Retrospective shed insulation

[Carlos0371]

Good afternoon all, firstly thank you for allowing me to join, been looking for a group with people who actually know what they're talking about after so much conflicting advice and opinions on Facebook groups.

Secondly, apologies for the rambling post, but I wanted to avoid as many additional questions regarding the construction etc.

Ok, so I have searched, and I have read Mike's way to build a shed, but I still have questions regarding my build. Alas it's not a workshop, but a pub shed (don't judge me, I am adding a pent roof shed to the side of it once I've completed the pub part lol), but hoping to get some proper advice here.

So, it's a pre built 14' x 8' shed (originally from Timberlux buildings) that I've purchased second hand, dismantled, moved, and reassembled. The walls are 3x2 framework with 22mm barrel board cladding, which is ALREADY fixed to the frame, thus making 4 x wall panels and 2 x roof panels which slot/screw together. It has 4 fixed and 1 opening single glazed windows on each side, which I'm hoping to upgrade later.

The roof is 13mm T&G, covered with roof felt, which I'll be replacing with a suitable alternative.

As it's already constructed, I am unable to wrap it in breathable membrane, despite hundreds of posts in other places of people telling me I need to do this.

I need to insulate, and the framework gives me a 70mm (yes I know, just under 3") gap.

Celotex/ Kingspan etc is out of my budget, so looking at a way to insulate as it will be used hopefully all year round, and will have heating (hopefully wood burner).

After trawling the web for days and days I've come to the conclusion I'm overthinking it, and need a simple option, so I'm looking at mineral felt / rockwool in the framework directly against the exterior cladding (external is being treated with Cuprinol 5 year ducks back paint), then OSB internal cladding, which I've read elsewhere on this forum acts as a vapour barrier due to the glue.

I think I also need to install 2 vents in the eaves, one at each end, to allow air circulation from inside to outside.

I'm hoping this will be sufficient to prevent condensation inside, but cannot get a definitive answer from anyone who actually knows what they're talking about.

Obviously if the OSB alone isn't sufficient for the vapour barrier, then I would use plastic sheeting as well, so:
Barrel board external, mineral felt/rockwool, plastic sheeting vapour barrier fixed to the framework internally, with OSB fixed directly over that.

Next up is if I use loft insulation as intended, I'm looking at 100mm knauf stuff on special offer in B&Q at the moment, but this will need to be compressed to fit the 70mm gaps, so is this better than 50mm which would have better airflow?

I'd appreciate any help or advice that any of you can offer, and once again, yes I've read Mike's post, but reiterate I cannot follow all the advice there due to the shed already being built.

Thank you all in advance, and thanks for bearing with my ramblings lol.

 

[NickVanBeest]

I used "Xtratherm PIR Rigid Insulation Board" followed by 18mm OSB, and it works really well

 

[Carlos0371]

I used "Xtratherm PIR Rigid Insulation Board" followed by 18mm OSB, and it works really well
View attachment 100175

As mentioned, PIR beyond my budget

 

[artie]

Given where you're starting from, I'd seal the cladding as well as possible inside and out, then fill with 50mm polystyrene. It cost just under a tenner per 8 by 4 sheet, last time I bought it. Then line with osb.
I'd put a small vent outside at the top of each partition and a small one at the bottom on the inside.
If you are using a log burner in a 14 by 8 you can afford to lose a bit of air.

 

[Carlos0371]

Given where you're starting from, I'd seal the cladding as well as possible inside and out, then fill with 50mm polystyrene. It cost just under a tenner per 8 by 4 sheet, last time I bought it. Then line with osb.
I'd put a small vent outside at the top of each partition and a small one at the bottom on the inside.
If you are using a log burner in a 14 by 8 you can afford to lose a bit of air.

Thanks for the reply Artie.

The shed is already treated by the manufacturer, the external paint is just belt and braces. Per square metre the poly works out considerably more expensive (currently over £16 per sheet in B & Q), and I've heard it's not great for fire resistance, although I do have the Jablite stuff in the floor.

With my budget constraints, I'm just looking for someone to advise whether my plan is OK, or a no-no. I've read elsewhere that loft insulation compression has a 95% thermal reduction when compressed, but as it's only going to be 25-30mm compression, I was hoping this would be negligible?

