How to Know What Insulation is Behind Your Walls

Insulation has come a long way. These days you can stuff your walls with everything from recycled blue jeans to soy-based foam to keep temps comfortable year-round. But if you live in an older home with insulation installed before you moved in, understanding what pads your walls is essential to keeping your family safe-especially if your spring to-do list includes cleaning the attic or renovating it. Here's what you need to know about three common old-house insulations. -Natalie Gingerich Mackenzie

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What it is: A lightweight, shiny mineral that resembles mica flakes.

How it works
: Vermiculite absorbs water and is fire resistant, giving it myriad uses, from fluffing up garden soil to soundproofing floors. The mineral was popular during much of the 20th century as insulation, thanks in part to its easy application: It could simply be poured by the bagful between ceiling joists.

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Also see: Insulation Education


What it is: Finely spun glass fibers create a lofted material that can be rolled as batting; it can also be sprayed into walls as loose fill. It remains one of the most popular types of insulation.

How it works: Fiberglass works as a barrier by trapping air inside pockets of fluff; that's why it loses its effectiveness when compressed by the weight of boxes or other heavy items.

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Also see: Cutting Batts of Fiberglass Insulation

Mineral Wool

What it is: Popular before World War II, mineral wool comes in two main forms: rock wool, which is made by spinning molten rock into masses of tiny intertwined fibers similar to cotton candy; and slag wool, which is made from the by-product, or slag, that forms on the surface of molten metal.

How it works: Formed into blankets, batts, or boards, mineral-wool fibers block both sound and temperature exchange through walls. It can also be found as loose-fill insulation.
Mineral Wool

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