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New Workshop Going Mouldy, Desperately Need Help!

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crazySteve

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Hi Chaps,

Just stumbled across this forum while desperately searching for answers, I really need some good sensible help and advise on my shed muck-up.

So during the summer 2019 I decided to build a 30m2 shed at the bottom of the garden as a games room/ snooker room/ workshop. The construction goes as follows:

Reinforced concrete slab with a visqueen DCP layer sat on a MOT compacted stone base. The walls framework is build from 4x2" C24 (none treated) CLS timber, the walls are 400mm centres with loft insulation stuffed in between. On the outside I have a breather membrane stapled to the 4x2" and I have fixed 12mm shuttering PLY over the membrane directly to the 4x2" frame (first mistake no air gap). The back and two side are clad with the PLY to save cost but the front is clad with 20mm treated timber gravel boards. Also to note the walls do overhang the slab by 10mm or so and the extra visqueen from the slab is stapled to the base plate of the timber wall before the membrane was added, all to prevent water from reaching the dry slab.
The roof is constructed with DIY 'A-Frame' designed trusses which are basically C24 4x2" CLS joined with an A-shaped piece shuttering PLY to give strength and to set the pitch, the design worked well and allowed for a shallow pitched roof that was strong. As with the wall the trusses are at 400mm centres and stapled to this again is the breather membrane and the 12mm shuttering PLY boards (no air gap), I then covered the shed with good quality roofing felt.
The roof is insulated with 100mm loft insulation, there is a slight gap in the A of the frame where to 100mm insulation didn't fill the void, but I left this clear. Internally I boarded both the walls and ceiling with with 9mm shuttering PLY.
There are two windows (none opening), double glazed units on the front and a door to the side.

As I say I started the project in the summer but progress has been slow and I only finished boarding the inside probably around Oct/Nov. Since then I've been only able to work on completing the shed in the evenings and weekends.

Because I still don't have any permanent heating in the shed yet anytime I go down there I usually just have my small 1KW electric heater which does a OK job of heating the room, after an hour or so the temperature will go from 6/7c to 14/15c, so I assume the insulation is doing some good!

The problem I've now noticed gradually over the past 6/7 weeks is green/blue fluffy mould appearing on the PLY boards around the bottom 100mm of the shed perimeter (internally), I have also noticed the odd small area on the ceiling and higher up the walls and corners, and overall it seems to be getting worse! Yesterday I unscrewed a couple of the internal PLY boards to investigate what is actually going on, I did notice the backs of the boards are no worse than the front, fortunately! But still the mould is there at the bottom. I also removed some of the insulation and the back of the breather membrane does feel 'sweaty' the 4x2" CLS frame also felt a bit damp to the touch. I then took a small section of the ceiling PLY down, the board had no signs of mould, but when I felt past the insulation to the membrane on the underside of the roof, it feels Very Sweaty!

So thats where I'm at with my problems.... I've pretty much spent all the funds I can afford on the shed and it's upsetting to see mould issues already. I realise now I've made errors with not introducing air gaps but the reality is it's a bit late in the day and too much money has been spent to start again so I need to somehow work with what I've got.

Can anyone please give me some advice on how best to tackle the problem, I realise there have been a few mistakes so please no 'if you'd have done it like this' scenarios...

I've read adding a VCL vapour barrier to the warm side of the insulation (between the insulation and internal PLY) will stop warm moist air getting through to the insulation and reduce condensation on the membrane? That would hopefully stop the 4x2" timber from getting moist also? Externally I could take the boards off in the summer and batten them off the frame to create an air gap, but the problem is what do I do with the roof? stripping the roof isn't an option.

Sorry for babbling on.. hopefully there's enough info there, but happy to answer any questions if it help to get this fixed.

Cheers
Steve
 

crazySteve

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Hi Jimmy,

No there not UPVC or Wood window frames with vents. I basically created the openings in the stud framework and bought doubled glazed panels which sit in. They have putty on the outside world to stop water getting in and are held in place on the inside with some timber lining. No leaks, and rarely do they get much condensation build up in the morning, so I assume thats fine?... To add, the windows are on the front of the workshop where I used the timber gravel board cladding (not PLY), which does have small air gaps due to the overlapping of the boards.

