What’s an effective, economical method for insulating the floor of a cottage on piers?
Filling the spaces between the floor joists under the cottage with insulation batts is the simplest and most cost-effective method. To get the highest R-value, completely fill the cavities between the joists. If you only use the cottage a couple of weekends each winter, adding minimal insulation would be enough to keep your tootsies from freezing solid when you step out of bed in the morning. But if you’re crawling under the cottage to insulate anyway, then make dodging the spiderwebs worthwhile by spending a little extra money and time and getting the most R-value for your efforts.
Cover your insulation with 5-mm (1/4″) hardware cloth. This size of metal screen should be small enough to keep out nuisance animals, such as mice. Place the batts against the underside of the floor, then cover with the hardware cloth, securing it to the joists with a staple gun. Make sure you haven’t overfilled the space—squishing the batts a little bit is okay, but too much compression will reduce their insulating properties. As an alternative to hardware cloth, you could screw a sheet of 1/4″ plywood to the bottom of the joists—or use any other material that will keep the batts in and the animals out.