Wooden shed insulation

Hello all, please excuse the long post but im sure someone will be able to offer some sound advice (I have gone round in circles reading both on this forum and elsewhere)

I am currently erecting a second hand wooden 16' x 12' shed in my garden to use as a workshop and im interested in hearing some real world experiences in how best to keep the condensation at bay in this type of structure.

Details as follows, the shed is sat on a concrete base with dpm present. It is in sections that are built using a 3" x 2" CLS square frame with vertical uprights and 20mm shiplap cladding nailed externally. I have screwed all the sections together, have added an additional sole plate and wall plate and have built a new roof consisting of 4" x 2" rafters followed by a breathable membrane then 2 x 1 laths for which to fix a metal sheet roofing. The roof space will be insulated with 100mm loft insulation between the rafters and clad in 3mm or 6mm ply internally (maybe with a vapour barrier before ply).

I'm pretty happy at this stage but it's the walls that are causing more head scratching. Now I understand the best way to construct if building from scratch with air gaps and breathable membranes to allow the cladding to breathe but I don't have this luxury so am after the most suitable compromise to offer some improvements.

To try and limit the amount of water that will come in contact with the cladding I have the rafters overhanging at the eaves by 12", that will also be plus facia and guttering. The verge overhang is yet to be determined but will be at least 12" if not 18" (depends how it works out with the roof sheeting).

My most recent train of thought with the walls would be to place 50mm polystyrene (PIR not within budget) between the vertical uprights, leaving a 15mm air gap between it and the outside cladding then a vapour barrier over everything, followed by a ply cladding internally. With this I just have visions of the vapour barrier sweating due to the lack of ventilation in the wall structure without drilling loads of holes in the framework and or cladding. Sometimes I just wonder if it would be better to just leave the walls open, draught proof and roll with it but once all the tools are in there's no taking them out again.

So does anybody have any suggestions of what needs or needs to not be done with the walls to help prevent anything I put in the shed going rusty and ideally keep the heating bills down during the winter?

Thanks, Craig

Edited By Craig Brown on 08/08/2023 20:45:16

My previous workshop was made of a 3"x2" timber frame, but with external ply on the outside, so less chance of wind-blown rain penetrating.
I lined the walls and roof with rockwool and plaster-boarded the inside. There was little issue with condensation, and the place warmed up quickly in the coldest of winters with an electric fan heater, though it was smaller than your new one.
Don't be tempted to use a gas heater as they give off water vapour.
Since you have shiplap walls, how about using a breathable membrane immediately behind them, such as one of the newer style of underfelts designed to go underneath slates or roofing tiles.
This would repel any moisture, still be breathable, and keep the rockwool dry.

Expect problems in the short term though as a new base will have quite a lot of moisture to get rid of whilst drying out.

Bill

Edited By peak4 on 08/08/2023 21:14:04

The only trouble with rock wool is that it acts like a sponge if there is any water leak and it won't dry out due restricted air movement so can build up over time without being noticed.

Although not ideal polystyrene is better in this respect and can possibly be done for nothing by collecting sheets or even fragments of PS packaging that can often be found in builders skips. The downside of PS is it is combustible but since a workshop, except in the distant past, is rarely slept in then no problem.

It is amazing what is offered for free on Freecycle sometimes including building materials as some traders would rather give away materials they can't use rather than have to pay for the disposal of trade waste.

CS

Seems like you are going in the right direction. Breathable membrane on outside of EPS. The 15mm gap just needs a few 1in holes at top and bottom with vermin mesh, one in each section. The EPS maintains the gap letting you put a bit of fibreglass in the remaining space to the vapor barrier and internal cladding (12mm OSB probably cheapest).

Of course run a dehumidifier at least during and for an hour after a work session.

I would also add a decent dehumidifier. Get one that has a drain pipe so it can empty under the shed through the floor so you don’t have to empty it every day or two.

Is the shiplap already fixed to the frame? You don't mention a breatheable membrane for the walls so I assume it is. Best option will be to take the cladding off and then fit a breathable membrane then refit the shiplap. That way water droplets then ge behind the shiplap won't get any further but moisture vapour can still get out.

Polly will do though not as insulating as PU, vapour barrier over that (polythene sheet) then your boarding. This barrier will stop air bourn moisture from breathing or burning propane from entering the wall construction and condensing within the structure.

As said fibreglass can act like a sponge so poly or PU would be better, either way don't fully fill your rafters/studs, leave an air space between insulation and outer surface. 

This is a job I went to put right after the builders got their vapour barriers wrong and other failures, nice £100K job for me. This is fibreglass pulled out through a downlighter hole.

Edited By JasonB on 09/08/2023 07:02:18

I have a wooden garage on a wooden base that I use as a workshop. The floor has a sheet of plastic DPM covered with interlocking rubber matting squares bought from B&Q. The walls have that foil-backed bubble wrap stapled across the main beams, leaving an air gap behind them. On top of this is 18mm or so flakeboard. The building remains dry and I have not experienced any trouble with condensation. There is reasonable ventilation under the eaves.

