Where to Place Wall Insulation in Florida
trevdawg122
| Posted in Green Building Techniques on
Hi. I’m building a home in central Florida. I plan on attaching a continuous layer of rigid insulation on the exterior, then placing furring strips over that for siding. However now I’m second guessing. The exterior insulation would allow me to take advantage of the block’s thermal mass, but now I’m wondering if the insulation will cause more heat gained to remain in the home, cooking the place. Would it be wiser in my climate zone to place the insulation on the inside of the block?
Thanks,
Trevor
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Replies
Trevor,
Almost everyone in Florida has an air conditioner. Do you?
The best place to insulate a concrete block wall is, indeed, on the exterior. I think your plan is a good one. If your house gets hot, you'll have to turn on the ceiling fans or the AC.
Insulating your block walls will indeed "trap heat" in the house, but it will prevent even more heat from entering in the first place, so it's a net gain by far. The summertime benefit of uninsulated masonry is really only present in places where the temperature drops substantially at night, allowing the temperature difference between the inside and outside to be high enough to actually pull some heat out of the walls. Where I live in the high desert, it can cool down to 55 degrees at night during the summer, so thick uninsulated masonry (adobe around here) can stay comfortable during the summer due to this effect of the walls cooling off at night. But if it only cools down to 70 or 75 at night where you live, this will barely happen at all, because there's almost no any temperature difference between the wall and the exterior, so the flow of heat will be very slow and very small. You're better off insulating the wall and preventing heat from entering in the first place, and running your AC when it gets too hot.
If you're planning on fastening siding to furring strips, you can get a small additional benefit if you choose foil-faced insulation board and have the foil facing outside. The foil plus the gap between the foil and the siding created by the furring strips allows the foil to act as a radiant barrier, which will make a difference in your climate. Polyiso wouldn't be a terrible insulation choice since it improves in performance when it gets hot.
Having the thermal mass on the interior doesn't prevent heat from getting into the place- it limits the amount of temperature rise for the given amount of heat- it moderates the temperature swings.
The vast majority of the heat gain is from windows & roofs, not walls, but the thermal mass moderates the temperature independently of the source of the gain. It's definitely a net improvement from both a comfort and energy use point of view to put the insulation on the exterior, keeping the thermal mass on the inside of the thermal envelope of the house.