Will soffit vents solve condensation problem?

[Glengyle]

My house is a semi-detached bungalow, the gable end facing South. Earlier this year I had the walls cavity insulated and I topped up the loft insulation to 250mm. I've just been up in the loft and noticed condensation on the boards above the rafters, but only down near the eaves - the boards at the apex are dry. I don't know if this problem has existed in previous years as I'd never checked before. I think my loft set-up might be slightly different to the norm as the East and West inner walls meet the loft inner roof around 9" above the loft floor and they are sealed along the roofs length. I am hoping that fitting soffit vents at front and back might solve the condensation problem but bearing in mind that the air from the vents will only enter and leave the main loft area at the 4 corners is this likely to provide adequate ventilation in the areas of condensation? I'm trying to get away with the cheapest and easiest option first

 

[JohnD]

increase ventilation and reduce damp load

//www.diynot.com/forums/viewtopic.php?t=160032

 

[Glengyle]

Checked out the neighbours loft earlier - same set-up with cavity wall insulation and plenty of loft insulation - dry as a bone. The main difference is that he has 4 round soffit vents at the front and 4 at the back, around 2000mm between centres. Been to Screwfix tonight and bought some vents, so will fit them tomorrow and hope they will solve the problem.

 

[JohnD]

what about the sources of moisture?

 

[Glengyle]

Two adults (+1 labrador and two cats) living in the house. Clothes are dried in tumble drier in garage. Always open bathroom window after shower (never have heating on in bathroom and close door but always get condensation on toilet in winter). Use extractor when cooking. Gas boiler and radiators. Since having cavity wall insulation and topping up loft insulation we have also been experiencing condensation in morning inside double glazed windows. We do have a hot water tank in the loft, directly under apex, but would not have thought this was the cause as the condensation is down at the eaves and boards at apex are dry?

 

[JohnD]

what sort of windows?

try leaving the bedreoom windows partly open during the day, and trickle vents at least (if you have them) all the time. Do this for a week and you should see an improvement. If so, you might like to add trickle vents if you do not already have them.

Showers cause an enormous amount of steam. A bathroom extractor fan is actually better than a window at extracting steam, as it sucks it out, to be replaced by warm dryer air through the gap under the door, and it does not depend on wind direction. The suction reduces the risk that it will leak into the loft. As some people have an aversion to turning fans on, one that comes on with the light switch is most effective. The bathroom will continue to be steamy until the last towel has dried out. The window does not need to be left open, so the bathroom will be warmer as well as dryer. A typical 20W fan runs for 50 hours for about 8p worth of electricity.

Water tanks in the loft should not cause damp if they are cold and have tightly-fitting lids. Plumbing faults can make them warm.

Have you checked for ceiling leaks like downlighters, pipe holes and loose loft hatch?

 

[Glengyle]

Thanks for the advice John. The windows are all double glazed with no trickle vents. I will try as you advised and leave windows open for part of the day. Is the condensation forming on loft sheets likely to cease in the meantime anyway with the weather turning a bit mild and no sub-zero temperatures at night?

 

[JohnD]

yes, it may do. Loft condensation varies with moisture load, ventilation, and roof temperature.

If you post a pic of your window frames in our "windows and doors" section someone will know if and how you can drill them for trickle vents, or use casement catches with a "ventilation" position.

If it is any easier for you, you can cut neat round roles in the gable walls using a hired Core Drill. They can make holes that the ducts and grilles as used for extractors and tumble driers will fit. It is a bit dusty but quite a pleasant job. Preferably work from the outside to avoid a scab on the brickwork showing. Holes at each side or end of the loft will encourage through-flow of air which is what you want. Check that pipes are well-insulated with the BS (thick) grade of Climaflex or similar, and your water tanks have good jackets and lids.

 

[Glengyle]

"Water tanks in the loft should not cause damp if they are cold and have tightly-fitting lids. Plumbing faults can make them warm.
Have you checked for ceiling leaks like downlighters, pipe holes and loose loft hatch?"

The water tank in my loft is a hot water tank. When I topped up the loft insulation I worked around light fitting cables and did not cover cables with insulation (as advised on insulation packaging). Therefore there may be some heat loss through room light cable holes. I have yet to insulate the loft hatch and there will be some heat loss through the hatch. But if these three things were causing condensation in the loft would you not expect it to be higher up near the apex rather than down near the eaves? (the hot tank and loft hatch are directly below the apex and all light fittings are a good six feet in from the eaves).

 

[JohnD]

the problem is not heat rising, it is water vapour (which is lighter than air so rises through gaps in ceilngs). bathroom ceilings are the worst as the air is hot and steamy.

I think you mean a hot water cylinder (not a tank) where the water is not open to the air so steam cannot come off it.

it is just possible that the upper part of your loft is warmer than the bottom due to heat rising, otherwise the damp patches might be where moist air is rising through the ceilings and condensing on the first cold surface it meets. Is there any cracking of the ceilings neat the walls?

the walls and wall cavities ought to be dry unless you have leaky gutters or something.

 

[Glengyle]

The ceilings are all ok - no cracks. It is a hot water tank in the loft, not a cylinder. Square tank with a wooden U shaped lid, meaning there are two 4 inch high rectangular gaps at two ends under the lid - the hot water is open to the loft space . The house is 30 years old and we have lived here for 12 years, the tank has always been like this I think although there may have been insulation draped over these gaps in the past. I'd imagine I should be covering these gaps?

 

[JohnD]

a hot water tank in the loft, not a cylinder. Square tank with a wooden U shaped lid, meaning there are two 4 inch high rectangular gaps at two ends under the lid - the hot water is open to the loft space

There's the source of the steam and condensation.

This is a very strange arrangement - do you have a coal stove? I take it you have no hot water cylinder?

Think about having it converted to an well-insulated cylinder - you will save lots of ££££

Meanwhile make up a tight-fitting waterproof lid to keep the steam in. WPB ply wrapped in thick plastic would do. Also buy a water tank insulating jacket if you can get one to fit - if not, wrap it all round in insulation. You can get white loft insulation in B&Q now, it is made from recycled plastic bottles and does not shed irritant dust and fibres and is very pleasant to work with.

 

[Glengyle]

It's just the hot water tank in the loft and gas boiler in the kitchen - no coal stove or hot water cylinder. Many thanks for your help and advice John.

 

[big-all]

may i suggest you stop looking for a week or so as every time you open the hatch you introduce warm moist air into the loft that may take days to evaporate

 

[Glengyle]

Fitted the soffit vents this morning - 9 at front, 9 at rear, 3 feet between centres. Was able to put my hand through the vent holes and feel the roof boards protruding out past house wall. The boards at the front of the house were fine (above bedroom and living room). However, at the rear (above kitchen, bathroom and bedroom) the boards were damp, some soaked. So it looks like this problem has been around for a while. Looking at the lowest board inside the loft at the rear it looks damp and has moisture droplets (or maybe just water dripping of the soaked board) on it while the second boards up are dry. The first board is directly above the rear wall. Is it possible that the main cause of the condensation could be not from the hot water tank but from moist air travelling up the wall from the kitchen and bathroom?
note, the wall meets the roof boards and is sealed. Along the length of the loft there is no gap to the soffits, only at the 4 corners.