Garage to workshop conversion

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The Gray Man

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Hello all. Sorry if this topic is a horribly common one...

I have a detached, triple garage (roughly 8.5 x 6m) which I'm going to convert into a workshop. Currently it has bare, single thickness breezeblock walls with occasional pillars. Height slopes from 2.65 at the front, to 2.20 at the back. Big wooden swing doors. Floor is reasonably smooth concrete. Ply partition down the middle (or rather, 2/3 way along). Existing power is surface mounted sockets. Ceiling is wooden planks (is that what's called the roof deck?) on 2*6 joists.

My plan (such as it is) is to insulate walls and roof for warmth & sound, put dado trunking all round for power etc (also allowing for future flexibility), and install a woodburning stove somewhere. Any comments, suggestions or questions so far?

Here's where the specifics need to start, so I have a whole bunch of questions for you clever people...

1. As far as I can make out, vapour/moisture barriers are an important thing. If I use foil backed insulation (Celotex & the like), that does that job for me - yes?

2. Plan for the walls is to have wooden battens screwed to the walls every 600mm, with insulation in between them, then a layer of 12 or 18mm OSB3, then 12mm Soundbloc, with joints taped and sanded, and painted on top. The OSB is to let me fasten things to the walls easily; the Soundbloc is to reduce irritation for the neighbours! Is 18mm OSB overkill? Is Soundbloc overkill - would normal (or fire resistant, or moisture resistant) plasterboard be more appropriate for this setting?

3. The pillars every couple of metres are really messing with my head in terms of how to deal with them. Ideally I want a continuous flat wall - both for cosmetic appearance and to make it easier to install the dado trunking. The pillars stick out from the wall by 11cm - so is it reasonable to just skip the insulation over the pillars? i.e. battens & insulation between the pillars, OSB & plasterboard over everything? If I do that, how do I avoid my battens and insulation needing to be 11cm thick (which feels a trifle excessive)? Is it ok to have an airgap in front of or behind the insulation?

4. The roof is simpler - I hope... Insulation between the joists, plasterboard over the top. Don't think I need OSB there as anything I fit to the ceiling (lights, dust extraction hoses etc) can be fixed to joists. Anything else I need to put up there?

5. Floor will have a 2 part epoxy floor paint applied. I'm not sure what order to do things in, though - floor, then walls, then roof? Or roof, the walls, then floor? Or... Or doesn't that matter?

6. I'm sure someone will say that there's no point in doing all that insulation if the whopping great big doors are just wood. I know, I know - I'll need to address that somehow, but that's a problem for tomorrow...

Really hoping the hive mind can ease my stress by answering some of those questions!

Many thanks, Ian
Forfar, Angus
 

eribaMotters

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I'd insulate the pillars and go with the 18mm OSB on the walls. I'd also use OSB on the ceiling as you are soon going to hit and put a hole in plasterboard. Dado trunking sounds expensive. I've used metalbox sockets and ran cable through 20mm plastic tube. What about fixing some sockets on chains from the ceiling. My last workshop had a similar height ceiling and it is one of the things I now miss. I considered 2 part epoxy for my floor, but after putting 140L on the walls and ceiling I was fed up of painting and could not face the cost for the 55m floor area. I used a 50% mix of exterior pva and water giving the floor two coats. It cost about £20 and had been done now 3 years. I'd do this again in preference to paint, which is how I have treated previous workshop and garage floors.

Colin
 

Fitzroy

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Ian, it is a question that comes up endlessly. Especially since COVID stuck us all at home!

As a start you are in a compromised situation. Retrofitting insulation to a building not designed for insulation is full of pitfalls.
You do however have the luxury of space with 6.5x8m to play with.

The key bit about insulation is that the inside is warm and the outside cold.Warm air holds moisture, cold air doesn’t. If the warm moist air get to a cold surface you will get condensation. If the place you get condensation is not well ventilated you risk dampness and all is associated problems.

The longer and more frequent a space is used to more opportunity for condensation to accumulate and the less time for it to dry out.

Your ideal solution from a warmth and sound proofing perspective is to build a free standing insulated box inside your existing garage. With all walls 50cm from the existing walls and a new ceiling 50cm below the old. Then make some nice bit vents in the old walls. The old building keeps the rain and bugs out, the new box is warm and sound proof. Any moist air has lots of room to find its way outside. You can inspect the box on all sides as you have access, and your old garage doors are no longer a problem.

However! This will look very odd/on entry to the space, it’ll waste floor area, it’ll have no natural light.

Each step you move away from this ideal you will:
- Make the job harder and reduce the insulation effectiveness as you add details for doors, windows, pillars etc
- Compromise the ventilation and carry more risk of condensation
- Improve the aesthetic and make a nicer work environment.
- Maximise the available floor space.

