What Is A Thermal Barrier (Yes, That’s Your Insulation)

by Erik North on September 30, 2011

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What is the thermal barrier and why do we need such a fancy term for fiberglass insulation? And for a change, instead of five sentences of contrived SEO mumbo jumbo, let’s jump into it.

What is The Thermal Barrier (Yes, I snuck in an SEO phrase)

The thermal ‘barrier’ or enclosure or control layer (whatever you’re calling it) is a portion of the building shell along with the vapor and air barriers. The thermal barrier slows the movement of thermal energy (heat) across the building enclosure.

My standard caveat for using the phrase ‘thermal barrier’ applies here. The word barrier strongly implies completely stopping something. And there are some building materials that count as ‘barriers’ but insulation (including the most commonly used insulation, fiberglass) is not one of them. A sheet of 10 mil polystyrene would be a barrier, entirely stopping water vapor. Nothing short of a vacuum entirely stops convective or conductive transfer of heat energy.

Tying it in conceptually with the building enclosure, the thermal barrier controls the temperature variations between the interior conditioned space and the nasty cold exterior space (or the air conditioned inside and sweltering outside). In other words, it is the building equivalent of the old McDLT…it keeps the hot side hot and the cool side cool.

6000 Years of Construction in 100 Words

Through history this taken many forms. 1500 years ago, better insulation just meant thicker walls. Want to keep the firewood heat in your hovel? Add another foot of wood/thatch/stone. There were variations like cob construction, wattle and daub (easily my favorite name of a building technique; I could say that all day wattle wattle wattle), evolving toward mason double wythe walls and lathe and plaster interiors. Then folks began stuffing the New York Times in the walls ( a prime era for the discriminating mouse connoisseur). Eventually the thermal control layer settled on fiberglass, a vast improvement over most other techniques for retaining heat but it runs into the ‘barrier’ misconception hard. (Thus ends my one paragraph butchery of the history of building insulation).

The Thermal Barrier in Modern Construction

Today the insulation material is matched to the job. The exterior of foundations are insulated with polyiso, XPS foam board or closed cell spray foam. Commercial (and some residential) construction which use building wraps to prevent thermal bridging, sheathing the entire structure in XPS foam board, polyiso, fiberboard and recently high density rock wool board.

A residential basement can be insulated with XPS, closed cell spray foam or if you’ve taken a blow to the temple, fiberglass. The vertical framing elements (your walls) are most often insulated with fiberglass batts, less often with dense packed cellulose, blown fiberglass or open cell spray foam. Attics are most often insulated with fiberglass batts but loose blown cellulose or loose blown fiberglass are fairly common as well. Whew, that’s a lot of choices.

The Thermal Barrier’s Arch-Enemy – The Thermal Bypass

This is where the thermal barrier misnomer comes in. The word makes it sound like 6″ of an entirely porous stringy glass fibers stops EVERYTHING. Well, outside of slowing down heat, it doesn’t stop much of anything. Contractors lay down 18″ of criss-crossing fiberglass batts in the attic…and the ‘thermal barrier’ creates a false sense of security.

Remember the other parts of the building envelope? No? Well, they’re the air and vapor barriers. Every seam, crack, light fixture and electrical outlet leaks air, carrying moisture and $4.00/gallon heat with it. And unless the insulation used is one which retards air flow, the conditioned air will whisk right out of your house.

A perfect example is an active sub soil radon mitigation system, like I covered in yesterday’s post. These systems often run vertical PVC tubing from the basement to vent sub soil gases like radon, through an interior wall or plumbing chase, through the ceiling (and the attic insulation) and out the roof.

Radon mitigation installers will drill a 4″ hole through your attic floor/ceiling making a thoroughfare for the vent tube. It’s plenty of space for the PVC and loads of warm conditioned air to escape. Will the fiberglass batts in the attic stop this heat loss? Not even a little. The fiberglass is your insulation, your thermal control layer. The drywall WAS your air barrier prior to introducing a fist-sized hole in it. When my audit customers mention a conspicuous ice dams problem, one of the first things I check is for a radon mitigation system, open plumbing chases or a similar large hole.

Control Your Building, Control Your Comfort

Your thermal controls are an extraordinarily important part of your house. Without insulation, life in Maine would be a mite less comfortable come January. But your insulation needs to work in conjunction with the air and vapor barriers for the best efficiency and comfort.

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