We get multiple requests a year for basement glass block windows with vents and we have to explain why they are a terrible idea.
In the summer, your basement tends to be cooler than the rest of the house. Warm moist air rushes into your basement when you open a vent and then it condensates. This contributes toward that mildew smell in the basement. Homeowners often try to mitigate that problem by running a dehumidifier which uses a lot of electricity and then has to be drained. Believe it or not, we’ve gone into peoples houses where a dehumidifier is running and they have window vents that are open. It’s a bit like trying to dehumidify the outside.
Plastic Vents Break
All of the vent manufacturers are making the vents out of plastic or vinyl these days. While this is good to reduce thermal conductivity, the opening and closing mechanisms (which are metal) have a tendency to break. One of the most common causes of broken window vents is pulling them open on one side. This torques the vent and those little arms or the plastic they are attached to, tend to break over time. They used to manufacture metal vents, but now they are all made out of plastic.
Worthless “Cross Ventilation”
They always start out with “some guy told me that we need cross ventilation”. Unfortunately “some guy” doesn’t understand the concepts of condensation, dew points and the varying needs of our climate. The vents tend to be abound 6″ x 12″, of which you lose one to two inches per side. The actual ventilation is small and doesn’t truly count as cross ventilation.
If ventilation is truly a need in the basement, consider a window that will have a larger opening such as a hopper window. Your better contractors will know how to mortar them in.
So when and where do they make sense?
Glass block window vents make very little sense except in two cases. If you have to put a pump hose though your window because you are in a flood prone area, you need a place to get the water out. Or, if you have a cigar smoker in the house and you want to get a small fan unit to blow the smoke out, they’re a temporary solution.
So why do companies offer these plastic window vent? They’re looking to sell something that the customer wants because “some guy” told them they were a good idea– never mind that they don’t really help.