Condensation Problems in Workshop

[John McCullaParticipant @johnmcculla]

Hi everyone,

I appreciate this topic has been covered many times on the forum before, however I’m after answers to some specific questions I have.

I have a large 6m x 12m garage. It’s a single skin metal building with a concrete floor, and I have problems, mainly in the winter, with condensation and rust. As a stop gap measure last winter I bought a dessicant dehumidifier and ran it beside the lathe. This helped a bit, but it ran constantly because the building was drafty enough that it could never pull the humidity down enough to cut out on the humidistat. As you can imagine, this was expensive and not a feasible option going forward.

I’m now thinking of essentially building a room within the garage at the back, having it well enough insulated and sealed that I can leave the dehumidifier on, but it will only need to fire up occasionally to maintain the humidity level. Originally I had though of buying a pre fab wooden garden shed, but these are surprisingly expensive and by the time I would insulate it I think I can built one for less. My plan is to build a frame of 2×4 timber, and sheet it with plywood inside and out. I could then insulate the cavity. Essentially building it of four stud walls, and a roof made of the same construction. If I made it 12′ long, 8′ wide, and 8′ tall, I could use 3 sheets of plywood for the long sides, 2 for the short sides, and 3 for the roof. Plus the same again because it’s double skinned.

This would give me a well sealed room, insulated on all sides apart from the floor. I like the idea of keeping the concrete floor, as while I only have a lathe and pillar drill atm, I might like a Bridgeport or something similar in the future, and in terms of weight bearing it’s hard to beat a concrete floor. Is that though likely to cause problems from the cold concrete floor, would I be better making a wooden floor with an air gap below it?

In terms of the insulation in the walls, is it necessary, or would the 4″ air gap between two layers of plywood be sufficient? If it is necessary, what would you recommend, rockwool?

Finally, do you think this is a good idea? Or am I barking up the wrong tree?

[norman valentineParticipant @normanvalentine78682]

I have a shed that I built using 3×2 timber lined on the inside with 6mm MDF and the outside with T&G boards. The floor is also a timber frame of 3×2 with OSB as the floor covering. The insulation throughout is rockwool including the floor and ceiling. I never have a problem with condensation and it warms up quite quickly using a fan heater.

[Neil WyattModerator @neilwyatt]

First off, kill all drafts.

Insulation, I use 50mm expanded polystyrene with a plastic membrane INSIDE and then a plasterboard (or it could be your plywood) inner skin.

I have a steel roof and use the ‘bubble wrap’ type insulation held on with double sided tape which has a poor reputation in some circles but is actually very effective from practical experience.

I use a dehumidifier and frost heater, just in case.

[BazyleParticipant @bazyle]

An inner section is a good idea as not everything needs to be mollycoddled. You might start smaller but with design for expansion. Also perhaps rather than small doors on the inside have large sliding wall sections for opening up in the summer.
Instead of traditional 2×4 closely spaced, since it is inside and non structural consider 6×1 to make more thickness for insulation and space at say 2ft to match covering. Rockwool is a lot less insulating than foam so you need more thickness. Better to be warm in 8×6 than cool in 8×12.
Insulate the floor but when you get a heavy machine cut holes though to the concrete under its feet.

Dessicant dehumidifiers put out extra heat which is wasteful in summer. Contrary to folklore regular dehumidifiers do work at low temperatures of the sort we get in the UK. Might be different in Canada or parts of EU.

 

[Dave HalfordParticipant @davehalford22513]

Most of your damp in the garage is probably coming from the floor as I’d bet there’s no membrane.

Seal the floor.

If you insulate the walls and roof leaving all the corrugations open for ventilation. That Jablite stuff that B&Q stock would do. I had a much smaller metal 6×8 shed and 1/2″ suspended ceiling sheets (they were free) stopped the condensation and the worst of the summer sun.

[bernard towersParticipant @bernardtowers37738]

I think one of the biggest problems is the speed of the temp change in uninsulated buildings. A metal clad building must be the worst as the heat/cold transfer is fast whereas a brick building changes temp more slowly. My larger machines are in an uninsulated brick built garage with edpm roof and no insulation and in the last 25plus years have had no problems. As I have said before on this subject the machines use hydraulic oil for coolant which gives the machines a condensation proof barrier if the temp rises quickly as it does a few times a year.