Are you thinking I might have a ventilation problem?

I was eventually planning on installing a heater/air con/de-humidifier unit, so I wasn't sure adding vents around the shed would counter act the whole idea of air con and heating?

At the moment the door isn't completely finished, in the sense that there is no frame on the outside with a foam seal to prevent cold air getting in... So I suppose there is currently a little air venting through the 5/6mm gap around the door?

Steve
 

Brandlin

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I'd guess you have a mixture of problems...

Its still a newish build so potentially any moisture in the slab is still evaporating, hitting the inside of your structure which has limited ventilation, and condensing on/in the walls.

Plus, you have no barrier on the inside of the insulationto prevent moisture passing into the walls where it is trapped.

And you have no ventilation behind the exterior cladding.

Add to that the fact that when you go in, you turn on the heater, creating warmer air that will carry all the moisture you breath out (better than cold air) then you leave, close the doors and the warm air cools, the moisture condenses and cant go anywhere because of the lack of ventilation.

I am sure others here with greater building and architectural qualifications will comment in due course, but I would be looking to strip the external cladding, adding battoning to create a void and reapply. Then provide ventilation above the roof insulation, but below the sheathing. Apply an internal vapour barrier, and then be prepare to wait while your slab (and any other construction moisture) naturally dries ... might take until the spring/summer?
 

crazySteve

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I should also mention since Christmas I have been using a Hygrometer (Xmas present!) to monitor the temperature and humidity levels since I noticed the mould. Overnight the humidity hits the 80's, during the day if I'm working on something in there and have the heater going it drops to the low 70's. Today I actually left the door open to try and purposely let some air in after reading up about ventilation, and while the heater was going I managed a record 68%. Still this evening when I took out one of the LED downlight and poked my hand up through the hole to feel the back of the roof membrane it was wet as usual... I'm just worried about the effects this is going to start having long term on the main 4x2" structure if it's constantly getting damp from the sweating membrane.
 

Brandlin

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that sounds like you have a lot of moisture still IN the construction (mostly the slab).

Are you sure you have applied a breather membrane around the outside and not a barrier?
 

crazySteve

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Brandlin":spz4vyk5 said:
I'd guess you have a mixture of problems...

Its still a newish build so potentially any moisture in the slab is still evaporating, hitting the inside of your structure which has limited ventilation, and condensing on/in the walls.

Plus, you have no barrier on the inside of the insulationto prevent moisture passing into the walls where it is trapped.

And you have no ventilation behind the exterior cladding.

Add to that the fact that when you go in, you turn on the heater, creating warmer air that will carry all the moisture you breath out (better than cold air) then you leave, close the doors and the warm air cools, the moisture condenses and cant go anywhere because of the lack of ventilation.

I am sure others here with greater building and architectural qualifications will comment in due course, but I would be looking to strip the external cladding, adding battoning to create a void and reapply. Then provide ventilation above the roof insulation, but below the sheathing. Apply an internal vapour barrier, and then be prepare to wait while your slab (and any other construction moisture) naturally dries ... might take until the spring/summer?
Thanks for the reply.

I do sadly realise the errors of my ways!.. some where perhaps naive mistakes in a rush to get it constructed before winter and other purely due to cost limitations at the time.

The slab was bone dry before and after the roof and walls where constructed and water tight on the outside, so I'm pretty sure thats not the problem. As you say ventilation would help overnight, the vapour barrier on the inside I think is going to be my main hope in slowing the amount of moisture inside the wall and roof voids?? Could I just treat the 4x2 timber with preservative to maybe slow the damaging effects down? This could be done when I remove the internal boards to fit the vapour barrier? If it means I get 10 years out of the shed instead of 3-5, then I'll just have to live with that as a botch fix.