 

[RobinBHM]

Maybe look at 75mm cavity insulation

Or 100mm loft insulation, but add on 2 x 1 battens to get the cavity to 100mm - that would give you better insulation. Squashing it isn't a great idea.

The disadvantage you have is no cavity behind the external cladding for airflow.

 

[Carlos0371]

Maybe look at 75mm cavity insulation

Or 100mm loft insulation, but add on 2 x 1 battens to get the cavity to 100mm - that would give you better insulation. Squashing it isn't a great idea.

The disadvantage you have is no cavity behind the external cladding for airflow.

Thanks Robin,

So if I used 75mm, would I need a vapour barrier on the inside, or could I just go with the insulation then OSB on top?

 

[RobinBHM]

Thanks Robin,

So if I used 75mm, would I need a vapour barrier on the inside, or could I just go with the insulation then OSB on top?

OSB is quite a good vapour barrier.

In any case I think a plastic vapour barrier in a shed is asking for trouble....it risks condensation and damp.

The methodology of vapour barriers to prevent interstitial condensation is based on the house always being the warm side....not so true in a shed. It's not uncommon for a shed to be unheated, get very cold during the night, then the sun comes out....bingo we have outside warm, inside cold.

 

[Carlos0371]

OSB is quite a good vapour barrier.

In any case I think a plastic vapour barrier in a shed is asking for trouble....it risks condensation and damp.

The methodology of vapour barriers to prevent interstitial condensation is based on the house always being the warm side....not so true in a shed. It's not uncommon for a shed to be unheated, get very cold during the night, then the sun comes out....bingo we have outside warm, inside cold.

Hmmmmm I see your point about warm outside.

OK, so how could I combat the risk of condensation effectively with my scenario?

As mentioned, it will be heated during the colder weather, but I need protection in the summer too.

 

[TheUnicorn]

just a thought, and not from first hand experience, but have you considered sheeps wool / fleece. I gather that they are virtually worthless to the farmers and so are often given away for little or nothing (at the right time of year of course), breathe well and offer very good insulation. It might be worth looking into, maybe some eco build forums etc?

 

[Carlos0371]

A lovely thought, and eco friendly, but timing is kind of out here, need to crack on and get this shed done now as it's been dragging on for longer than I had hoped.

 

[Jameshow]

Second sheeps wool....

Being a wood shed and doing woodworking I don't think you will have a moisture issue.

The shed walls and timber from your wood working in will absorb any moisture and release it slowly.

Cheers James

 

[RobinBHM]

Hmmmmm I see your point about warm outside.

OK, so how could I combat the risk of condensation effectively with my scenario?

As mentioned, it will be heated during the colder weather, but I need protection in the summer too.

I think your biggest risk is rain water getting behind the cladding. In an ideal world you would have a small cavity then breathable membrane. Without that, I would go for fibreglass or Rockwell insulation which can't rot or harbour damp much and don't compress it.

OSB will limit vapour movement but isn't a cold surface so it wont encourage condensation.

 

[TheUnicorn]

have just been reading around a bit about using unprocessed fleeces for insulation (it was my idea, but it was just something I'd come across in passing, no experience of it). it seems that there are quite a few issues with smell and moths, but a manuafactured sheeps wool product might still be an option, though I'm not sure how it compares cost wise

 

[Carlos0371]

I think your biggest risk is rain water getting behind the cladding. In an ideal world you would have a small cavity then breathable membrane. Without that, I would go for fibreglass or Rockwell insulation which can't rot or harbour damp much and don't compress it.

OSB will limit vapour movement but isn't a cold surface so it wont encourage condensation.

Ok so I'm looking at 75mm rockwool/fibreglass, and OSB internally, no chapter barrier needed?

 

[Fitzroy]

I think you know your idea has compromises vs the ideal, the trouble is no-one can quantify them. Each compromise on the ideal increases the likelihood of condensation or penetrating moisture (wind blown rain, melting ice) remaining for long periods and allowing rot to set in, or getting to the internal finished surface and causing unsightly damp patches in your work environment.