[vic neweyParticipant @vicnewey60017]

My shed is an uninsulated 14X8ft lean to against the house, the floor is flagstones and it has corrugated transparent sheeting for the roof which has shade netting inside the shed. The shed sides are shiplap boards which have been there 25 years so it’s now rather draughty in places. I normally don’t have any condensation problems in the winter but there again I don’t have any heating in there either.

To be fair I have to say I’m not a model maker as such and won’t spend much time in there if the weather is very cold, apart from a small workspace I have the shed filled with 5 vintage lathes and other machines.

[Chris CrewParticipant @chriscrew66644]

I keep an old convection heater, switched to its lowest possible setting, on 24/7/365 with a small upper window ajar for ventilation. The energy cost doesn’t seem to be prohibitive and keeps the rust fairy at bay. Anyway, it seems to work for me and my workshop.

[Neil Wyatt]

On Neil Wyatt Said:

First off, kill all drafts.

Insulation, I use 50mm expanded polystyrene with a plastic membrane INSIDE and then a plasterboard (or it could be your plywood) inner skin.

I have a steel roof and use the ‘bubble wrap’ type insulation held on with double sided tape which has a poor reputation in some circles but is actually very effective from practical experience.

I use a dehumidifier and frost heater, just in case.

Bubble wrap on a steel roof? Interesting idea. Might try it.

 

I spray 3 in one oil over anything that can rust and also have a diesel heater if it gets too cold. Helps loads. Apart from that I do have a single skinned wooden shed on concrete blocks and with a concrete floor. Condensation is an issue but I manage it with oil and heat.

[duncan webster 1Participant @duncanwebster1]

1. I have a single skin brick garage with concrete flag floor. First thing to do was get the floor screeded with mastic asphalt. This makes it flat and waterproof. Then built a structure from 3*2 spaced 1″off the brick with 75mm polystyrene between the timbers, lined with visqueen, covered internally with plasterboard. I don’t see any reason for plywood on outside of this (ie facing the bricks). The roof I put plasterboard across the horizontals, covered with more visqueen, then a lot of insulation.. On the floor I put 25mm polystyrene covered with flooring grade chipboard. Where the heavy machines are I cut holes and put timber pads down to the asphalt. Result, no condensation, very little heat required, but I have a balanced flue gas heater (surplus from the house rebuild)

[RainbowsParticipant @rainbows]

also have a diesel heater if it gets too cold. Helps loads.

It’s worth remembering that most hydrocarbon based space heaters blast water vapour into the area you’re heating. Electric or flued furnaces have an advantage here

[Rainbows]

On Rainbows Said:

also have a diesel heater if it gets too cold. Helps loads.

It’s worth remembering that most hydrocarbon based space heaters blast water vapour into the area you’re heating. Electric or flued furnaces have an advantage here

No, diesel heaters (I run mine on kerosene) of the sort used in caravans and trucks exhaust to the outside world, not inside the workshop.  Obviously a lot of space heaters do exhaust internally though. Using those would be a bad idea.

[John McCullaParticipant @johnmcculla]

I like the idea of using 6×1 instead of 2×4 timbers to give a bit of extra insulation depth for less cost in timber (albeit more in insulation). Thanks Bazyle.

I’ve been trying to get my head round insulation options, and would something like this (https://insulation4less.co.uk/products/knauf-150mm-combi-cut) work inside a stud wall for these purposes? Everything I read online says it’s a bad idea because it will slump down over time, but this isn’t a house I’m building. If it was well packed, maybe into a cavity only 140mm deep, do you think it would be suitable? It certainly seems to be the cheapest form of insulation I can find, so it would be good if I could use it.

OSB would be cheaper than plywood to clad the inside and outside walls with, can you think of any disadvantages to using OSB? I will probably end up putting up shelving on it, possibly both inside and outside.

[Charles LamontParticipant @charleslamont71117]

I agree with waterproofing the floor, but I would then cover it with interlocking chipboard flooring panels. This means you could just lay a DPM under the boards. They provide a certain amount of insulation and are easier on the old feet and the occasional dropped part or tool. I also run a dessicant dehumidifier during the winter even thought the shop is quite well insulated, apart from the roller shutter double garage door.

