This cold weather has really shown up a problem with an unheated converted agricultural implement shed. The building is used to store generators, lights and small tools and has had a plasterboard ceiling added, to stop dust and bird crap spoiling the kit.
The gable ends are ventilated and the roof is simple 4 by 2 rafters with corrugated iron, the corrugations vent at the eaves and the ridge capping.
This roof is plainly a good radiator on cold nights and vapour from ambient air is condensing, and sometimes freezing, on the inside then dripping onto the plasterboard below.
I'm considering stapling tyvek in between the rafters or even full length under them but worry about the effect this will have on old untreated roof timbers?
You could simply increase the ventilation in the roof space above the plasterboard just as much as is needed to stop the condensation.
But if you want to keep the area beneath the plasterboard ceiling "warm", in comparison to the space between the plasterboard and the tin roof above, then consider adding a strong thermal barrier against the ceiling. (And this may possibly stop daylight warmth escaping from the tool area into the roof-space, so that you then won't have to increase the ventilation up there.)
here is good ventilation as the gable ends have the equivalent of 4 air bricks in each end and the eaves and ridge have the area under the corrugations. we have a lean to adjacent which is open fronted with no ceiling and the condensation is visible on the inside of the roof.
In anticipation of wishing to warm the area when maintenance is done I did put 100mm fibreglass insulation between the joists above the plasterboard when it was fitted in March and it is this that is getting wet from the drips.
Not sure what tyvek is but it sounds expensive. Why not use normal breathable roofing felt? - it's cheap and can be stapled to the joists just the same and will also cut down the amount of condensation reaching the tin sheeting, with the added bonus of allowing the old timbers to breathe.
Yes this is what I was asking, would it be better to staple it in between the rafters or staple it across the rafters horizontally, as one would fix slater's felt on the top side of the rafters.
I have used the proper Tyvek, under profiled plastic coated steel sheet, on rooves where we have replaced a certain fibre reinforced corrugated concrete sheet, and this seems to be working fine but this is on top of the rafters and in loose but direct contact with the steel sheet.
This is a hard one, and it's not simply a question of ventilating which could even make it worse. As you correctly identify, the roof is likely to be colder than the (outside) air. Ventilating more (outside) air under it is just going to create more condensation. The only circumstance in which ventilation would help is if you are generating significant moisture inside the building which is getting into the roof (and this could simply be a moist floor which is staying warmer at night).
You really need a layer of air-tight insulation on the underside of the metal roof, with no air gap or circulation inbetween. That's not viable with roofing timbers in contact with the metal.
Spray on would be the obvious way to stop air getting to the underside. Depends what is in the vicinity. I remember dealing with a claim once for about 100 cars that got affected by a contractors efforts. We paid a local bodyshop so much per car to clean them up. Only a few needed painting.
Andrew, I am surprised by some of the advice you have been given by other posters above. I studied the whole business of insulation two years ago when we were refurbishing this house by importing an excellent book from the States. What I learnt is that condensation will occur on the underside of roof-tiles, tin-roof, or the felt or canvas directly beneath it if the air in that space is not the same temperature as the air outside. Hence, the advice was that one must make absolutely certain that your attic has good and free ventilation. Now, obviously, beneath the ceiling you want to keep warm, so beneath the ceiling you want none of that cold that is directly above your ceiling. Hence a highly effective thermal barrier must be created a ceiling level.
From my understanding all you will achieve in the circumstances you have described by adding canvas adjacent to your tin-roof is that the condensation on the underside of the tin will be caught by the canvas and dribble downwards to the eaves. However, you are likely to get much the same problem on the underside of your canvas! In fact, I distinctly remember that being one of the issues of one of the chapters: how do you ensure you do not get condensation on the underside of the canvas over which your roof-tiles are laid? Answer: increase your ventilation.
You say you have four air-bricks in each gable, but are these actually allowing free and immediate movement of air? We were so concerned about this matter when this place was being refurbished that we forced our builders into putting one bloody great big vent, 12x12" at one gable end of our roof. It's as good as 1-foot hole but for mesh which stops birds etc. getting in. It's damned cold up there in our attic, as cold as it is out in the garden, but we don't get condensation on the underside of our canvas.