I really want to avoid removing anything externally... I think I could use some thinner insulation in the roof (50mm Celotex) to create a gap and keep it away from the breather membrane that is sweating and then drill air holes at the eaves on both sides of the roof to maybe introduce a bit of air flow on the inside of the roof? I could do the same on the walls, but the cost of that amount of insulation is almost impossible for me to afford, which is why I went rockwool in the first place...

:roll:
 

crazySteve

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Brandlin":190spo6t said:
that sounds like you have a lot of moisture still IN the construction (mostly the slab).

Are you sure you have applied a breather membrane around the outside and not a barrier?
Slab is definitely dry, it was poured around July time and was left for a week initially before the bare frame started to go up, all-in-all I reckon it had 5-6 weeks of good hot dry weather before it was covered and the frame water tight.

I used Protect A1 roof underlay as the membrane... pretty sure its breathable? I hope so anyway.

Thanks
Steve
 

crazySteve

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I'm worried about this membrane.... Checked on Protect website and it states 'completely water and air tight'
 

Brandlin

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crazySteve":2fkt6gkl said:
Brandlin":2fkt6gkl said:
that sounds like you have a lot of moisture still IN the construction (mostly the slab).

Are you sure you have applied a breather membrane around the outside and not a barrier?
Slab is definitely dry, it was poured around July time and was left for a week initially before the bare frame started to go up, all-in-all I reckon it had 5-6 weeks of good hot dry weather before it was covered and the frame water tight.

I used Protect A1 roof underlay as the membrane... pretty sure its breathable? I hope so anyway.

Thanks
Steve
One quick google... from the manufacturers website...

"Protect A1® is a third generation roofing underlay which has been developed specifically to overcome the disadvantages of traditional Type 1F felts and second-generation plastic sheet materials. Proven time saving over Type 1F and 5U felts. Can be used as a temporary roof covering. Completely waterproof."

It isn't a breathable membrane.

The moisture you are seeing is your slab drying (and you breathing) and all that moisture being trapped in the building by the waterporoof and airtight plastic sheet you've wrapped the building in.

Heating and cooling (either day / night, or you running the heater) is causing the change in humidity, because warm air holds more moisture than cold air does. You're simply not getting any of the moisture out of your building.

manufacturers product details
 

jimmy_s

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I've just lost my post somehow.

I Agree with Brandlin - you need to get some ventilation in.
 

Brandlin

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jimmy_s":3ue2obd2 said:
I've just lost my post somehow.

I Agree with Brandlin - you need to get some ventilation in.
Ventilation isn't going to solve the problem of having wrapped the outside of the whole building in an impermeable sheet.

You're going to have to take it off.

Then follow manufacturers instructions on breather membrane, including ventilation gaps for walls and ceilings on the cold side of the insulation.
 

crazySteve

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No.. I've just read that myself :shock:

I actually can't believe it, I purchased from Screwfix after searching 'breathable membrane', and shows up top of the list. Assumed as it was roofing it would be breathable! I'm surprised the moisture hasn't actually been 10 times worse after discovering this..

SO! In theory... completely remove the internal boarding and insulation, cut out the membrane from the underside and dispose, that should be Step 1 right? Step 2 let the shed dry out for a few days. Step 3 do I cut up some breathable membrane and patch inbetween the 4x2"? Does it actually need it if the PLY cladding has no gaps for rain to get through? Or just fit in anyway? Step 4 replace insulation as before. Step 5 add a vapour barrier. Step 6 re-fit the PLY

What do we think?

So glad I know what one of the major problems is! Thanks so much
 

Brandlin

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Breather membrane wont do any good if it isn't continuous. You need to stop taking short cuts. Take off the boarding, remove the current plastic and reinstall breather membrane. Then batten over the top of that then reclad - that will give you the ventilation in the walls.

A friendly resident architect here started this thread to help people, complete with diagrams.

If he contradicts anything i suggest - listen to him not me.

On the other hand -contact screw fix and complain that you searched for breather membrane and they recomemended you the wrong product - kick up a fuss - you never know they might throw you a bone.
 

Inspector

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I know we do things differently over here but not too much.

