[Novus]

From another Forum about Forensic Architecture

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Ventilated Attics are a waste of your money.

The purpose of a ventilated attic is solely to prevent ice damming; and before insulation was widely used, and houses were heated with chimneys and stoves - it worked. It still works, for that matter, if you don't care about your energy bill. It works so well, that it's been Codified. Even if your roof design prevents ice damming on your roof (which is relatively easy, these days), your Code Offical is a mindless bureaucratic jackass who will refer you to Section 1203.2 of the 2003 IBC (Internation Building Code), or the equivalent in whatever code your local officials have adopted.

Anyway, legally, you're stuck with something that sucks at your air conditioning bill for no good reason, and creates an environment in your house that shortens the longevity of your roof and attic space.

What is Ice Damming?

Ice Damming is caused when the attic space is warmer than the exterior, and you have a pile of snow on your roof. The snow will melt, due to the radiant heat from inside your house, flow down to the edge of the roof, or into valleys, and refreeze there. As the frozen portion builds up, a pond of melted snow (i.e. - water) will back up the roof line, and capillary action will bring it under the shingles where it will then enter the house and flood the rafters and ceilings.

The sole valid purpose of ventilating an attic is to provide a "Cold Roof" so that the inside of the attic is the same temperature as the outside, so the snow will just sit there. Now, this is just fine and dandy, if it weren't for all of the other problems that a ventilated attic causes, and the fact that the "Cold Roof" solution doesn't really work.

"The Cold Roof"

There's one good reason a "Cold Roof" doesn't work - ever. Generally, a roof is black or metal, and, generally, snow and ice are transluscent. When the sun shines, the absorbed heat is enough to cause ice damming regardless of the temperature in your attic (unless you refrigerate your attic during the winter, but that's your problem). All that ventilating an attic does is slow down the ice damming process, hopefully enough so that the water doesn't come in under the shingles; or that it doesn't noticeably flood your house (i.e. - it will still increase humidity levels and create potential mold & mildew problems, as well as rot your roof out).

Solving the Damn Ice Problem

There are a couple of ways to stop ice damming, or even to just prevent it from coming back in the house. Most of them involve removing your shingles, though. One doesn't (#3a), but it will shorten the life of your roof and your shingles (i.e. - stop the water from coming in while buying you some time to save for a new roof).

1. Add a vapor barrier under your shingles (all over the place - not just halfway up). Use something like WR Grace Ice & Water Shield (R) with a very low permeability. Assuming your roofer is smart enough to tape the seams properly (and not lazy or crooked enough to not to), if you have this stuff on your roof, the ice can dam all winter and you won't notice it.

2. Add a heating cable system around the eve of your roof and in the valleys. Something like Bylin's RIM (Roof Ice Melt) products. Then, ice won't dam at all, regardless of #1 or #3. Relying on an exterior electrical long-term solution, however, is haphazard at best. I've never owned anything that didn't break, but if you can get a warranty on the doohicky at least as long as the one on the roof, then go for it.

3. Enclose your attic and insulate between the rafters. If you use an insulation rated R19 or better, and have also implemented #1, then unventilate your attic, and insulate it. The result is that your roof will have the same properties as a "Cold Roof", without the cold attic. If you haven't done #1, though, water will still come in through the shingles and destroy your insulation and defeat the whole purpose of insulating it to begin with.

3a. Use an insulation that has vapor barrier properties. Closed Cell Foaming Insulation is the only thing I know of right now that is also water impermiable. You can achieve an R30 with about 3" of the stuff, and it sprays right on inbetween the rafters. It's not cheap at all, though - about $3/s.f. You can apply this material without doing #1, but you have to realize that water from ice damming (and vapor pressure during the summer) will still get under the shingles and may shorten the life of your roof deck and the shingles (but it damn well won't flood into your attic or ceilings anymore). How much it will shorten it is an unkown, though. Still, it's better than having roof leaks; but you'll still have to do #1 when you do re-roof.

What's Ice?
Live in a part of the U.S. where ice and snow don't ever collect? Like, anywhere in the South? Then why does your attic need to be ventilated? Because Northeastern know-it alls wrote the code, that's why.

Here's why you need to ventilate your attics in the South: because you didn't insulate inbetween your rafters and treat your attic as another conditioned room.

If you don't ventilate without insulation, your attic will superheat. The rafters will warp and lose their structural strength. But guess what? Ventilating doesn't solve this. Have you ever been in a comfortable uninsulated and ventilated attic in 90+ degree heat on a sunny day? Hell no. They don't exist. Without rafter insulation, an attic space can be up to 20 degrees hotter than the ambient exterior temperature, regardless of how much air you push through it.

And what's separating you from the 120 degree air in your attic on a hot summer day?

Blown-In Blow Hards

Whether your ceiling is insulated with Batt Insulation (rolls of pink stuff) or Blown-In Fiberglass/Cellulose; it's effectively worthless after about one month in a ventilated attic. Two things happen in a ventilated attic that ruin your insulation.

1. Humidity. Unless you have plastic taped all over the floor of your attic, there is no vapor barrier. Imagine: it rains outside, and the humidity increases. Oops, your attic is ventilated. Humidity absorbs into the air spaces in fiberglass and cellulose, and reduces the R value.

2. The Filter Effect. Unless you have plastic taped all over the floor of your attic, there is no ventilation barrier. Air is shared between your house and the attic, through joints at the wall/ceiling, through your ceiling fan attachment, light attachments, etc. etc. What happens is air gets filtered through the insulation. The insulation's insulating air gets filled with dust, and loses even more of its R value.

So, you could tape plastic all over the floor of your attic, and prevent these two problems (and have you ever seen a house where this is done when ceiling insulation is used? Why is that?).

There's one thing you can't prevent, though: insulating from 120 degree air vs. 100 degree air. So let's figure this out: why do we have to ventilate the attic again?