Minimising condensation on outbuilding roof?
Discussion
I'm I'm planning on replacing an old corrugated iron roof on an enclosed outbuilding. The current roof gets a lot of condensation on it even though there is plenty of ventilation at the eaves. Whatever I use as a replacement I want it to be as condensation free as possible in an unheated out station.
One part of the outbuilding actually has a wooden roof with the corrugated iron above it and this does not seem to have the same condensation issue as the bare corrugated iron elsewhere. It's a much more usable/dry space.
The roof is largely flat so I was thinking of putting 18mm OSB3 down then EPDM rubber membrane above (where it's largely flat ish). Any better ideas? Anything else I could add to reduce condensation (like a breathable membrane below the OSB3)?
I know I can add insulation between the rafters later if condensation becomes a big issue.
Thanks for any advice.
One part of the outbuilding actually has a wooden roof with the corrugated iron above it and this does not seem to have the same condensation issue as the bare corrugated iron elsewhere. It's a much more usable/dry space.
The roof is largely flat so I was thinking of putting 18mm OSB3 down then EPDM rubber membrane above (where it's largely flat ish). Any better ideas? Anything else I could add to reduce condensation (like a breathable membrane below the OSB3)?
I know I can add insulation between the rafters later if condensation becomes a big issue.
Thanks for any advice.
The air gap between the corrugated and the OSB means it will outperform OSB with no air gap..
Insulation board is quite cheap unless your outbuilding is huge!
A low heat capacity surface will be better than a higher heat capacity surface for a given thermal conductivity, so any sort of insulation board or foam generally scores.
You may want to take care of condensation above the insulation which can be trapped by the insulation and rot the OSB or whatever.
The other thing is, if there is excess moisture, to some extent it will condense 'somewhere' and that's the coldest surface. Take away the wrinkly steel and it will condense somewhere else? (gross simplification but food for thought?).
I used to have a shed with a metal door, the door attracted all the moisture which then dripped outside the building.
With boats, we always say the first thing to do is eliminate the water at source. Zero tolerance of rain leaks, dry up any bilge water etc. With a shed, check the floor is not damp and ideally has a DPC. When the sky is colder than the ground is prime time for water vapour coming out of the floor and hitting the ceiling!
Insulation board is quite cheap unless your outbuilding is huge!
A low heat capacity surface will be better than a higher heat capacity surface for a given thermal conductivity, so any sort of insulation board or foam generally scores.
You may want to take care of condensation above the insulation which can be trapped by the insulation and rot the OSB or whatever.
The other thing is, if there is excess moisture, to some extent it will condense 'somewhere' and that's the coldest surface. Take away the wrinkly steel and it will condense somewhere else? (gross simplification but food for thought?).
I used to have a shed with a metal door, the door attracted all the moisture which then dripped outside the building.
With boats, we always say the first thing to do is eliminate the water at source. Zero tolerance of rain leaks, dry up any bilge water etc. With a shed, check the floor is not damp and ideally has a DPC. When the sky is colder than the ground is prime time for water vapour coming out of the floor and hitting the ceiling!
Scarletpimpofnel said:
I'm I'm planning on replacing an old corrugated iron roof on an enclosed outbuilding. The current roof gets a lot of condensation on it even though there is plenty of ventilation at the eaves. Whatever I use as a replacement I want it to be as condensation free as possible in an unheated out station.
One part of the outbuilding actually has a wooden roof with the corrugated iron above it and this does not seem to have the same condensation issue as the bare corrugated iron elsewhere. It's a much more usable/dry space.
The roof is largely flat so I was thinking of putting 18mm OSB3 down then EPDM rubber membrane above (where it's largely flat ish). Any better ideas? Anything else I could add to reduce condensation (like a breathable membrane below the OSB3)?
I know I can add insulation between the rafters later if condensation becomes a big issue.
Thanks for any advice.
I had a similar issue. An ancient concrete garage down the end of the garden with a corrugated roof that was wet with condensation all winter. All I did was baton it with 2x2, lay some cheap 50mm insulation boards (not pir some kind of cheap flooring sheets) between the batons and then reroofed with some new corrugation. Bone dry this winter. One part of the outbuilding actually has a wooden roof with the corrugated iron above it and this does not seem to have the same condensation issue as the bare corrugated iron elsewhere. It's a much more usable/dry space.
The roof is largely flat so I was thinking of putting 18mm OSB3 down then EPDM rubber membrane above (where it's largely flat ish). Any better ideas? Anything else I could add to reduce condensation (like a breathable membrane below the OSB3)?
I know I can add insulation between the rafters later if condensation becomes a big issue.
Thanks for any advice.
OutInTheShed said:
The air gap between the corrugated and the OSB means it will outperform OSB with no air gap..