The odd thing though is your saying that you have a totally open lean-to with a tin roof and that there is condensation on the underside of it! Do you live in a very moisture-laden area? Are you in a dingle in the hills of Wales, Cumbria, or Scotland? Do you have a stream and/or ponds in your garden? Are there constant mists? If you have a significant condensation problem inside a totally open lean-to, I don't think there's much you can do, except lay some large plastic sheeting over the upper-surface of your plaster-board ceiling. Surely there isn't enough condensation to drop onto such and form massive puddles that would need to drain away? You might find that the moisture that gathered on the sheeting will also be constantly evaporating.
I suggest trying this easy and inexpensive route first. Nip into Wilko and buy a plastic-dust or two first. They're massive and they only cost £1. Lay it over your plasterboard, wait a month, and observe.
Fine, so start by laying a large dust-sheet over the insulation. If you need to use more than one sheet, edge it upwards where they meet so that you create two potential "paddling pools". And then wait a month and observe.
The thought I had was that there would be a limited amount of air to supply extra moisture in the gap between tyvek , rafter and tin, the fear would be this staying damp between the tin and timber and causing rot. Most of the time it would be a ventilated space and become dry, the problem only arises on cold nights. a limited amount of vapour would pass through the tyvek also. Whether the tyvek would become cold enough itself for droplets to form I guess I'll only find out by trial and error.
It's deciding whether to risk the tyvek applied to the rafters or attempting to staple it up at the joint twixt timber and tin.
I've considered this, it is expensive, makes for maintenance problems as well as difficulty in recycling the roof (buildings are conversions and only limited life) and doesn't embedding the timbers in foam risk rotting?
That information is wrong, and is key to the OP's problem. Condensation forms on the underside of the roof if the _roof_ is cooler than the air. The roof will often be cooler than the outside air (if no cloud cover, it radiates it's heat into space, and cools much faster than the air). If the same air ventilates under it, then it will be cooler than the inside air too, and hence the condensation, which will be worse the more ventilation (air changes) there is.
That works for a heated house, but not for an unheated shed.
You mustn't put a moisture barrier on the cold side of permeable insulation. It will form condensation from the moisture in and blow the insulation and slowly saturate the insulation with water, which has the opposite effect than intended.
I've seen that very thing on our attached "mistal" after building works - seems oK now things have settled (1+year on)
do experiments? doubt you'll discover which bodge works *best* unless you do... i'd do em loose so your membrane "sags" down from rafters at sides so any moisture collected drains away from rafters rather than soaking them...a la sarking felt in traditional roof structures...
I think risks of rotting are probly "in the mix" whatever you do "bodge wise"..
If you don;t want to bodge it then I can't think of any alternatives apart from take the roof off and redo it "properly"?
BSRIA (I think it was) published a research paper on condensation the underside of steel roofs. It was a common occurence in 1970s buildings. The metal roofs radiate a lot of energy into the sky on a clear night, causing them to cool far below the ambient air temperature.
I came across the paper about 20 years ago. I can't recall what the recommendations were though.
This is intriguing. Can a *black body sink* extract energy in this way?
It is possible to buy roofing sheet with the anti-condensation finish pre-applied. I've not seen any but believe it is a *felt* which retains the condensate until it is evaporated by the next bit of sunshine. This is said to be the way cement fibre roofing works. Retro-fitting might not be practical.
I doubt Andrew has funds or authority to renew the entire roof. One issue will be that the roof sheets may be secured to purlins rather than rafters so attaching membrane to the underside will tend to run water down to the timber. I have heard that trapping a layer of relatively static air can work to minimise condensation but at the risk of encouraging rot in the wood:-(
More ventilation? The moisture content needs to be below the level at which *dew* forms or zero in a frost!
Dead right on both counts, anyway being on enforced holiday and then 4 day week I'm in no hurry to spend any money!
There are no purlins, it's screwed directly to un treated rafters.
This is my worry but I will try a section with tyvek, the difficult decision is whether to spend the considerable extra time laying the material up to down between the rafters, tight to the roof. With the hope any runoff will be down the tyvex and out via the eaves.
Yes extra ventilation is a red herring in this case. As the roof is a good radiator I had considered a split aircon unit in to extract heat on sunny days but the cheap ones don't allow a difference in height between the two units.
BTW even ice has a vapour pressure (that's how freeze drying works in a vacuum).
When I had some slates off our roof I noticed that the roofing felt (laid over the rafters) still sagged enough to cause potential puddling where it lifted over the gutter board. Difficult to imagine doing this as retrofit down near the eaves!
When we bought the yard opposite the farmhouse, we found lots of office type suspended ceilings. T section steel hung on wires with 4' x 1'6" insulated slabs between. Long gone now but a possible cheap source of fireproof rigid insulation.
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