cavity wall full of wet sand

toejumper

none of my air bricks are below the dpc.

David_Aldred

Hi Steph,
Sorry for the delay in responding. If the house was built with a dpc then failure of it should be the last thing considered especially if there have been other things going on such as the debris within the cavity bridging the dpc and condenstion within the cavity too.

Painting the outside masonry will tend to reduce evaporation outwards as will a hard dense cement mortar compared to a breathable true lime mortar the latter of which many houses were built with. If the house was built with a true lime mortar then the preferance is to re-point defective mortar with the same type and when cured apply a breathable silicone clear water repellant (availaible form good builders merchants) rather than paint the walls - sorry if that comment is a late in the day. Trapping moisture within the wall with hard dense cement renders, mortars and paints makes the pores in the bricks wetter and if they are wetter they are colder an dif they are colder the problems of condensation within the wall itself increase along with freexe / thaw damage of the outer face of the cavity wall.

The vents to the suspended timber floor ideally should be positioned such that there is cross ventilation (i.e on at least two opposing walls) and ducted all the way through the wall as described above. The Building Regulations suggest a vent every 2m but this does not take account of differing sub floor conditions and there needs to be sufficient vents to keep timbers at a moisture content below which they will not decay.

Dry and wet rot spores are in the air all the time and only require timber of a sufficient moisture content to produce growth and decay. The key to preventing decay is therefore to keep the wood relatively dry at which point decay will either not occur or if it has occurred it will cease. This why venting the sub floor voids throughout is very important to keep these timbers dry.

With a cavity that is fully insulated there is often condensation on the cold side of the insulation and the amount of condensate should not be under estimated. I am not a fan of full fill cavity wall insulation perhaps because I see so much of it that is like a wet sponge within cavities of problem properties I see. If you wish to retain the full fill cavity insulation in a property then you must accept that there is a risk that the wall will be wetter than if it was not full filled cavity wall insulated especially if evaporation outwards has been compromised.

The salts you describe will require a plaster specification to hold these salts back within the wall and this requires a high standard of wall preperation and workmanship. Remedial treatment product manufacturers such as Safeguard, Wykamol and Sovereign have websites you can browse and technical staff who will advise on such specifications / supply appropriate plaster additives etc. Alternatively there are waterproof grade dry lining systems. Hope this helps, kindest regards, David Aldred Independent damp and timber surveyor

toejumper

Thanks David already painted the outside wall, but will look at the vents you mentioned and will replace the other one. the wall has changed colour dramatically over the past few days but we haven't had any rain for the 3 days. so will see if the wall gets wet again after it rains. after doing everything as i said in the above post not sure what else we can do. should the air bricks be below the dpc. Thanks steph

David_Aldred

Hi Steph,
As you say there is a difference between me trotting out what may be ideal and what you in all practicality can achieve but at least you may have a bit more background information as to what the goals should be. As regards the airbrick being below or above the dpc it doesn't matter so long as it is ducted all the way through to the sub floor void - it needs to be a certain height from the ground to stop ground water running into it and into the sub floor void and the dpc can run over the vent or underneath it. Kindest regards, david

toejumper

Thanks David you have been very helpful. lets hope everything we have done has cured the problem. Thanks again Steph.