Is it normal.....

[jeds]

It isn't necessarily due to wind blowing through from anywhere. Clear cavities will always have air movement caused by natural convection. Cut a hole in a SVP boxing and stick your hand in and you'll feel what appears to be a draught. Nine times out of ten it isn't a true draught it's just air movement due to convection. Same applies to the void behind dot and dab plasterboard.

 

[WabbitPoo]

No, its the wind alright - we get v strong ones here and when it blows outside, you can feel it blowing inside! Some of the places the wind exits from:

Around CH pipes coming up through the floor upstairs (solid floors downstairs)
From behind the bath panel
One or two light switches
Around the edge of the upstairs carpets

I need a way of sealing off the cavity from the wind. There are two or three air bricks at the upstairs floor level: presumably these were designed to vent the floor space or cavity? Given we now have blown cavity insulation do I still need these or can they be blocked?

 

[theprinceofdarkness]

You should really read up on green forums, where they are designing houses that require 4KW of heating at the coldest part of winter (-10 degs C) and absolutely none for 10 months of the year (See Passiv house). Even with our c**p building standards about 30% of the domestic heat is wasted due to drafts and dot and dab is the main cause, along with badly fitting windows/doors. . . and "trickle" ventilation.
Vented cavities have no technical reason for being vented, the vents are to vent the under floor void when you have wooden joists and flooring. if you have block and beam you don't need them. if you have solid floors you don't need them. The cavity is there purely to stop driven rain percolating through the brickwork into the inside of a room. We have something like 17 million houses in this country without cavities and driven rain damp does happen but not very often. Sort of like 1 in 10,000 houses and they are over a hundred years old.
The problem with cavities is that poor workmanship allows them to get filled with muck which eventually builds up and bridges the damp course then you DO get damp.
The continentals all get by without cavities.
Frank

 

[noseall]

Agree, it's a quality issue and a lack of understanding air pressure by most builders.

By addressing these things as you build, you end up with a more thermally efficient building with a wee bit more effort. I encourage pointing of block-work or at least filling all perp's as well as being uber diligent when dabbing boards and finishing reveal cavities etc.