Flat roof ventilation

[Lime]

I see your point, Lime, about long-term damage to the OSB. Unfortunately, it doesn't look like exterior grade OSB, it looks like interior grade. I can only try to dry it all out and hope that it will be OK long term.

If it is an interior grade OSB I'd spray it when dry with a sealer of some sort.
Preferably something that will penetrate the fibres.
Have you considered getting the insulation sprayed in place?
http://www.sprayseal.co.uk/what-is-sprayseal/
Normally I would not think of using it but the sprayed insulation will adhere to the underside of the OSB and to the rafters obviating the need for ventilation of the loft space.

I've taken the insulation out now, and I can see where all the moisture is coming from. Although I taped and sealed the insulation boards, there were some tiny gaps at the edges where they met the wall. The moisture, mostly from the shower and the kettle, must have been working its way up through these tiny gaps. I'm amazed how bad it is, with such tiny gaps. It goes to show the importance of thorough detailing with airtightness!

I've installed some vents in the east fascia, and am drilling through the firrings to get some cross flow. I will be very thorough in sealing it all up again when I put it back.

One of the drawbacks of ventilating the roof is that you enable a difference in air pressure on either side of the tape/seal.
If the wind direction across the roof creates a decrease or increase in outside air pressure that is transferred directly to the inside of the loft space.
Almost certainly a differential pressure will exist between the loft space and living area and will put your seals and tape to the test for the life of the roof.

The problem though , as you say Lime, is how to get it all to dry while I'm living in it.

Yes, your life style may need to be altered until a solution is reached.

 

[Zebra]

I assume your OSB has large inkjet print all over it? If so, somewhere it should say OSB1, OSB2 or OSB3. OSB3 is the exterior stuff.

No, it doesn't have any print on it. It just doesn't look like it's been treated.

If it is an interior grade OSB I'd spray it when dry with a sealer of some sort.
Preferably something that will penetrate the fibres.

That sounds like a good idea. Some sort of exterior timber sealer, which protects against fungus and rot, perhaps? Should the wood be able to breathe though?

Have you considered getting the insulation sprayed in place?
http://www.sprayseal.co.uk/what-is-sprayseal/
Normally I would not think of using it but the sprayed insulation will adhere to the underside of the OSB and to the rafters obviating the need for ventilation of the loft space.

I'm not sure about this, as you said yourself:

My gut feeling is that it isn't healthy in a cold roof to have insulation directly under OSB because it needs to breath/dry if it gets damp. Insulation will hold any moisture in the OSB.

I'm really not sure.

Almost certainly a differential pressure will exist between the loft space and living area and will put your seals and tape to the test for the life of the roof.

I hadn't considered that. How come everybody with a flat roofed extension doesn't have this problem!

 

[Lime]

To a certain extent the type of OSB has sealed the fate of the roof were a leak to occur from the roof itself, whether the roof space is ventilated or not.
I do not think you will be able to provide satisfactory ventilation to adequately dry out saturated OSB caused by a roof leak if one were to occur.
By the time you become aware of a leak the damage will be done to the OSB and that may lead to serious sagging and consequent ponding on the flat roof or worse.
(I do not know how rigid the fibreglass finish actually is.
Nor do I know how accommodating it may be if sagging of the OSB were to occur.
Much depends on how well it was applied and how it is treated subsequently... it should be treated as a no go area unless the surface is protected.)

You also need to keep in mind that damp/saturated OSB will expand and disintegrate eventually in the freeze/thaw cycle of a normal winter.

If I were to use the sprayed insulation it would be with the assumption that the roof will maintain integrity in the foreseeable future and keep the OSB free from water ingress.
Any leaks could lead to major problems whether the roof space is ventilated or not.
Some compensation may be gained by the fact that the sprayed insulation may also offer some support to the OSB if it were to become soft.

How come everybody with a flat roofed extension doesn't have this problem!

Many do from the start albeit unsuspected or discovered.
Their life expectancy is often short compared to conventional roofs.

 

[Zebra]

So let's assume the OSB is toast in the event of a leak, whether ventilated or with spray on insulation. But hopefully the roof won't leak unless it is otherwise subject to movement or stresses. Presumably if it were ventilated and condensation didn't occur because I got the airtightness really right and moist air couldn't get up there, then it stands half a chance. I will certainly lose sleep in the future over whether it's going mouldy up there though.
Would you mind, Lime, saying what sort of product you would recommend to treat the timber with? I'm well on the way to getting it dry using an electric heater.
Thanks!

 

[DJH]

I've been reading along with this topic as it touches on part of my earlier life when I was involved with the failure of lead sheet roofs on both new and historic buildings. Invariably the failure of the lead was due to condensation which, trapped on the underside of the sheet, forms a white carbonate as the moisture is, in effect, distilled water which corrodes lead. Most of the problems came about because of changes of use of the buildings, but that's another story.