Insulation board is quite cheap unless your outbuilding is huge!
A low heat capacity surface will be better than a higher heat capacity surface for a given thermal conductivity, so any sort of insulation board or foam generally scores.
You may want to take care of condensation above the insulation which can be trapped by the insulation and rot the OSB or whatever.
The other thing is, if there is excess moisture, to some extent it will condense 'somewhere' and that's the coldest surface. Take away the wrinkly steel and it will condense somewhere else? (gross simplification but food for thought?).
I used to have a shed with a metal door, the door attracted all the moisture which then dripped outside the building.
With boats, we always say the first thing to do is eliminate the water at source. Zero tolerance of rain leaks, dry up any bilge water etc. With a shed, check the floor is not damp and ideally has a DPC. When the sky is colder than the ground is prime time for water vapour coming out of the floor and hitting the ceiling!
Thanks some good advice there. Yes I aim to remove all the current sources of damp and hope that the new roof will then stay dry inside. If not I'll look at the insulation boards you mention but I am not sure how I'd prevent damp getting between them and the OSB3 in the fullness of time which might be worse than just leaving the OSB3 bare on its underneath side to breathe.Insulation board is quite cheap unless your outbuilding is huge!
A low heat capacity surface will be better than a higher heat capacity surface for a given thermal conductivity, so any sort of insulation board or foam generally scores.
You may want to take care of condensation above the insulation which can be trapped by the insulation and rot the OSB or whatever.
The other thing is, if there is excess moisture, to some extent it will condense 'somewhere' and that's the coldest surface. Take away the wrinkly steel and it will condense somewhere else? (gross simplification but food for thought?).
I used to have a shed with a metal door, the door attracted all the moisture which then dripped outside the building.
With boats, we always say the first thing to do is eliminate the water at source. Zero tolerance of rain leaks, dry up any bilge water etc. With a shed, check the floor is not damp and ideally has a DPC. When the sky is colder than the ground is prime time for water vapour coming out of the floor and hitting the ceiling!
OutInTheShed said:
I'd consider a gap between the OSB and the insulation board, breathing to outdoors.
So long as you don't have an impermeable layer holding water against the bottom face of the OSB, I expect it would last many years.
OK makes sense thanks. I'll try just the EPDM and OSB3 and if condensation is still an issue then I may look at an insulating layer. But I think with all the other changes I am making the sources of moisture will be eradicated.So long as you don't have an impermeable layer holding water against the bottom face of the OSB, I expect it would last many years.
Magooagain said:
Something like this could do the job.
https://www.roofingoutlet.co.uk/products/cladco-32...
Thanks. I did look at that. Not sure how effective it is compared with having a "warm" layer like OSB3 under the external roof?https://www.roofingoutlet.co.uk/products/cladco-32...
Scarletpimpofnel said:
Magooagain said:
Something like this could do the job.
https://www.roofingoutlet.co.uk/products/cladco-32...
Thanks. I did look at that. Not sure how effective it is compared with having a "warm" layer like OSB3 under the external roof?https://www.roofingoutlet.co.uk/products/cladco-32...
If you want to leave the existing roof in place then just baton above it, insert polystyrene or a similar cheap insulation sheeting and then fix the new roof on top extending the overhang a little.
You really don't need much insulation to prevent the roof condensation but if fixing it to the underside, as a cold roof solution I think you'll still be getting condensation but you won't see it and it won't dry off as quickly.
Personally, for a cheap outhouse that is basically a storage shed, I've found that the battening over the original roof, filling the space with the cheapest form of sheet insulation and then placing an inexpensive but suitable sheet roof on top works everytime and is by far the cheapest solution.
I have a couple of outbuildings with corrugated tin roofs which suffer badly from condensation at this time of year so I wanted to avoid the issue completely when I built my new workshop.
I boarded out the roof with sarking boards (rough sawn 150mm x 22mm pressure treated planks) and then put on a decent roof membrane. Then counter batten with treated timber before putting on the metal sheets.
This means you create a ventilated gap under the tin that drains down to the eaves if any condensation does form. Important that this gap is open at the top and bottom to allow cross ventilation to let the condensation dry out.
Completely bone dry despite often parking a snow covered warm tractor etc in there.
I boarded out the roof with sarking boards (rough sawn 150mm x 22mm pressure treated planks) and then put on a decent roof membrane. Then counter batten with treated timber before putting on the metal sheets.
This means you create a ventilated gap under the tin that drains down to the eaves if any condensation does form. Important that this gap is open at the top and bottom to allow cross ventilation to let the condensation dry out.
Completely bone dry despite often parking a snow covered warm tractor etc in there.
Edited by Snow and Rocks on Thursday 12th February 12:44
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