The old condensation in the loft problem again

[michael]

Hi All
I have a felted and slated ‘lean to’ roof overlooking a small courtyard at the rear of my property. As the property is built on the side of a hill, the rooms below this particular roof (corridor, bathroom and a bedroom), are about 10ft or so below the height of ground level on the other side of the wall.
In addition to the brick wall that this roof is leaning against (a part retaining wall against the hill) it is also enclosed at each end by vertical brick walls which are neighbouring buildings so ventilation of the roof space is quite restricted.
At the moment there are three black plastic roof vents incorporated into the slates, venting a loft which is about 30 ft long x 7ft wide x 4.5ft high at the highest point of the slope but in this cold weather the inside of the bitumen felt builds up with condensation and drips down saturating the Rockwall insulation and ply board crawl way.
Question:- Would it help if I installed something like Celotex insulating board up between the rafters, flat against the felt to prevent any warm air that escapes into the loft condensing on the cold felt surface?

 

[Flyfisher]

If the rockwool insulation and plywood crawl-way is being saturated from dripping condensation then that implies quite a bit of water. I think my first concern would be where is it all coming from?

If I understand correctly, the rooms under the roof are partly below ground level. This is not automatically a bad thing, depending on the construction. If the inside walls to these rooms are showing no signs of damp penetration then the ground level is probably not a problem. So where else could all the water vapour be coming from in sufficient volume to saturate things in the attic, especially as you have some ventilation already?

Is the attic access hatch a tight fit? Is the bathroom ventilated via ducting through the attic? Could this ducting be leaking into the attic? Is there any structure penetrating the roof, such as a chimney or stench pipe? If so, is the flashing working corrrectly? I'm just trying to guess how so much water could be getting inside the attic space.

Once full of humid air, there will of course be condensation during cold weather (and not so cold as well). I'm not convinced that insulating between the rafters would solve the problem because if the attic is already ventilated and the ceilings are already well insulated (with the rockwool) then the whole of the attic space will remain cold anyway, so condensation will not be prevented (unless you have so much warm humid air getting in there that it raises the attic space temperature!). The celotex would certainly slow down the rate of temperature change through the roof but, with no actual heating in the attic, the temperature would depend on the balance between heat entering from the ceilings (something you are trying to avoid with the rock wool) and being lost through the roof - and given the roof is ventilated anyway (rightly so) I'd guess the attic space would remain pretty cold, in which case wouldn't the humidity just condense on the celotex and drip down anyway?

I know it's easier said than done, but if you can prevent the humidity getting into the attic then the condensation problem will go away. After all, basic tile-and-felt is a fairly widespread form of roof construction and is not an inherently bad thing.

 

[michael]

Thanks for your comments Flyfisher.
When I say saturated, The Rockwall is wet on the surface to the touch and there are small puddles of water on the ply board crawlway. There are beads of condensation on the inside of the roofing felt which you can see dripping from time to time. This part of the building never really gets that hot. The heating may be on at moderate temperature for about an hour or two in the morning and only if that particular bathroom is used but as we have showers elsewhere in the house the bathroom only gets steamy about once a week. The small bathroom ceiling extractor linked to the light switch (Manrose extractor I think) is ducted through the loft and out through the roof via flexible ducting. I’ll have to get up there and check this for leaks next time the bath is running but seeing that this bath isn’t used that often, I doubt that this is the problem however it could perhaps be leaking warm air into the loft when the light is switched on.
In addition to this there is a high level Combi boiler in the corridor below. The extracting flue passes directly up through the loft and out through the roof.
The loft hatch is certainly not a very tight fit. No hinges just push the hatch up and deposit it in the loft when access is required. I think I need to look at making a better seal here. Any suggestions, draught excluder around the hatch frame perhaps?
The bottom line however is that I’ve discovered the condensation up there today at a time when the rooms below are cold, the high level combi boiler is off and no steam is being produced in the bathroom but i will take all your points on board.
I wonder if the surface of Celotex insulation board would get as cold as roofing felt?