Proceeding with your plan as stated will likely be fine for multiple years, even untreated wood sitting wet will give you five. However as the years push on 7, 10, 15 you’ll start to see the issues with your compromises.

There is also an active post on here about a condensation issue that has made a space unusable after only weeks. However the factors effecting that build are quite specific.

No one can assure you your idea is problem free as else all buildings would be built as simply as your proposing. Folks will likely give you some suggestions to minimise the compromises you have to make. After that you make a decision and history will be the judge. My workshop build was not precisely ‘Mikes Way’ but for each compromise I made the decision based on what I could achieve and the advice given.

Personally for your situation I’d be trying to get a ventilated gap between the insulation and outside boards. This will give the best chance of stuff drying out.

The main problem with loft insulation in walls is it slumps, getting a gap behind something that slumps is difficult.

My 2p worth. Add a 50mm batten internally to each stud increasing the depth to 125mm. Staple in a breathable membrane from the inside into each wall section with a 25mm gap to the boards. Use 75 or 100mm rockwool rwa45 between studs, it will self support. OSB on the inside.

Same on the roof, venting at the eves. EPDM on the roof to replace felt.

Fitz.

 

[johnnyb]

I reckon your method is really poor tbh. what will happen is moisture will pass through the rock wool and condense on the cladding causing mould.

 

[Carlos0371]

I reckon your method is really poor tbh. what will happen is moisture will pass through the rock wool and condense on the cladding causing mould.

Not sure that's the case to be honest, how will moisture pass from the warmth inside to the colder outside if the OSB is acting as a barrier?

 

[Carlos0371]

I think you know your idea has compromises vs the ideal, the trouble is no-one can quantify them. Each compromise on the ideal increases the likelihood of condensation or penetrating moisture (wind blown rain, melting ice) remaining for long periods and allowing rot to set in, or getting to the internal finished surface and causing unsightly damp patches in your work environment.

Proceeding with your plan as stated will likely be fine for multiple years, even untreated wood sitting wet will give you five. However as the years push on 7, 10, 15 you’ll start to see the issues with your compromises.

There is also an active post on here about a condensation issue that has made a space unusable after only weeks. However the factors effecting that build are quite specific.

No one can assure you your idea is problem free as else all buildings would be built as simply as your proposing. Folks will likely give you some suggestions to minimise the compromises you have to make. After that you make a decision and history will be the judge. My workshop build was not precisely ‘Mikes Way’ but for each compromise I made the decision based on what I could achieve and the advice given.

Personally for your situation I’d be trying to get a ventilated gap between the insulation and outside boards. This will give the best chance of stuff drying out.

The main problem with loft insulation in walls is it slumps, getting a gap behind something that slumps is difficult.

My 2p worth. Add a 50mm batten internally to each stud increasing the depth to 125mm. Staple in a breathable membrane from the inside into each wall section with a 25mm gap to the boards. Use 75 or 100mm rockwool rwa45 between studs, it will self support. OSB on the inside.

Same on the roof, venting at the eves. EPDM on the roof to replace felt.

Fitz.

Sound reasoning there Fitz, just really not keen on extending all the framework by another 2 inches either side all around, due to the additional work / cost involved. RWA45 is also prohibitively expensive to my budget. Trust me I've looked at all the options on this, and because of my gaps being various sizes (465, 550, 620, 700mm are just some of the random spacings) I'm really trying to negate waste as much as possible, which I imagine is going to mount up considerably with the batt stuff.

I'm just trying to clarify your details, do you mean install breathable membrane in the gaps between the frame leaving a 25mm gap between that and the OUTSIDE boards? This would be quite fiddly to make sure the membrane stays taut between the frame wouldn't it?

How about using polystyrene insulation as someone has mentioned to me instead?

Also, venting each partition could prove awkward as well, as to vent every partition would require 7 vents each side, and 4 on each of the end panels.

 

[Jameshow]

Another thought.

How about no insulation.

When you put the OSB board on your creating a sealed unit which in and of itself will insulate. Of your working in the shed an oil filled rad and your body heat will warm it.

Or what about foil bubble wrap it's what the camper van community use.

Cheers James