is there a reason the mash process should be closed lid other than to not lose any water vapour? im doing a lager and will be topping up after the first 30 of 90 min boil. to keep the temp somewhat constant i have the lid open a lot (for when i turn on the heat and stir and hold the bag off the bottom of the kettle). it's cold outside
mashing has a lot to do with temperature, Once you have your mash at the desired temp. you may find it hard to keep it there with no lid I would think. I wrap my pot in a blanket when I am mashing, lid on of course.
I would grab a couple of blankets and wrap that baby up. I use 2 blankets down to 45* and lose 2-3 deg over 90 minutes. These are cheap moving blankets from U-Haul at that. I just take an elastic cord and wrap around the blankets once to hold them onto the kettle.