 

[Lime]

I think in any loft subject to a large temperature drop there is always a possibility of condensation forming on the colder parts.
However if air from the living quarters is entering the loft void then that is likely to cause condensation problems particularly if the loft is insulated.
Good ventilation in a well-insulated loft is alway important.

I think that I'd employ a two-pronged attack on the condensation.
~As has been suggested, prevent any air from condensation sources entering the loft space.
e.g.Holes in ceilings (particularly look for holes hidden by light fittings), leaking flues, loft hatch leaks and gaps in the ceiling plaster over open lath and plaster partition walls.

~Fit a couple more tile vents but further down the roof nearer the eaves.

I wonder if the surface of Celotex insulation board would get as cold as roofing felt?

Using celotex to cover the felt may just hide the problem.
If condensation is being cause by low ventilation and warmer air from the house entering the loft condensation may collect between the felt and the celotex.

 

[Flyfisher]

I wonder if the surface of Celotex insulation board would get as cold as roofing felt?

I think it would have to, given enough time and good ventilation in the attic space. Of course, it will never feel as cold, but that's a human touch thing and depends on the thermal conductivity of the material being touched. Hence, metal and wood will feel different even though they may both be at the same temperature. Anyway, even if the celotex prevented condensation on the cold felting, what about the cold timbers?

My instinct (and it's just that, I'm not a building professional) is that celotex may only hide the problem, as Lime describes. I think my feeling is largely based on the fact that there's nothing inherently wrong with a plain tile-and-felted roof, there are millions of them that seem to be fine (OK, that's a guess, but you know what I mean :wink: ), therefore there must be something else going on.

 

[chuckey]

" I think my feeling is largely based on the fact that there's nothing inherently wrong with a plain tile-and-felted roof, there are millions of them that seem to be fine " - NO, most felt under tile roofs suffer from condensation but its not seen and does not cause any short term problems,tho' in twenty years times there will be a tremendous amount of re-work to do. The building regs call for a continuous slot 10mms wide under the total run of the roof (or equivalent in air bricks or what have you).
My Victorian house was re-tiled and felted about 33 years ago. we suffered from condensation problems in the loft (things going mouldy etc.)for about 25 years. When we had new fascias fitted with soffit vents, all the condensation disappeared.
We had a stone slated roof re-roofed with so called "breathable" stuff. It now suffers condensation very badly, newly painted bedroom wall and ceiling badly stained, gable end walls wet. Gable end walls now have air bricks - no difference. I'm at a loss what to do, have tried to make sure over laps in the "felt" are opened up, no soffits or fascias for additional vents. I guess I'll bite the bullet next year and remove sections of the felt to improve the ventilation.
Frank

 

[michael]

Thanks for all your advice
My plan of action then is to better seal the loft space from the living space and improve the loft ventilation.
There is a ceiling hole in a high level gas meter cupboard where air from the living space could be leaking up into the loft. Also the loft access hatch needs a better seal. I would also like to add additional Rockwall insulation above the ceiling at right angles to that that is already there.
As previously mentioned the lean to roof containing the loft leans up against a retaining wall in the side of a hill. I have checked the measurements and I can confirm that whilst the rooms are below ground level and are all quite dry, the loft space is above ground level. I was wondering therefore if an air brick on either side of the retaining wall connecting the outside to the loft space would solve the ventilation problem. If so would one air brick be sufficient for my 30ft L x 7ft W x 4.5ft H loft?
I suppose I could put additional tile vents in but soffit/facia type ventilation cant be achieved because the ceilings in the rooms below follow the pitch of the roof for about 2 or 3 feet before flattening out to form the loft void above.
In the interim is there any way I could dry the loft space out quickly perhaps with a fan heater or dehumidifier? The woodwork up there (rafters and ceiling joists are all quite damp and this worries me (potential wet rot?) Also I don’t want to lay new Rockwall over damp Rockwall and damp ceiling joists.

