These inserts might work well as single panels but I do not recommend them with the use of mullions. Mine were exceptionally hard to get in due to having to place the panel over the lip of the mullion. Apparently, this condition was not factored into the construction of my inserts Also, if you want to have fresh air in your home on a regular basis do not use these inserts. Taking them in and out on a daily basis is a hassle. Unless you are willing to do your work twice to convince them that panels do not fit you will not receive a refund. Apparently they don’t have confidence in their laser measuring device or your ability To use such.