The roof is currently left as simply the wooden T&G with no cover. Initially, I had polystyrene insulation slabs between the rafters and hardboard over the top, but this caused serious condensation. I removed it about two years ago and have had no trouble since.

James.

Thanks for the replies so far. Bill, it's interesting to hear you had success with a full fill rockwool with plasterboard over. At the end of the day it is just a shed after all, the majority of which was free and it is only intended to be stop gap before I construct a masonry workshop, but who knows when I will get to that.

Bazyle, thanks for confirming my suspicious as regards to the finer details regarding the ventilation. There may be a way in which I can channel the top of each section into the eaves to ventilate there.

Edited By Craig Brown on 09/08/2023 08:17:18

Definitely insulate walls and roof with tockwool or gl;ass fibre.

My internal walls are lined with 12 mm ply. and shelf brackets are fixed through the lining to the verticals.

To maximise storage space, being only 10'9" x 6'9" externally, and for security, there are no windows.

ventilation is by a cpule of fixed vents at floor level, and a 6" fan near the ceiling, with a weatherproof hood on the outside.

If you have windows, fix grilles over them, retained by coach bolts, with the domed heads outside and the nuts inside.

The door hinges should be retained in the same way. I made hinge bolts by nturning off the heads of woodscrews and screwing them into the back of the door with a drill chuck, to leave about 25 bmm protruding. trying to cvlose the door marks the p;ost where to mdrill the holes..

In this way with the door closed and locked (The door has a 6 lever mortice lock, although 5 should be adequate ) the hinges cannmot be unmscrewd, and the door cannot be lifted out, even of the hinge pins have been knocked out.

Heating shopuld be dry (NO combustion heaters) If there is a combustion heater, the re must be a flue leading to outside, so that no water vapour can escape to the inside (Also, you don't want to gas yourself) Probably safest all round not have one, in the longer term )

Lighting, LED, lamps or battens, for energy saving and to have plenty of light., particularly over the machines and the fitting bench.

HTH

Howard.

Yes Jason the cladding was already nailed liberally to the framework when I got it and I understand that removing it to create proper ventilation and installing a breathable membrane would be the preferred method but for multiple reasons it just isn't something I can dedicate the time and effort into

This is a stop gap workshop that has come earlier than I was expecting due to the fact it was a freebie and on the door step so I am just trying to make the best with what I have to give me a usable work space.

Howard, security will be the next thing I'm sure I will be fretting about, shed break ins are not uncommon in my area so will be something to consider. Like you say I have toyed with the idea of doing away with the windows for both insulation and security reasons

I did go to quite a bit of trouble to ensure good waterproofing; e.g. where plywood sheets met on the walls was sealed, and an outer batten added, as well as regular treatment with one of the waxy water repellent shed finishes.
The one water ingress issue I had was with the roof on one corner.
I made the single pent roof from ¾" shuttering ply, covered and sealed with bituminous paint, and topped with corrugated bituminous sheeting from Wickes, with angled ridge pieces used as seals along the top edge and sides, and a gutter along the front.
Unknown to me, the local squirrel population chewed a hole on one corner, under the bituminous sheet, behind the gutter, but through the wood to make a larder for the peanuts that my neighbour used to leave out for them.

The other issue was a break in attempt one night, where a neighbour's shed had been burgled.
They nicked a cordless drill and 1" tank cutter and chain drilled a hole through ply and plasterboard, big enough to crawl through.
Fortunately they picked a spot immediately behind the pillar of my bench drill and came face to face with a big steel tube, equally fortunately, they didn't realise it wasn't bolted down.

The windows already had steel bars on the outside, and cable gridding (similar to concrete reinforcing weldmesh, but smaller holes) on the inside. The main door was my old house door, with a crossbar of angle iron and a protected padlock; worth having an outward opening door, both for space saving internally, and it's harder to shoulder charge through.
Windows were a pre-made double glazing unit from a local DIY shop, which had been made the wrong size for the customer, so was sold off cheap.
There is now overlapping sheets of weldmesh inside the insulation of the back wall.

I think now, I'd consider Firestone rubber roofing sheets, but they weren't around 35 years ago.

Bill

Edited By peak4 on 09/08/2023 11:38:54

Built my workshop from scratch nearly 50 years ago now. Shiplap board over a light 2" x 1" frame with 2" x 2" corner posts. No membrane under board (not sure it was around then. 2" rockwool insulation in the walls and 4" in the roof. Never had problems apart from having to replace the felt a couple of times. Needs doing again soon and am contemplating putting steel profile sheet 34/1000 over the felt screwed directly to the 1/2" shuttering ply I used originally.

