefore, I assume I need to apply the best insulation level to the roof (50mm celotex?). However, I'm not sure whether the floor or the walls is the bigger potential source of heat loss, given the same level of insulation, and thus which one warrants investment in more expensive insulation. - or perhaps they both warrant the same?. Given the narrow nature of the room, the walls have a much greater surface area and thus on economic grounds it makes sense to aim for lower insulation levels there if physics backs this up.

Thanks again!

kind regards.

 

[diynovice99]

Doh! Just realised that my comments re the extra surface area of the walls lifts them up quite considerably in the potential heat loss stakes, and thus they must be equal - or probably higher - on the heat-loss spectrum than the ceiling. But I get I'm trying to generalise a complex system here, with lots of variables coming into play, so I should probably do my best with both of them... Although, is it at least fair for me to say that I can get away with the lowest level of insulation on the floor?
kind regards
Neil

 

[oldbutnotdead]

Heat rises so do the roof first. You've got deep enough joists to stick 250mm rockwool between them and still have 50mm airgap- rockwool is much cheaper than celotex and easier to get a draught free fit.
Your partition wall- use 4 x 2 and fill the void with rockwool.
Side walls- its a small room already, external insulation would be better but not cheap (£80/sq m was the last quote i had). So internal- glue your celotex to the walls (board adhesive or non-solvent gripfill), tape the joints then cover with 8mm OSB (long screws through the insulation into the wall).
Floor- put rubber tiles down, anything else is going to cost lots and reduce headroom.
Wiring for sockets- don't bury it in the wall insulation, surface on the osb will be easiest.

 

[Tigercubrider]

What are you doing in there? When you work you may get hot so don't go mad.
My attached garage has the gas boiler in it and is not insulated other than good insulation over the door.
It's bloody cold mid winter at the end away from the boiler, and spanners are chuffing freezing.

However if you wear a jacket it warms up quite quickly

If you were doing stuff that needed physical effort you "could" over insulate it if you're nit careful