[BazyleParticipant @bazyle]

Fibre insulation does stand up ok in vertical walls. However it is cut into widths that fit the standard 16in spacing (14in gap) of studwork with a little compression to grip the timber. However you don’t need so much timber in a shed. Look for varieties that are not precut (top-up rolls for going over already filled joists maybe). Don’t overpack as it is the air gaps that provide the insulation but 150 in a 140 gap is fine.

OSB is fine inside but not good outside as you can’t seal the water out. Even some smooth ply is hard to get waterproof. Consider shiplap or even fence weatherboarding but prepainted corrugated iron doesn’t look bad and is good for decades.

[Howard LewisParticipant @howardlewis46836]

Insulating bthe walls and the roof with 50mm or so of glassfibre is a styarting point.

My shop has the walls and roof insulated with 50mm thick glassfibre. The 3/4″ ply floor stands on 8″ x 2″ bearers which are protected on three sides, by walls or fencing.

Putting a waterproof membrane on the concrete floor and then adding the floor on which you will walk is the way to go. It will be much warmer if you can stand it off from the membrane. An air gap will provide insulaton. Unfortanately, gaps will provide a home for swarf, which will need regular removal!

Condensation will be avoided by having ventilation. You don’t need huge grilles. My 10’9″ x 6’9″ wooden shop has two  1″ grilles at floor levels, and the high level vent is via a 6″ diameter ex equipment intake fan, with a weatherproof hood outside.

Howard

Condenstaion is not a problem

One vent should be at floor level, and another high up, (preferably diagonally opposite the floor level one,) to encourage

[Howard LewisParticipant @howardlewis46836]

Hopefully, I got it right this time.

Insulating bthe walls and the roof with 50mm or so of glassfibre is a starting point.

Condensation will be avoided by having ventilation. You don’t need huge grilles. My 10’9″ x 6’9″ wooden shop has two  1″ grilles at floor levels, and the high level vent is via a 6″ diameter ex equipment intake fan, with a weatherproof hood outside.

My shop has the walls and roof insulated with 50mm thick glassfibre. The 3/4″ ply floor stands on 8″ x 2″ bearers which are protected on three sides, by walls or fencing.

Condensation is not a problem

Putting a waterproof membrane on the concrete floor and then adding the floor on which you will walk is the way to go. It will be much warmer if you can stand it off from the membrane. An air gap will provide insulaton. Unfortunately, gaps will provide a home for swarf, which will need regular removal!

Howard

 

 

[File HandleParticipant @filehandle]

My workshop I usually describe as a wooden shed built inside an asbestos garage. perhaps better described as being lined with whatever was lying around, timber boards, MDF and chipboard and painted. There is some insulation. Typically not heated, but I do have an electric fan heater. The walls are decorated with shelves, storage containers, plastic drawer units, bins and copious tools (some of which get so little use that they really are just decorative, e.g. I can’t remember the last time I used a 7/8 Whit spanner). Whenever I use a tool I do wipe it with an oil impregnated rag in a can. Condensation, rust, isn’t really a problem. Possibly due to the oil, anything that I think might suffer gets a liberal spray of 3in 1.
We do have 2 dehumidifiers in the ding room, these get our washing dry quickly, so do work well.

[Terry KirkupParticipant @terrykirkup37827]

My little workspace has a shiplap timber outer shell with 25mm slab insulation inside with no air gap, covered by 1/2″ rough plywood. The floor is raised from ground level and is constructed of 20mm timber boards on 4″ x 2″ joists covered with 3/4″ plywood and 3 or 4mm sheet vinyl floorcovering. The ceiling uses the same insulation as the walls and is uncovered as shown in the photos. I have a double socket controlled by a thermostat set at 12 degrees Celsius with a 12″ tubular heater in the swarf tray under the lathe (chuck end, see photos) and a four footer (both 60 watts per foot) mounted just above floor level behind the lathe stands (just visible left hand side of lathe photo). I also keep the chuck and toolpost clothed in old tee shirts from autumn on. The Warco mill that now shares this space has no heating near it and the vise and table ends have just suffered their first rust patches in two years so I’ll be sticking another one over there soon. There is little or no ventilation in this space so I may be tackling this the wrong way!