The amount and speed of this corrosive action depends on the frequency of the condensation conditions occurring, which can be when the air from below has a high level of warm moisture and the external temperature of the roof changes. I was involved with research into the failure of lead roofs many years ago and the method used to prevent it happening.

I agree with Lime's comments and I think he has summed up your problem exactly. There are just a few other things I would like to mention one of them being that I consider a roof with anything but a traditional covering as temporary.

Lime has mentioned the effect of wind in roof causing pressure variations. Sub atmospheric conditions can occur in a roof space when the roof is subjected to high temperature from sunshine and then suddenly cooled by a shower of rain. This can happen frequently and, when it does, can lead to thermal pumping when as the pressure drops, moist air can be sucked exacerbating the damp conditions and further wetting the structure under the roof, which in effect, your OSB board.

There is another little known cause of condensation, clear night sky radiation, which can cause condensation under a roof covering. As with any roof structure which is to be covered like yours Zebra, the structure has to be as dry as it possibly can be. Timber left in the wet or soaked by rain during construction will give conditions for condensation above it at some stage.

Interstitial condensation can also occur and, if there is inadequate ventilation, as we know there is on your roof, the moisture will affect surrounding timbers especially the OSB you have. It can occur in the space between your fibre glass roof and any underlay beneath it and if the decking was at all damp when it was covered it will condense.

One day, maybe soon, you may have to replace your roof and it's covering. When you do I recommend you use a ventilated warm roof where a vapour barrier is positioned above the joists, rigid insulation above that with a breather membrane over that. The ventilation area is formed by 2" square battens over this with the roof decking on top. I'm not a lover in anyway of any laminated boards for roofing whether it's so called marine or water resistant. The only way to reduce interstitial condensation is to ventilate the underside of the roof covering, whichever one you decide to use, and use planed square edge boarding with a penny gap. The air circulating below the boarding and above the breather membrane will dissipate any moisture which might occur due to the reasons I've mentioned.

My objection to proprietary boarding is firstly that it doesn't allow air to the underside of the roof covering unless you drill holes all over them and the amount of glue or whatever is used to bond the fibres or sheets together gives off vapour. This can be proved by placing a piece of new lead on a board overnight and seeing the resultant white carbonate coating on it the next day.

Doug

PS Sorry, I banged on a bit there......

 

[MdB]

Interesting. Thanks.

 

[Zebra]

Thanks for all that information, it's very interesting.

One day, maybe soon, you may have to replace your roof and it's covering.

The flat roof was only built this summer, so I would hope it wouldn't be so very soon. I would have loved to have had a traditional pitched roof on the front of the house, but I'm afraid Planning would not allow any increase in volume at all.

I'm sure a warm deck roof would be much better, and I hope others can learn from my mistake, but cold deck flat roofs have been around for a long time. My Mum's utility room has one that's not had a problem in 40 years. My old one didn't form condensation. What's the secret to making it work? Do you think I would be better off with spray on insulation under the rafters?

 

[DJH]

The secret to making it work Zebra is ventilation, ventilation, ventilation!!

Cold roofs have been as you say around for many years and I can only assume that those which have lasted are well ventilated (intentionally or not) and do not leak. Traditional materials such as timber decking are far more durable than compressed sawdust or wood flake boarding is and if rot was occurring in the timber for any reason it's invariably just that particular plank and maybe it's near neighbours suffering while the rest of the roof maintains it's integrity.

I have no experience of insulation filling the void of a roof up to the underside of the decking but I would be reluctant to use that method simply because the cellular structure of insulation will have air in it which in turn when heated by the sun from above could cause interstitial condensation.

I have to agree with Lime and say that I suspect water is gaining access under the decking some how. How are your flashings?

In haste as Mrs DJH wants to go out...... :shock:

Doug

 

[Lime]

............
Do you think I would be better off with spray on insulation under the rafters?

That must depend on how certain you can be that moist air from the house is not going to enter the roof space.
The sprayed insulation would give a greater certainty that moisture will be kept from the OSB.
On the other hand if you are able to provide a good barrier to moisture it may be an additional expense that you can do without.
It is worth bearing in mind that the average ceiling properly installed will provide quite a good seal against moisture and prevent air easily passing into the roof space... just make sure there are no holes left in it under light fittings etc.


With regard to your earlier question about sealing the OSB.
I suggest a varnish or similar spirit based sealer diluted with a suitable thinner to make it more penetrating.

 

[DJH]

OK, I'm back from the shopping expedition so carrying on from my last post I have to say that from the description you give of the amount of wetness on the underside of your OSB boarding that a leak in the roof covering has to be the main suspect Zebra. Is there any ponding on the surface of the roof covering and if there is is it anywhere near a seam or joint?

