I would recommend against blocking up those vents as it will lead to a build-up of moisture which then leads to mould. A property my friend rents has those types of vents painted over and sealed and she is constantly dealing with moisture and mould, it’s not pretty. An alternative solution is to maybe get something that can go over those vents that can still allow for ventilation but keep out dust and other particles. - Dan
I would be guessing that you have limestone footings which act as wicks drawing water up (especially if you are close to the water table). These vents are extremely important to allow the moisture to vent out of the cavities of your home preventing water damage and more importantly, mould. This is currently a major problem with a heritage building in Fremantle we are currently working on. - darylhewston
It's important to remember that static vents serve a purpose, especially in older homes. They provide ventilation to indoor spaces, help regulate indoor temperatures and prevent the build-up of excess moisture in wall cavities and rooms which can lead to mould outbreaks and damage to building materials.
Newer homes typically do not have wall vents as they are of better construction, including materials with higher insulation properties, and don't suffer as much from moisture issues. Also, if the rooms have a wood fireplace or older unflued gas heater, the vents would have been installed to provide fresh air and inhibit carbon monoxide build-up. Please remember that there are building codes for the ventilation of a dwelling. I would advise you to check with your relevant building authority to ensure that blocking ventilation in these rooms complies with regulations.
If you haven't suffered from moisture issues, you can attempt to close these vents. Remember that if this is a bedroom, you'll need to crack a window at night instead; otherwise, you'll have condensation running down the window by morning. This condensation enters the window frame and begins the decay process.
I'd suggest that screwing athin timber board, cut to the same size as the vent, over the larger exterior holes would be the most straightforward solution. A neat solution for the internal vent would be to remove the cover, clean the inside withalcoholand then adhere someadhesive filmover the holes. The cover can then be placed back into position. If you experience a moisture build-up issue, you can reverse the process to open the vent again. - MitchellMc