Flat roof ventilation

[Zebra]

Now this is not strictly speaking a Period Property issue, so forgive me if it's naughty to ask you. I've tried the normal DIY forums, and got the answer, I'd just like to see what you lot make of it, because, well, I trust you lot a good deal more than those folks on those other forums.
The issue is condensation on the underside of the new flat roof deck. There is a 50mm gap below the deck, then 100mm of rigid foam insulation between the rafters and 25mm of well-sealed-and-taped rigid foam insulation under the rafters. There are circular soffit vents in the fascia on the west side, and soffit-strip ventilation under the tiled detail on the south side. Nothing on the east side. Joined to the house on the north side. Clearly, not enough cross-ventilation.
Now the answer, it would seem, is to fully fill the voids between the rafters with rigid foam insulation, and do away with the gap altogether. If there is no air, then there is no moisture brought in, and no condensation, so the thinking goes.
Doesn't seem right, what do you all think?
PS, for those still reading, to further complicate things, rafters run one way in one half, and the other way in the other half, so whichever way you ventilate it, you run up against a section going the other way.
I could install a number of mushroom vents on the flat roof, a bit ugly.
To install vents in the fascia on the east side would make no difference, because there is a rafter right up against the wall, so the air wouldn't get through.
I could pepper the rafters with holes to allow more of a cross-flow, but this weakens the structure.

Sorry about that, I'll now return you to your normal programming, and get back to posting about limewash and laths and beams. And chickens. And Kit-kats.

 

[MdB]

Our new roof is 100% solid with no ventilation. From bottom to top it is sterling board, vapour barrier, insulation, felt. It is what comes recommended and seems sensible to me.

Condensation occurs when warm air gets cold so you need a vapour barrier on the hot side of the insulation to stop this happening. I'm not sure how this is happening in your set up but if you can work this out, it may be point you in the direction of a solution.

 

[DJH]

Zebra, you don't say what the material is covering your flat roof, it's pitch and how many walls form abutments to it and how is the 50 mm gap formed. Does it have counter battens or cross battens on the rafters to support the deck? Is there any ventilation detail at the head of the roof at the abutments to allow air flow from the eaves? A picture would help of course.

Doug

 

[Lime]

If there is any new brickwork under the flat roof it is worth remembering it will give off a great deal of moisture until it dries out.

 

[Lime]

If there is any new brickwork under the flat roof it is worth remembering it will give off a great deal of moisture until it dries out.

This was built in the summer, wouldn't it have dried out by now?
Very grateful for any advice!

Whether it has dried out or not depends on how much water was in the brick to start with and what opportunity for drying the brickwork has had.

Put it this way, I replaced some brickwork on the outside of my house where a kitchen drain had broken out (Edwardian plumbers, removed in mid 60s improvement) over 6 years ago.
Only recently has the lime mortar become white indicating it is now dry!








edit for spelling

 

[DJH]

Using firring pieces on top of the rafters is the problem here Zebra. Different size battens across, or counter to, the rafters sized to give the required fall would have been the better way of construction thus allowing air to circulate and move over and between the rafters along with an abutment ventilation detail but that's too late now. If you go down the route of 'mushroom type vents then you would ideally need to place them over each space between the rafters which, as a retrofit, could leak and would of course be unsightly.

Drilling holes in your rafters would help the situation but to work properly the number and size of the holes required to allow air to percolate would no doubt weaken them. Can you introduce any ventilation from the East side at all? I think you are going to have to consider venting at the North wall to solve your problem. Do you have a picture of this junction you could post? I have somewhere a detail of a continuous vent which I used to use for lead roofs. I'll dig it out if I can.........

Doug

 

[Zebra]

Thanks for your advice. Hmm, yes, rather as I feared. Introducing ventilation at the east side has two obstacles. One problem is that once you cut holes in the fascia, then you'd have to drill through the the wall outer leaf itself, and then through the first rafter too. Then the rafters run north to south anyway, so you'd still have to drill through all those rafters to get the cross flow.
As for introducing ventilation at the abutment, I'll have to do you a sketch of the junction, it's complicated. I'll come back to you.......

 

[Zebra]

Here is the abutment detail.



The lead is fixed down to the upstand on the fibreglass roof with (sorry Doug!) a sticks-anything adhesive. Not shown is the 100mm of celotex between the rafters, with the 50mm gap above it.

Thanks for any thoughts!

 

[Zebra]

the answer, it would seem, is to fully fill the voids between the rafters with rigid foam insulation, and do away with the gap altogether. If there is no air, then there is no moisture brought in, and no condensation, so the thinking goes.
Doesn't seem right, what do you all think?

So does anybody have any thoughts on this? This is the link to the site where I heard of it:
http://www.diynot.com/forums/viewtopic.php?p=1309467#1309467

 

[DJH]

The detail shown in that link with along with some more detailing would be the one I'd use Zebra .

Glue for lead?? Where can I get some? :lol:

 

[MdB]

I still don't see where the moisture is coming from. If your insulation is all taped up and with two layers, I wouldn't expect much warm air to get into the roof space. Your new lime might be producing some but this would go with time. Or you may have a leak....

I think personally I would leave if for 6 months or so. If it is the construction just drying out then the problem will go away. If there is a leak then you want to know about it and not hide it. If it is moisture coming up from below then plaster boarding your ceiling may sort the problem.