Our framing is sheeted on the outside and the house wrap/building paper (breathable but sheds water) is stapled on the outside. Then depending on the kind of siding, battens or some kind of rain screen over the wrap with the siding over it. Some siding is nailed directly on the house wrap.

Inside the fibreglass/mineral wool fills the cavities between the studs touching the sheeting. Vapour barrier (polyethylene) stapled on the inside edges of all studs and sealed with acoustic sealant. Drywall sheet over the vapour barrier. Moisture doesn't get into the walls from inside but any that does will migrate out of the wall through the wrap.

Unless it's a cathedral ceiling, insulation bats would be pressed into the spaces between the truss with vapour barrier stapled to the bottom of the trusses. Blown in loose insulation can be used with or instead of the bats. The space between the insulation and the roof sheeting is vented from the soffits to roof vents along the ridge to carry any moisture away and to remove summer heat to keep the asphalt shingles happy. There will be roofing paper or membrane on the top of the roof sheeting with the shingles on top.

I would do as you want and remove the interior sheets and insulation. But after you cut the barrier from between the studs I would not replace it. After the insulation was dry, put it back in the stud bays and then put a vapour barrier across the walls followed by the drywall sheeting. Sealing around all penetrations like electrical boxes etc. Then take off the exterior siding (actually I would do this before the interior) and put the house wrap on the outside followed by the siding. The roof needs space between the insulation and sheeting with ventilation along the ridge from the eaves. Vapour barrier between the drywall and insulation.

You didn't mention it but did you put a barrier between the cement and bottom plates the studs sit on to keep moisture from migrating in that way?

Maybe there is a simpler and cheaper way to fix the issues but I don't know one.

Pete
 

crazySteve

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Practically though I can't start stripping the external parts of the shed this time of year...

So for the walls I'll remove interior boards and insulation and cut out the membrane? At this stage treat the timbers just to be stop rot or damp that may already be kicking in? Let it dry out. Could I then drill holes to vent air up and out of the wall into the eaves, then out to the outside world. If I could somehow create an air gap between the insulation and the back of the exterior PLY could this serve as 'the air gap'?. Re-install the insulation then add a vapour barrier and re-fit the interior skin of PLY, with some air vent holes at the bottom to create an airflow up the new cavity? Could that work?

What about the roof? Same process, cut out the membrane? Or do I leave the membrane and concentrate on improving the ventilation by creating and air gap again and adding vents at the highest point of the roof on the two gables, and more vent at the eaves?

I realise this is never going to be perfect, but I'd just like to make the best of it without going back to square 1.

Thanks!
 

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For now, If I do cut the membrane out and just put the insulation back so that it touches the external PLY and add a new vapour barrier on the inside. Would this be worse or better then what I have now in terms of being able to breath? Does PLY wood even breath anyway? The internal PLY is bare wood at the moment, but the external PLY is painted on the outside skin with exterior wood paint.

If its better the membrane comes off then I'll do that and then come spring time do a full strip off and sort out the external cladding and fit new membrane battens, air gap etc. If however it makes the condensation build up worse because its touching wood not plastic then it'll make more sense to leave the membrane on for now? Just weighing up the options...

With regards the roof, I'm struggling here, was I suppose to batten off the timbers to leave the same type of air gap as the walls? Although the roof is pitched there is virtually no space in the apex, so creating a traditional style vented roof space is difficult unless I can compact the loft roll insulation down to give an air gap.

I'll try and get some photos up to help explain some of this.

And to answer the other question Pete, no the walls are sat dirctly on the slab, but the slab has a damp proof sheet beneath, it's the one thing in the workshop that is actually dry.
 

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Brandlin":n76zyy3n said:
.......A friendly resident architect here started this thread to help people, complete with diagrams.

If he contradicts anything i suggest - listen to him not me...........
No, I think you've got it about spot on. This building need stripping back to the frame. Ideally it would be lifted up onto a plinth at that stage. Then it needs to be put back together with the right materials in the right locations following the principles in my "How to Build a Shed" thread, and it needs some ventilation. The insulation doesn't need touching......it's not the issue.
 

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