Couple of things I would do differently now. Up the timber frame to 3" x 2" and increase insulation, possibly using Kingspan or Cellotex (order of magnitude more expensive). Originally I fitted thin printed hardboard inside lining (cheap) and I put the wiring in the walls. If I did it now I would use 12mm OSB on the inner walls and use surface mount trunking for the wiring and sockets. Much stronger both for mounting shelves & cupboards and to increase security. Wouldn't use plasterboard, messy and not particularly good for load bearing. Engineering equipment and stock is on the heavy side!

John

My workshop is mostly a 100mm (or more) cocoon of PU insulation board with edges sealed with tape. It’s built inside a sectional concrete garage. Some of it was built with old garage doors and now with insulation both inside and out. The floor is 100mm (or 150mm) concrete with a damp-proof membrane about 150-200mm below the 50mm screed. It was extended with full sheets of 2400 x 1200mm Recticel (or similar) which, conveniently, fitted just below the roof members. The end , with only insulation, has strips of weld-mesh behind. The inside has been sheeted out with 12mm ply.

Moisture control is by a couple of small dehumidifiers (usually only one running, for up to a couple hours each night) in winter). It is warm in the winter and cool in the summer. Extra warmth is supplied by a chinese diesel air heater and I have installed a CO monitor, for extra safety. I don’t weld or angle grind inside my workshop.

Even the steel door is insulated inside and out. I’ve yet to see the temperature below 4 Celsius and it is usually 6-7 Celsius or above at a relative humidity of 60%ish.

My workshop is a combination of concrete block and timber frame with weatherboarding, inside the weatherboarding is 100 mm Celotex and in the roof is 50 mm cellotex under sterling board with two layers of torch on felt, I found Celotex so easy to cut and fit between frames. I also run a dehumidifier which drains to outside, during the winter months I run an oil filled radiator on low setting to keep the temperature above 0 degrees. If I built another workshop I would use the same format again. Dave W

No. 1 action that I found extremely effective, was to cocoon the wooden walled shed in roofing felt so as to exclude rain or snow from accessing the timber walls. Timber is like a sponge. Past experience of a wooden shed with no exterior waterproofing, was a very damp interior and plenty of rust problems. I cut the felt into large squares and nailed it on the outside walls with about a 2" overlap. Felt being heavy but weak and therefore liable to tear away from felt nails, hence using it in squares tends to match the weight of each 'tile' to it's strength. Deal with apex roof in the same way and the door. I did the overlaps so that the top was sort of back to the prevailing winds that drive heavy rain. All still working successfully after several decades. Roof interior filled with polystyrene sheets, mainly ex - packaging. Patches of glue to roof. Walls lined with thin 3ply recovered from elsewhere, voids stuffed with rock wool. Door had a close fitting perimeter frame covered by glued and screwed 3ply and interior voids stuffed with rock wool. Lathe and one bench mill covered in plastic sheeting with a small heater under sheeting. Heat inevitably escapes, warming the rest of the shed, which is 10 x 8ft. Almost no rust and comfortable to use anytime. Interior temperature still goes up and down like a whores drawers. Humidity usually around 60%. Currently growing hedging/ bushes next to South and West walls to keep the sun off the walls as heat is problematical on hot summer days. Location, Brighton, in the 'sunny south.' (Sometimes!)

Edited By DMB on 09/08/2023 19:45:00

Forgot to mention roofing!

Flat pitched roof, with 6" fall from front to back over a 7'6" length.

The original felt, lasted for 11 years before it began to show signs of deterioration. Replaced it before any chance of water ingress.

Material, was EPDM rubber from Rubber for Roofs.

Put the measurements into the on line calulator, speciafied where the gutter would be and ordered, mid afternoon

0800 next day rubber (ample! ) trims, adhesive, application roller, tray, and plenty of ring shank nails arrived.

It took two us with scrapers and a heat gun to remove the previous bitument adhesive, before laying one half of the sheet of rubber and then rolling it over to lay the other half.

Ring shank nails are almost impossiblke to remove, so do put them in the right place first time.

Final job was to trim off the excess rubber from below the trims. So much that bits are still being used for little jobs!

Adter nearly ten years still looks as good as new. The garge roof is also EPDM and has been ikn nplace for at leastbv thirty byears without any deterioration, so quite confident about the workshop.

Howard.

I replaced the gently sloping roof of my garages, total 22 x 17 feet asbestos with the cheapest possible roof. Single layer industrial building roofing with two strips of fibreglass, one for each adjacent garage. The consensation was bad until I bought sheets of expanded polystyrene to fit between the joists next to the underside of the roof. There was still some condensation from the translucent fibreglass panels, but 95% had stopped. The modern roofing of this type is usually double skinned with about 75mm of expanded polyurethane and the translucent panels are also double skinned. Cutting and fitting expanded polystyrene, if it is still available, inside the building, even 12 mm thick will provide very good insulation.

"Cutting and fitting expanded polystyrene, if it is still available, inside the building, even 12 mm thick will provide very good insulation."

But ensure that you have enough ventilation. I put polystyrene between the roof rafters, impeding the ventilation and suffered chronic condensation. Removing it stopped the condensation. I now have just the uninsulated wooden roof and no condensation.