 

[Zebra]

To be honest MdB, I don't know where the moisture is coming from either. It can't be coming from underneath. I've taped and sealed the foil-backed celotex which is under the rafters, so it can't be coming from indoors. I haven't plasterboarded yet, and won't until this is sorted. It must be coming from outdoors. I don't think I have a leak. I think that moist air blows in from the only vented sides, the south and the west, which is where the prevailing winds generally are, and then doesn't escape again but condenses in the gap.

So DJH, you think it would be OK to fully fill the gap with celotex, and do away with the ventilation gap?

I've so far adopted my usual philosophy of "try a bit and see," and have wedged some celotex up tight to the deck in the bit that's exposed, and will give it a while and see if it's drier or wetter than elsewhere.........

 

[worms]

Sorry if I have missed something here, but how do you know it is getting damp in there? If it is all finished and sealed, you surely can't see in? Are there damp patches appearing?

 

[Lime]

Sorry if I have missed something here, but how do you know it is getting damp in there? If it is all finished and sealed, you surely can't see in? Are there damp patches appearing?

That is just what I was wondering.
How is it possible to see the water on the underside of the roof?
From the description of the roof construction it appears ventilation wasn't considered with any seriousness.
Ventilation should have been considered at the design stage.

The damp could be entering in the form of a leak. In such a roof it is difficult to be sure it is not a leak.
Adding foam into the air spaces if it is a leak will make matters much worse.

To be fair in this very cold weather I'd expect some damp under a flat roof particularly if it isn't exposed to sunlight.

I don't think it will be easy to ensure a complete fill of the air gaps with foam insulation either.
As the plasterboard is not in place I suppose it would be possible to inject large quantities of foam through the insulation and into the air gaps.
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.
The suggestion to drill large ventilation holes in the beams is quite wrong, dangerous and could lead to the roof collapsing under heavy snow.

 

[Zebra]

Sorry if I have missed something here, but how do you know it is getting damp in there? If it is all finished and sealed, you surely can't see in? Are there damp patches appearing?

I forgot to wire for the thermostat, so I had to take some of the insulation down from the ceiling. There is moisture, not much, on the bottom of the OSB board, all over, not in a specific patch as if it were a leak.

 

[Zebra]

From the description of the roof construction it appears ventilation wasn't considered with any seriousness. Ventilation should have been considered at the design stage.

Well, the builder, my brother, thought it would be adequate. There are small air gaps between rafter sections where the roof meets the house, where the firrings are slightly shorter than the rafters, and he thought the air would find its way through. I also drilled some 20-25mm holes through the firrings every two feet or so (he thought I was mad and going completely over the top) to let the air through. So there is a path, albeit a small one, where air can move through from the west side and connect up with the air coming from the eaves at the front. But it's not a howling gale.

To be fair in this very cold weather I'd expect some damp under a flat roof particularly if it isn't exposed to sunlight.

Ah! This is encouraging. So it might not be abnormal, and might go away all by itself? The flat roof is south facing, and gets lots of direct sunlight in the summer. The worry though is whether the rate at which moisture can get in, in the winter, is greater than the rate at which it can get out, and for how long the timber up there could stand a bit of condensation before it started to rot. There's a bit of mould on the OSB now.
All the materials used on the old house are breathable. So there's even a possibility, thinking about it, that the moisture could move, through the woodfibre insulation, from the new roof into the old house, into the sheepswool insulation between the old house studs, and out through the breathable lime render higher up. I don't know whether that's a good thing or not.

As the plasterboard is not in place I suppose it would be possible to inject large quantities of foam through the insulation and into the air gaps.

If necessary I'd take the insulation down and do it again.

 

[Zebra]

G'ah! Worse than I thought. Mould forming on underside of roof deck. Hate modern, non-breathable materials!
The verdict is that it's condensation, and must be coming up around the edges of the sealed foil-backed insulation, from the new floor slab which is has evaporated litres and litres of water vapour into the air; the drying brickwork; and from general living.
The plan is: 1. Do nothing, don't board the ceiling, wait until summer and see if it goes away. 2. Take all insulation down and drill more holes through firrings, and drill circular vents into east side fascia. 3. No more showering, or boiling the kettle indoors. The showering I can do without, I am sure, but how am I to manage without tea?!

 

[Lime]

G'ah! Worse than I thought. Mould forming on underside of roof deck. Hate modern, non-breathable materials!
The verdict is that it's condensation, and must be coming up around the edges of the sealed foil-backed insulation, from the new floor slab which is has evaporated litres and litres of water vapour into the air; the drying brickwork; and from general living.
The plan is: 1. Do nothing, don't board the ceiling, wait until summer and see if it goes away. 2. Take all insulation down and drill more holes through firrings, and drill circular vents into east side fascia. 3. No more showering, or boiling the kettle indoors. The showering I can do without, I am sure, but how am I to manage without tea?!

The concern I would have is that the moisture may soak into the OSB if it hasn't already. If moisture penetrates into the board it will soften and the roof deform.
Good quality marine ply would have been my only choice.

It is a "cold roof" so moisture in the interior air will continue to condense on the underside of the OSB even with all the insulation removed.
Unfortunately because the living areas under the new roof are being used the required cold airing and drying out of the roof will be impossible and the alternative combination of dehumidifiers, ventilation and extra heating of the underside of the roof expensive and not as effective.

 

[Zebra]

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.

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.

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

 

[MdB]

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.

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.