Thinking more about filling the roof void with a sprayed on insulation between the joists, if the decking boards aren't perfectly dry, and that includes the top of the board beneath the fibre glass covering, any moisture trapped there by the insulation will undoubtedly cause a problem from then on.

As Lime says, the average ceiling properly installed, for example foil backed plaster board in conjunction with a separate vapour barrier such as polythene sheet, the amount of warm moist air penetrating into the roof structure above will be negligible. You have to bear in mind though that there could well be a source of moisture from the wall at the abutment to the house as I don't expect you have cavity trays at the flashing line above your roof which would prevent moisture seeping down the wall. Lead flashings when properly fitted, will prevent rain from getting into the roof at the junction of the roof with the wall but they won't prevent wind driven rain from soaking the wall above the flashing percolating down and causing dampness below the roof deck and between the joists.

This dampness, if present, could give rise to ideal conditions for condensation to occur and with no ventilation to dissipate this moist air the cycle will just go on and on .
I would also urge caution about sealing the underside of the decking just yet if at all unless you can be absolutely certain that it is dry between the underside of the fibre glass and the top of the OSB board including any underlay. You could be just sealing in moisture exactly where you don't want it.

A simple way of demonstrating what happens with a sealed space when subjected to heat and cold is when an inflated balloon gets smaller when cold and how it returns to it's original size when it warms up again in the sun for example.

That's what I think Zebra for what it's worth.

Doug from windy Tipperary storm force 9 they said it could reach.. :shock:

 

[Zebra]

Firstly may I thank you both for your patience and for your willingness to chip in and help on a subject which, as I said at the outset, is not strictly related to old houses.
No, it's not a leak. I can see exactly what's happening. I sealed all the joins between the foil backed insulation boards, except where it met the wall, and this is where the steam from the shower and the kettle has been finding its way up. It's too general a problem, over too large an area, to be a leak. If it were a leak it would, I'm sure, be more concentrated. No, there's no ponding, and it's been raining quite heavily and the problem is getting better not worse.
Yes, I can see that timber decking would have been much much better than "compressed sawdust or wood flake boarding" as you aptly put it. Still, it's there now. I allowed the contractors concerned to do what they usually do rather than specify something better. We live and learn.
I'm glad you shy away from the idea of filling the void to the top with insulation (and give a good reason why) because that idea doesn't seem to sit quite right with anybody so far.
So, ventilation, and sealing the moisture from beneath are the key. This I think I can achieve. I am creating much more cross-flow by installing vents on the east side, and peppering the firrings with holes (not the beams, clearly, but the firrings, which are not under stress only under compression). When I put the insulation back I shall use silicone, squirty foam or foil tape to seal around all the edges and around the cables that come through. (I'm so glad I was insistent on not having downlighters!) The plasterboard will help also, so it's a question of keeping the moisture out until that's done.
The abutment to the house I don't believe is a risk area. No, there are no cavity trays but a similar arrangement as the lead flashings go all the way through the wall to the timber frame - it's a detail specific to the woodfibre insulation boards. However, there is an internal wall in the middle of the flat roof - could moisture from indoors soak into that wall and wick upwards? Hopefully that would be at such a low level that the increased ventilation would cope with it.
As far as sealing the underside of the decking is concerned, I shall wait until the end of a hot sunny week before doing anything to it. So you think varnish - an actual physical seal - rather than just an anti-fungal wood preserver? Yes I suppose that would be the very thing, if I can be totally sure that the boards are totally dry when it goes on!
Much reassured,
'With thanks,
Deb

 

[Flyfisher]

I know that flat roofs generally have a bad reputation for longevity but our previous house had a small section of bitumen-felt roof on a single floor extension and we never had any problems with it in the 20 years we lived there and I think the roof was actually built in the early 80s, so that suggests about 30 years, which doesn't seem too bad to me. Perhaps we were lucky?

 

[DJH]

No problem Zebra, underside condensation leading to corrosion of lead sheet was more my line. Perhaps you could consider offsetting the holes you drill in the firrings to improve circulation of the air so that it doesn't sort of blow through in a straight line so to speak.

Doug

 

[Zebra]

Have spent the day scrubbing the underside of the offending flat roof deck with a fungicidal wash and have got it really clean. Have drilled holes in all the firrings and installed extra soffit vents. Have bought some well-known-brand penetrating timber preservative to go on once it's dry, then will give it a top coat of either varnish or bitumen (recommended by a local builder's merchants....). Have closed the front extension off from the main house again so that the flat roof area is now not heated at all. Have spent a while this afternoon, cup of tea in hoof, pondering the possibility of moving the shower into the shed....
Feeling rather better, but what a lot of faff and hassle when the job could have been done better in the first place.
Still, nobody to blame but myself....