***I put these images in a gallery but don’t know how to display it (help?), so fresh uploads this time.

[BazyleParticipant @bazyle]

Terry – try and cover the whole lathe rather than just the chuck to create a micro-climate for it. A smaller heater placed in the lower shelf would also be really good for this and give you spare heating budget capacity to do the same for the mill.
Since you are prepared to use electric heating you could also put in a dehumidifier perhaps on a timer or instead of the heaters. As it pulls moisture out it will also provide more heat than the energy you are expending, though the COP is less clearly defined than a heat pump. Win-win.

As for the people who keep promoting vents. Have you never heard that hot air rises – and it will go straight out of your top vent. Then when a warm front comes by as it does every couple of weeks in winter in comes damp air to condense on your cold steel. Always always do you level best to seal your workshop and and use a dehumidifier.

[Terry KirkupParticipant @terrykirkup37827]

@Bazyle Thank you for that Sir. Yes, hot air does indeed rise and seek the fastest path to cold 🙂 As luck would have it my daughter has only just commandeered my dehumidifier for her front porch! I will take extra measures though including tents over both machines.

[John McCullaParticipant @johnmcculla]

I don’t think I want to put vents in this, it would defeat the purpose of creating a sealed area to run a dehumidifier in.

I can line both the inside and outside walls with 9 or 11mm OSB then, water shouldn’t be an issue since it’s inside.

In terms of the floor, I’m thinking I’ll lay a DPM on the concrete, and lay chipboard (or maybe just more OSB?) directly on that for a floor. Unless you think the extra insulation provided by an air gap is worth the extra effort and timber

Regarding the stud wall frame, is 150mm deep insulation overkill? I don’t plan to heat the area, just whatever heat the dehumidifier supplies, the insulation is only there to avoid big temperature swings. Could I make the wall from 4x1s? The loft roll comes in widths of 1140mm or 570mm, I assume I would need to space the studs at 570mm as 1140mm would be too far apart?

Finally, the roof. Initially I was just going to make essentially a 5th insulated stud wall and set that on the top for the roof, but is there a better way? It needs to be insulated, but doesn’t need to be load bearing (unless I could use it for an upstairs storage area?!) or waterproof.

[John Mellor 1Participant @johnmellor1]

Kingspan or equivalent at 50mm should be all the insulation you need. Rockwool is lower cost but needs a thicker layer. You do not need ply wood outer skin, just inside for all your fittings.  Use the steel shed as your outer skin.

For the floor I have used floor tiles in one situation. Anything you cover the concrete with will attract the damp. Another place I laid 25mm insulation board with flooring directly on top, no air gap. The insulation board is my vapour barrier.

[John McCullaParticipant @johnmcculla]

Kingspan seems to be a lot dearer than fibreglass loft roll though, and I’m keen to keep costs down as far as possible. From looking at online prices I could insulate all the walls with 100mm fibreglass for around £70, 150mm fibreglass for £105, and 50mm kingspan for £234.

The reason I was thinking of plywood both inner and outer was because I wanted it to be a stand alone structure you could walk around, however there might be something to be said for putting it into the corner and using the steel shed as the outer skin for two walls, plywood or osb for the other two.

[BazyleParticipant @bazyle]

A dpc on the floor would be fine if you can avoid it being punctured by nibs and specs of gravel that are magnetically attracted to concrete floors. The moisture will be diverted to the edge so be aware of where that is and if it is ventable.
In theory 2in of insulation in an 8×8 foot cube (excluding floor) wants about 100W heating for a 10 degree temperature difference. A 16×8 only needs about 60% more so it is not directly in proportion to floor area.
However I would still go with your idea of 4×1 frame and filling it with insulation but you could make alternate frames of 1/4 ply if cheaper. In the verticals it tends to sag so needs to have the extra frame members to ‘grip’ even they are not structurally needed. Also put in some noggins so it is not a single vertical run. What you sometimes see is builders cramming the insulation in, especially if retrofitting a ceiling from below to make it grip but this defeats the object of having a loose fill with lots of air. It is the air that gives the insulation not the material, even for Kingspan etc. The only time air hasn’t been the best insulator was for the space shuttle.

The dehumidifier may be all the energy it takes but thermal mass of the contents will make a difference to how it feels. A remote controlled switch for prewarming the machines would be the ultimate to prevent your breath condensing on them.

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