Condensation in roof

Early 70s house, gable ended roof made from prefabricated trusses.
I have a problem with condensation in the roof space. I suspect the house was probably OK as built but since then previous owners have fitted double glazing which I assume has reduced the ventilation and resulted in more moisture in the air. Also a double thickness of loft insulation (the second layer looks quite recent) meaning, I assume, the roof space is colder than before and more liable to condensation. The underside of the felt is damp and where nails poke through they have a drop of water on the end. Storage boxes have gone soft.
I started fitting soffit vents but had access problems and decided to look and see if I could fit them from inside. I realised that the insulation was tucked right into the corners effectively blocking any airflow from the eaves. On rolling back some of the insulation I see that the outer leaf of bricks has been built up between each truss so that it comes right up to the roofing felt. An even more effective block to any ventilation.
Is this normal practice? The cutting of the bricks and cement work looks a bit gash between the trusses so I wonder if it was a bodge by the previous owner to keep birds out.
Should I just remove these extra bricks and allow airflow through the soffit vents?
A previous house had an airbrick high up in each gable end. Would that be a good solution?
Andrew
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Andrew wrote

snip
No
Yes
Yes - excellent idea.
Peter
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On Wed, 28 Jan 2004 19:02:09 -0000, "Peter Taylor"

I am still trying to understand this loft ventilation concept, my, semi-detached, house is 1862 build, probably re roofed after a fire, and the bedrooms have a 60cm ceiling sloping to the eaves, there appears to be no ventilation at the eaves nor any where the joists join the rafters, visible when I pull back the tatty nominal 2" fiberglass insulation to expose the area of lath and plaster. Mind there appears to be no condensation. I have been put off using celotex between the rafters because of your comments that ventilation of the cold space above the insulation should be to the outside. Prior to that I thought an internal circulation would be adequate, albeit slightly less energy conserving than a warm roof.
AJH
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Andrew Heggie wrote

Andrew - the design of your roof, with bricked up eaves and sloping ceilings sounds just like mine. I understand exactly your difficulty.
I am not clear what it is you are intending. Are you just wanting to upgrade the insulation, or do you intend doing any work requiring Building Regs approval, such as a loft conversion? If it's just upgrading then you do not need to achieve the very low U values now required by Part L1 and you have more options.
Unless you are stripping and reroofing completely (in which case insulation above the rafters is better), Celotex between the rafters is definitely the best route for you. It is the ONLY way to insulate the sloping ceilings. Also it will allow you clearer access in the loft and remove the worry of frozen pipes/tanks etc.
You are probably not getting too much condensation now because of the low level of insulation. If you improve this the temperature above the insulation will drop and more vapour will condense, which could rot the roof timbers. To avoid this the Building Regs require a clear air path 50mm deep above insulation between rafters. This means if you have 100mm deep rafters you have space for 50mm insulation. You can quite easily cut 50mm Celotex board to fit neatly between the rafters and slide it down the sloping ceiling sections to meet the brickwork filling the eaves. Above this, in the loft, you can continue the Celotex between the rafters, but also add further thickness underneath if you wish.
For ventilation at the eaves you need to allow air to pass between the top of the brickwork and the underside of the tiles. This will then be above the Celotex. This can be done simply by fitting plastic over-fascia vent trays on the top of the brickwork. It will be necessary to take up the tiles. Either you can fit the trays over the rafters, which will raise up the tiles, or you can cut out a small part of the brickwork and sit the trays between the rafters.
Ventilation at the ridge is best achieved by having a ventilated apex section in the loft, formed by making a "ceiling" of Celotex fixed to new 50 x 50 noggins fixed horizontally between the rafters, like a letter A. The apex can then be ventilated by ridge vents or vent tiles, or by IMM's idea of openings in the gable walls.
I hope this is helpful. If you don't understand or want further help please mail me - (remove NO & SPAM) and I can send you a sketch detail. A picture of your house would be helpful if possible
Peter
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On Sat, 31 Jan 2004 13:34:21 -0000, "Peter Taylor"

Yes, I am looking at increasing insulation in general, as the loft has a smaller "footprint" or net area than the whole house (because the roof forms part of the bedroom ceilings) it's too small to make more than a box room. ATM it is piled with junk.

Yes I see this is the best solution, from our previous dialogue, but too expensive and not yet needed.

As I thought.

This is what I thought, a sort of dry lined area on the sloping part of the ceiling?

OK
I had wanted to avoid removing the lath and plaster, I had hoped to "glue" an insulating board to the old ceiling and feather it in at the edges.

Understood.
At this stage the cost outweighs the benefit as my fuel costs are low. I can still not quite fathom why a ventilation path, to prevent condensate settling on the roofing timbers, cannot be formed by fitting the celotex in between the rafters as described but allowing an internal gap at the bottom and top. a convection current would be set up from the warm apex down the cold roof (under the tile+felt and between these and the celotex) and then vented pack at the eaves and over the ceiling??

I'll take a photo tomorrow and put it on a website.
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Andrew Heggie wrote:

Only if you want a warm loft space. Otherwise why not use the ceiling as the iinsulatin barrier. Less area to cover with expensive celotex. AND you can use cheaper rockwool

Thats true for teh sloping bits, bu don' feel you have to carry on on teh slope.
My house is very similar - sloping ceilings and a flat bit in the middle.
What the BCO came up with was celotex up the slopes, with an air gap above to allow eaves ventialtion to circulate, and then rockwool over foil backed plaster board on the flat parts of the ceiling.
This works extremely well, now we have made sure that the rockwool is stuffed right up to the celotex, and all draughts are sealed.
Its a LOT cheaper than celotex up the sloping parts. About 1/4 the cost.