If you were occupying the space full time I’d say you need to seriously do your homework and all the details you mention will be the difference between success and failure. If your spending 5-10hrs a week in it then you’ll likely never be impacted by any poor decisions.

Regards your questions:
1. Yes if you get the details correct. Spray foam the celotex into any spaces and foil tape all the joints.
2. 12mm OSB is plenty. Regards sound proofing, depends on you tools and your neighbours. The more mass the more sound reduction. It’s hard to make any promises as gaps and details are very important.
3. Brick/block is a terrible insulator. You need to insulate the pillars. You also need a ventilated gap behind your insulation.
4. Your design is a hybrid roof, without very good moisture barriers you risk condensation. A better design, as you have the space because of high ceilings, is insulation over the joists, plasterboard/osb over (under) this, then ventilate the joist area at the soffits.
5. Depends, it’ll likely be clear once you have all your other bits worked out.
6. That is the elephant in the room. I’ve seen a few YouTube vids where people have had secondary doors inside.

Hopefully some of that helps. It really is a minefield! The other option is just get a bigger wood burner and scrap the insulation idea.
Fitz
 

The Gray Man

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Fitz - that's both extremely helpful and mildly depressing at the same time. o_O What started out (in my mind) as a fairly simple operation is becoming more and more complex - and hideously expensive - as I look into the specifics. Not sure if you were joking or serious about the alternative option, but actually that begins to look more attractive... Hmmmm.

Setting that aside, what would the implications be of changing from Celotex (is PIR the generic name for that?) to Rockwool or Earthwool? I understand that's breathable - is that a good thing or a bad thing in this context? Or is it largely irrelevant?

Perhaps yet another alternative is to insulate the ceiling (can't help but feel that must be where a vast proportion of the heat will escape), but not worry about the walls beyond boarding them. If there were no insulation in the walls except osb & soundbloc, is condensation still likely to be an issue?

Last question (until my next one) - re ventilation of the roof space via the soffits. The joists run front to back, and I have access to the front and one side, but not the back or the other side. Presumably any ventilation would need to be at both ends to be effective?

Cheers,

Ian
 

TomGW

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The simple principle is, condensation will be an issue on the solid block walls and roof unless you can keep the heated, moist air away from them. The only way to achieve that is to provide an insulated sealed inner compartment and vent the (cold) gap between the two. Three elements are necessary; moisture barrier, insulation and a vented gap.
 

Fitzroy

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No impact changing insulation type, you still need a good vapour barrier to keep the moisture away from the coldside. PIR boards will act as more of a barrier but you'll not be relying on the insulation as the moisture barrier.

If you don't insulate the walls then any moisture in the air will condense on the inside surface as it will be cold. The classic example of this is single glazed windows in a modern warm, high humidity home. If you are in your workshop, on your own, doing nothing very strenuous and not boiling a kettle for endless cups of tea, then the amount of water vapour you emit is limited and similarly the level of condensation is low, I doubt you'll see any impact of any condensation on the walls. If you put a treadmill/exercise bike in there and ride for two hours I expect you'll see moisture droplets on cold spots, do this every day and I expect you'll get mould fairly quickly. However add some ventilation after each use and the place will probably dry out effectively.

Roof space ventilation needs a cross flow to be effective.

Personally for me it all comes down to space use and expectation. I've been through the same dilemma for my shed which I use for c. 5-10hr per week as a woodworking workshop. What I want is:
- A space in the winter than I can keep at 5-10degC and bring up to 10-15degC relatively rapidly ie within an hour. This will keep condensation off my tools is easily warm enough to actively work in (but nowhere near warm enough to sit in).
- A space that I can work in an evening before 9pm and use power tools without bothering my neighbours. I think realistically I will never manage to soundproof it sufficiently to work after 9pm as any level of tool noise past this point could be considered bothersome.

My plan to achieve this is in my 6mx3.5m space.
- 100mm rockwool insulation in walls and roof between studs, moisture barrier with taped joints, 12mm plywood over the top.
- Roof timbers are 150mm so I will ventilate the 50mm above (likely not a big enough void and noggins will impact the effectiveness although I will drill vent holes through them)
- Stud walls are 100x50 with 11mm OSB on the outside, so the back of my insulation is not vented, I plan to drill some holes in the OSB but as there is no cavity again this is likely to be somewhat ineffective and I risk condensation.
- Oil filled rad to maintain low temperature, and 3kw fan heater to bring the temperature up.
- I have a large door that I will open after using the space and a relative humidity meter, if I find the RH stays high even after venting I'm going to get a dehumidifier to run whilst I'm working/afterwards for an hour or so.

Sorry essay, I probably think too much about these things!

Fitz
 
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