Re-pointing victorian house

[stuart45]

In fairness to ree, the advice about air bricks being used for ventilation of the cavity was changed later on. From W B McKay in the 30's.
' An important matter affecting the heat insulating value of a cavity wall is the extent to which the cavity should be ventilated. Formerly, it was generally considered desirable to ventilate the cavity by means of air bricks just above ground level and also at the top. Whilst this circulation of air ensured a dry cavity and reduced the risk of defects arising in floor timbers, it also destroyed the insulating value of the wall to such an extent that an 11 inch cavity wall afforded less heat insulation than a 9 inch solid wall.
Therefore the only ventilation of the cavity which is now advocated is that provided by weep holes near the best of the cavity and at the head of door and window openings'.

 

[mrrusty]

@stuart45 yes, that makes perfect sense; a ventilated cavity wall with only a 2" cavity is likely to have worse insulation than a solid 9", but that focuses on insulation comparison of a dry wall. IMHO the cause of much reported damp and cold walls is caused by condensation. The more we keep our houses warmer, the more water vapour that warm air contains, the steeper the temperature gradient between inside and outside and the more likely it is that interstitial condensation results. Condensation within the wall fabric dramatically reduces the insulation factor.

It is no accident that so much "damp" is found in old houses around the colder base of external walls - the dew point of ambient room air at say 22 degrees and 60% RH is around 12-13 degrees. Any wall material colder than that will be accumulating condensation. The drying/evaporation process in itself also causes cooling, and wet walls conduct heat away from the inside much faster than dry walls. Ironically keeping our houses toasty increases the risk of condensation forming within the wall fabric.

Comparing the insulation of an empty cavity wall with a 9" solid wall is all very well, but ignores the point within the temperature gradient of the wall structure where the dew point is. IMHO the best way to achieve the best insulation in a wall is to keep it dry and not allow it to get damp, and here the ventilation in a cavity potentially has a higher impact than the base insulation provided by the structure of the wall. i.e. a wet 9" wall will have far worse insulation than a dry 4 1/2" (inner skin) wall, and this is what should be compared, not dry with dry.

Once you factor in internal insulation and move the dew point inwards, evaporating interstitial condensation becomes a priority, or damp will accumulate exacerbating any heat loss and potentially causing other issues. This is why internally insulating solid walls is fraught with problems - there is no way to evaporate any interstitial condensation caused by migrating humid warm room air once the wall is much colder so damp accumulates within the wall structure. If it is externally rendered it is even worse - water vapour can't then even evaporate through the outer surface!

I'll keep my ventilated cavity because it works alongside the internal insulation in controlling both heat loss and damp control by mitigating interstitial condensation. The theory I applied to condensation control when we rebuilt this house is working out in practice - it's 5 years since we moved in.

It's a really interesting topic and there is a huge amount of conflicting information out there. The basis of the theory I applied is from what I studied for condensation protection of ships cargo when it was carried in holds rather than containers. It used to be a mega- important topic, and still is but to a slightly lesser extent with containerised cargo. Still very relevant to e.g. bulk grain carriers. My theory, my house, but there are so many variables that each circumstance should be evaluated. Anecdotally, good friends had CWI put in a 1930's house and now have damp walls.......

 

[stuart45]

I think the main point that the later info made was that the cavity should be ventilated, but the amount of ventilation given by air bricks was excessive, and enough was given by weep holes. I can see how it works on your house, as you've got internal insulation and well placed 9x3 air bricks. It's possible the thermal performance might be improved with weep holes instead of air bricks, without sacrificing any moisture control. Nobody can say without trials.
I've worked on houses that only had one 6x9 air brick low down and one hi level one, and they did seem to cause a condensation issue in that area.

 

[mrrusty]

That opens up a whole new line of thought - there must be a balance point where the cooling effect of too much ventilation outweighs the benefits of the drying effect on internal condensation that would otherwise compromise insulation.

The size of the optimum ventilation holes would vary depending on which wall faced the weather more...and of course would vary depending on the temperature differential inside and out and the relative humidity of the ventilating air (because of course air bricks can also introduce condensation because of thermal inertia of brick/stone - if the brick outer skin is still cold from overnight and the weather suddenly becomes warm and wet, the dew point of the external air may be higher than the temperature of the fabric of the building!). Perhaps someone should invent a variable vent with sensors and intelligent control purely for controlling the atmosphere within cavities of old houses....

Think we've done it to death - beer o'clock!

 

[stuart45]

The problem with theoretical designs is that sooner or later some bricky comes along and fills up the weep holes when he repoints the house, or spiders build their nests in them and block them up. Maybe air bricks were the better option.