Also bear in mind the celotex, properly applied, is a vapour barrier. You will ONLY get condensation if warm moist air percolates up through the ceiing into the roof void and meets cold timbers.
You can combat this three way - a warmer roof (wastes energy: what you have now)
Ventialtion, as decsribed, to remove the cold moist air Pervent moisture movement at all, with a vapour barrier.
With insulation, the better the inpermeability of what covers the ceiling (BEFORE the insulation please) the less condesnation there will be. Then less venting is required to remove the rest.
With thick insulation, condenastion on the roof timbers is less likely than condensation on the joists, since these will now be as cold as the roof timbers were. Laying some membrane beteen them before adding insulation is very effective at reducing vapour transfer.
Even bits of black palstic bag cut up and sliped between the joists and taped together will be better than nothig, and there are products designed for this as well.
Otherwise the rest of the advice is totally sound.

Do you mean inside the room? You could do that with celotex.. Otherwise pushing slabs down the top of the slopes is fairly acceptable, if hard to get perfect fits.

Again, I queston teh need to do it this way rather tha insulate the flat portion of the ceilings.

I cant see where the gap at the bottom is coming from.
You need eacves vents of some sort, otherwise there is no way for air to get to the base, unless you are going to let cold air run down the front of the celotex, which meanas its not insulating the room anymore.
I am sruprsed there are no asoofits. However if ypou do as desribed and lift the tiles at the eaves line - only by a little - maybe a couple of cm - and then fit anti rodent mesh, the job is good enbough t comply with regs.
Ypu shopuld have some high up vents as well - ridge vents mean replacing a couple of ridge tiles, or vents in the gable ends.

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wrote:

Just to save a bit of trouble really, the loft has a modicum of fiberglass insulation and is part boarded, it just saves removing the boards.

I meant only the sloping bit exposed in the bedroom.

Lost me again there, my roof has a soffit (and a fascia to hold the gutter) and no visible ventilation in either.

Yes
There just does not seem to be the space to do this, I will go up and look again tomorrow.

Again it avoids disturbing the boars forming the loft floor, not to mention moving the junk.

I would leave the gap where the rafter meets the joist and floor boards, for the outside vent Peter gave a gap of 5mm, I assumed the same internal gap would work.

It is what I meant and I know it would be lossy, I considered it a possible compromise as the air changes between the underside of the felt and the celotex would be small.
AJH
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On Sat, 31 Jan 2004 13:34:21 -0000, "Peter Taylor"

Peter please have a look at
http://homepage.dtn.ntl.com/andrew.heggie/roof.JPG 118kb image
The house is a semi, was originally 2 up 2 down with the stair running up between the rooms direct off the then front door, walls seem to be 9" solid brick. There have been extensions which are rendered, probably cavity walls. these effectively cover 1/2 the downstairs front elevation and all the right hand elevation.
From other posts I take it I am losing 2W(t)/degC/m2 from the walls and the only obvious solution without losing the feature brickwork is to hack off internal plaster and dry line. Probably a bit too much for me. I can see a benefit in cutting heat loss through the upstairs ceilings direct through the roof as well as the loft roof.
AJH
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<............that the outer leaf of bricks has been built up between each truss so that it comes right up to the roofing felt. An even more effective block to any ventilation. Is this normal practice? ..........>>>>>>
Just the same in my bungalow 1950's and seen it done often in various houses. Sure would be a hell of a job for me, shifting all those bricks. As for normal practice, well that seems to change over time. I bet at least half the building regulations at present will be scoffed at in ten years time.
If it's such a bad idea to brick up to the felt all round the roof space now, why was it OK in 1952.? Must be different damp :)
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The common way these days on replacing a roof seems to be fitting a ventilator or two in place of a slate or tile. Might be easier than hacking out bricks.
--
*Oh, what a tangled website we weave when first we practice *

Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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London SW 12

I belive that you would need to be very careful with the positioning of the vents if you were just using tile vents. The use of high level vents alone is not recommended , for example see the first article of this page http://www.sandtoft.com/technical_library/tm-view.php4?Tile %2F20&Type=Related%20Topics&Articles=|41|45|46|47|48|51|52|54|55 apparently all still requires some form of soffit ventialation to allow a flow of air in the structure.
There are some other docs on the sandtoft website that are also relevant, and most other tile manufaturers, and some insluation suppliers provide information on appropriate roof ventialation.
cheers
David
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Yes - however removing insulation etc from round the soffits would IMHO allow enough air in, in combination with high level vents, unless the soffits were otherwise sealed. I've also seen tile vents fitted at the bottom and top of some roofs.
--
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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wrote:

I wouldn't be totally convinced that just removing insulation from the soffits would be sufficient without any additional soffit vents. My understanding of the situation is that you require a greater vent area at the bottom of the roof than at the top, and I'm not sure that this would achive it. My soffits are not particularly well sealed but there was still some signs of condensation before I fitted soffit vents.
With regard to tile vents at top and bottom of the roof, this is something I'll be discussiing with building control, as I intend to convert the loft at some point and I would prefer this option rather than the 25mm continous gap at soffits that is apparently the standard.
cheers
David
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Have a look at a product called lapvent
http://www.lapvent.co.uk /
This is actually designed for dealing with condensation after modernisation of very old buildings with roof spaces that were traditionally uninsulated and is very effective. But I can't see why it won't work in more modern buildings.
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Hi,
Removing several bricks instead of the whole lot may do the trick. Perhaps they were put in originally in a misguided attempt to minimise heat loss.
You may want to cover any holes with steel mesh to stop any local fauna getting in.
cheers, Pete.
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