 

[vicky whitworth]

Hi Michael, we have much the same problem down here in Damp Devon with our loft. The condensation drips off the rafters exactly as you describe and pools on the crawl boards and soaks the rockwool. We are having lap vents installed at ridge level and have been told that we need one every 2 metres on either side of the ridge. Also my understanding is that you need low level ventilation as well so that the air enters at eaves level and exits at ridge level, and that movement of air helps the drying out. We currently have eaves vents blocked by insulation so come the Spring we are having that removed (long story but it involves the removal of six rows of roof tiles). We have also been advised to seal any gaps were warm air from the house gets into the loft. We have FOUR loft hatches in a small cottage so we are having three sealed up, and any other little gaps we can find. But I think that we all produce a lot more moisture these days with our washing machines, etc. And some parts of the country are just damp.
I wish you well with it.

 

[Flyfisher]

Airbricks sound like a good idea - anything to improve the attic space ventilation. The general idea is to have ventilation holes on multiple sides of any space, so that the wind creates differential pressures around the building that helps push/suck air through the space to be ventilated.

In a long, thin attic I doubt that a single airbrick would be optimum. Better than nothing but not ideal. I'm not sure about latest building regs regarding soffit ventilation grilles but whenever I've noticed them, they seem to be about 4-5 feet apart. That might be a clue regarding the best number of airbricks to use.

I've never used a dehumidifier, but plenty of people here have. I don't see why it wouldn't work in an attic but if you have to leave a hatch open for the power cable then it might be self-defeating.

I agree about not adding any further insulation on top of already wet insulation.

 

[AMc]

We have the same problem - not much has changed in the roof but it used to vent into a derelict part of the house that is now converted, insulated and sealed up.
I noticed the condensation and some light grey mould on the rafters during the snow last year and went into panic but my electrician told me he was seeing that in almost every roof as it had been cold and still for weeks. His advice was to not panic and look at it again when things warm up.

Having been up there yesterday there seem to be some small drips on the insulation and some beads of condensation on the inside of the 70's non breathing felt but not much new and no indication of a significant change from last year.
The big problem I have is there are no significant overhangs at the edge of the roof to add ventilation so if I were forced to it would be vents in the felt.
Of course I may be storing up a problem for the future but there is no need to act in haste.

If you've got room to add more insulation at 90 degrees to the existing then you can pick it up very cheaply at B&Q as part of a subsidised deal with British Gas (IIRC) that, sealing the loft hatch and applying some wait and see would be my suggestion.

 

[michael]

If I go up there now and do the above works to my loft (venting/sealing etc) then I might as well do a bit of patch pointing to the internal brickwork where it’s needed.
Do you think a 3 sand to 1 hydrated lime mix would set / cure in the current cold damp conditions of the loft?
I guess that I’ll have to leave the airbricks until the weathers a bit warmer because the mortar on the outer airbrick could be exposed to frost and ice

 

[michael]

P.S. Do you think planning permission is needed to install 2 or 3 air bricks on a grade 2 listed property?

 

[MatthewC]

P.S. Do you think planning permission is needed to install 2 or 3 air bricks on a grade 2 listed property?

No for planning permission; technically you need listed building consent, as it's an alteration; However, we won't tell on you... :wink:

 

[Flyfisher]

I don't think I'd bother with pointing in this weather, even 'internally'. I doubt the damp will cause any problems but the low temperatures might. I recall reading that lime work shouldn't be undertaken below 5C.

Of course, that wouldn't prevent you removing a brick or three in readiness for the airbricks and get the air flowing through the attic (nice cold dry air at that) as soon as possible.

If you're worried about LBC then an email to your CO should clarify things. But unless you've got the world's rarest bricks, I'd be very surprised if there's any need for formal consent, especially as you're trying to resolve a bigger problem.