The walls under some of our air vents sweat in the warmer months causing water marks and streaking. Is there anything we can do about this?
The problem is usually caused by air filtration into your walls. Moist air from outside is leaking in near the vents and condensing. You can start by sealing all obvious gaps and cracks, but if the problem persists, it may be time to check the wall insulation. This will be costly, but it will improve your energy consumption which will go a long way to mitigating the costs.
Water marks can be wiped away if they are relatively new. If they are stains, you will have to repaint. Use a primer made to cover stain to prevent streaks from showing through.