Insulated Attics

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   Insulated attics are emerging as an excellent new way of building a better home. Insulated attics possible because of the development of spray foam insulation, which allows insulation to be installed between the rafters and not fall out. Fiberglass, which is only friction-fit, would fall out almost immediately and therefore has to be installed on top of the ceiling, which holds it in place... so long as nobody ever goes into the attic and disturbs any of it.

   You might say “Wait, but now you have more insulation, because you’re having to run up the slope and back down again, and you’re conditioning more space (the attic); doesn’t that cost more to operate?” The answers are “Yes,” and “No.” Here’s why:

   A. It’s definitely true that you’re using more insulation because there are more square feet on the roof than there are on the ceiling, because the roof is on a slope.

   B. You would think you’re spending more on utilities because that extra area of insulation on the slope will lose more heat, right? That would be true if everything else were equal, but it’s not. Spray foam seals tight around framing, pipes, wires, or whatever else is in the way. Fiberglass is notorious for leaving gaps and holes everywhere. So the performance of the spray foam is better than the fiberglass, meaning that you’re spending less. And that’s only the beginning.

   C. Ever been in an attic on a sunny summer day? The heat is stifling. Matter of fact, it’s a huge heat trap. Attics can easily heat up to 140 degrees or higher; in hotter parts of the country, they sometimes reach 180 degrees. So that less efficient fiberglass insulation has a much taller order to fill: insulating against the 140 degrees or higher in the attic, rather than the 80 to 90 degrees outdoors. Because spray foam only has to insulate against the outside air temperature, an insulated attic might only be 10 or 15 degrees warmer than the inside air, and somewhat cooler than the outside air... a far cry from the 70+ degrees of difference between inside air and attic air in a heat-trapping uninsulated attic.

   D. Uninsulated attics are ventilated, but only just enough to supposedly allow moisture to escape. But that ventilation is problematic... you’ll never convince me that rain doesn’t blow in the vents during a bad storm. Spray foam, meanwhile, doesn’t require the attic to be ventilated because it installs tight to the roof deck, leaving no air space for condensation on the unconditioned side of the insulation. Eliminating attic ventilation doesn’t just keep out a blowing rain. It also lets you avoid those unsightly ridge vents that crush so easily every time someone is on the roof for maintenance.

   E. There are other benefits as well: Horizontal-flow heating and cooling units in the attic save a lot of money, because at $200 per square foot (a common price in many places) a closet in your floor plan for the air conditioner can easily cost $2,000 or more. Putting the unit in the attic lets you actually use those square feet for living rather than equipment. But units in the attic are problematic if the attic is 140 degrees or more through the summer. How insane is it to try to make cool air in the hottest part of the house? Try putting your icemaker up there and see if it makes any ice. But if it’s an insulated attic, then the problem is solved, and you save money from now on with equipment that runs more efficiently.

   F. Here’s another one: Water pipes run through an uninsulated attic must be heavily insulated, and still run the risk of freezing in winter if any of the insulation is improperly installed or damaged accidentally by humans or other creatures. Anyone who has had frozen pipes knows the repair costs only begin with the plumber.

   G. Finally, there’s some really cool stuff that becomes possible on your top floor that’s impossible with an uninsulated attic. I’ll blog about it later; it’s worthy of several blog posts, actually. These are ideas that I’ve been developing for several years. Recently, I’ve discovered that some of my colleagues have independently been developing these ideas as well. One of the leaders is New Urban Guild architect Eric Moser. Eric has promised to start blogging on these developments shortly; I’ll link to his posts when he does, so you get his perspective as well.

   Stay tuned!

   ~Steve Mouzon

Legacy Comments

Carl Seville

Steve - Excellent points about insulated roof lines vs. ceilings.  I have been a strong proponent of spray foam rooflines for many years, but as I continue to learn about sustainable building, I find that my opinion has evolved.  Spray foam is a petroleum product and does contain many toxic chemicals that can do harm to installers and occupants.  Fiberglass, mineral wool, and cellulose, while none are perfect provide alternatives to consider, although none provide the air sealing of foam.  As to conditioning the attic, this makes sense, particularly if there is HVAC equipment there, but that in itself can be considered a design flaw.  If you don't put stuff in the attic that needs to be heated and cooled, then you don't need to condition it.  I recently saw the results of a study showing that the interior space below a well insulated and air sealed ceiling did not suffer measurable temperature changes from unconditioned attics.  This does require, however, that the ceiling be perfectly sealed and insulated and avoid recessed lights and other penetrations.  Most applications of ceiling insulation are very poorly installed, if they were done properly, they would perform much more closely to spray foam, but they would probably cost much more.  Basically, there is no single answer to the question of where to insulate the top of a house.  It depends on the design, budget, mechanical design, and other factors.  To me, the great thing about construction is that it's complicated.   I get bored with simple solutions.

Monday, November 29, 2010 - 04:50 PM

Allison A. Bailes III, PhD

Like Carl, I'm also a big advocate for insulated rooflines. I built a house with one and have been involved in many home performance jobs where we sprayed foam on the roofline, too. 

In addition to spray foam, though, it's possible to insulate a roofline other ways. The house I built had structural insulated panels as the roof, which, in addition to the air sealing benefits, also reduced the amount of thermal bridging because the 2x8 splines holding the panels together were every 4' apart instead of every 16" or 24". SIPs also save a step because the framing and insulation happen at the same time. You can use fiberglass or cellulose as well, but you have to have a rigid sheathing below, as you do in vaulted ceilings.

Regarding the temperatures, they're not quite as high or as low as you said, though. The Florida Solar Energy Center (FSEC) found that ventilated attic temperatures get up to around 130 degrees F, not 140 or higher. There's also significant stratification, so the temperature near the top of the ceiling insulation will be even lower.

Also, the temperature that the foam sees will be higher than the outdoor air temperature. Without insulation, the bottom side of a roof deck can get up above 150 degrees because of heat conducting down from the shingles. That's the temperature that the foam will see.

Here's a related article I wrote about shingle temperatures based on a study from FSEC called "Literature Review of the Impact and Need for Attic Ventilation in Florida Homes."

Monday, November 29, 2010 - 09:34 PM

Michael Anschel

I'm inclined to agree with my curmudgeonly brother from another mother on this one. Unvented attic assemblies have been around for some time and I am generally a fan. I've installed more than a few, even my own home, ignoring state codes to do so. However, sealing and insulating across the lid allows for greater R-value at significantly reduced cost and less volume to condition. (Recessed lights in insulated ceiling should be illegal, and sure as heck won't make it into one of our projects.)

Where unvented attic assemblies really shine are in vaulted, scissors truss, ceilings and existing 1 1/2 story homes. That upper half story is a nightmare waiting to happen with current code requirements for insulating. They are the perfect candidates for spray foam insulation to seal the many penetrations, get around that birds mouth, seal up that shiplap subfloor, and get decent insulation levels on the sloped ceiling and tiny section of collar tied ceiling.

Petroleum is a resource. Using it for insulation and air sealing doesn't bug me, but using it in the right location at the right time rather as a catch all is important. Blown place fiberglass is less toxic, less damaging to the environment, and achieves air sealing at levels similar to foams. Permeability is another story, but I've already chewed up enough space here. Nice post.

Monday, November 29, 2010 - 09:34 PM

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