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I added insulation to my attic rafters and plugged the space between the outdoors and my attic with insulation. It lowered my heating bill by $150/month, but I've seen some information that I may have made a mistake by cutting off the airflow in my attic.

Did I make a mistake, and if so, how can I properly insulate my attic? There's already insulation in the floor.

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Wow, $150 per month, that is substantial. I see that you're in the mid-Atlantic United States. How large is your house, and what is your monthly heating bill now? How did the insulation affect your air conditioning costs? –  Vebjorn Ljosa Sep 21 '10 at 16:56
    
$150 a month is my entire electric bill. O.o –  Doresoom Sep 21 '10 at 19:44
    
Wow. My house is underinsulated and my worst heating bill was just under $90 (for the month that had that week long stretch of -15F). My electric bill is around $30 in the Winter and only goes up to $80 with A/C usage in the Summer! –  Brian Knoblauch Sep 22 '10 at 13:10
    
My electric bill at worst got up to $400/month in a small 2BR townhome. Now it's around $250/month. The HOA is looking into upgrading all the houses in the neighborhood. –  ristonj Jan 30 '13 at 18:57
    
Most of the rest of the year, including summer, is around $100. –  ristonj Jan 30 '13 at 18:58

5 Answers 5

You can buy air ducts that staple to the inside of your roof above the eave vents and fit between the rafters. The will hold open an air gap of approximately 1 inch to allow airflow past a layer of insulation. The ones I've used are made of styrofoam and IIRC they cost around $2. You should be able to find them near the insulation in your home improvement store.

EDIT: They are called baffles or rafter vents; this is how they're installed:

metal rafter vent between wooden roof and insulation

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Thanks, how would you install these? Get a ladder and install from the outside or install them from the attic? –  ristonj Jul 22 '10 at 16:02
    
@ristonj: They go in the attic, at the point where the roof goes past the exterior wall. It sounds like your insulation is blocking the gap there, so with one end below the blockage and the other end above, you get airflow from the eave vents into the rest of the attic. –  Niall C. Jul 22 '10 at 16:12
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any attic room that is finished should have these installed. Housed like cape cods and finished rooms above garages usually have the sloped ceilings, and would need baffles. –  mohlsen Jul 23 '10 at 12:03
    
You only need those if you have vents in the soffits under the eaves. If you have vents in the walls of the attic or in the roof ridge, then the insulation can go all the way to the roof. The idea is to allow some air to circulate through some part of the attic under the roof but outside the insulation to prevent moisture build up. –  Reed Hedges Dec 10 '10 at 20:06
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@Reed, you need vents at the bottom AND the top of hte roof, so ridge vents are not enough on there own –  Walker Dec 18 '10 at 8:38

There are a few different types of attic designs. One is insulating the floor and venting the space above the floor with soffit (eave) vents and an attic fan or two. One reason for the air flow that hasn't been mentioned is that, since the hot air will exit out the top of the roof (whether the attic fan is powered or simply turned by the force of the hot air rising), then it has to be replaced by something. If your soffit/eave vents are plugged up, it will suck air out of your house, which, of course, is not good for your electric bill.

There is another type called the CATHEDRALIZED ATTIC, which is where the insulation is put on the underside of the roof itself. CATHEDRALIZED ATTICS SHOULD NOT BE VENTED. Since it sounds like you have cathedralized your attic, you should not install soffit/eave vents or baffles.

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In the UK a CATHEDRALIZED ATTIC is called a "warm roof" –  Walker Dec 18 '10 at 8:39

I've always been confused by this, and have delayed insulating my own attic until I understand this properly. Doesn't expanding foam insulation, sprayed directly on the underside of the roof violate this? Or, when foam is used, are these baffles run from eave to ridge in order vent properly?

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IF you get spray foam done (expensive but very effective) they should install the baffles so the insulation is not stuck to the underside of the roof (which can cause the roof tiles to overheat and not last as long). –  auujay Sep 21 '10 at 14:49
    
Thanks, that's exactly what I was wondering. –  Coleman Sep 21 '10 at 19:57

The point of allowing airflow to reach the outside is so hot air can escape. If it doesn't escape then it will heat up the roof/rafters and cause snow to melt and then turn to ice. This can damage your roof in a number of ways. Also, moisture will be trapped in the attic and can cause mold or damage to the structure.

I believe the optimum way to insulate your attic is to insulate the floor of the attic very well but allow the air above that to escape (ie. insulate the ceiling of the attic poorly)

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Allowing the warm moist air to escape also prevents mold. –  Tester101 Jul 21 '10 at 20:08
    
Thanks. I've added that –  Joe Philllips Jul 21 '10 at 20:17
    
In Australia, we always insulate the "floor" of the attic and usually have the roof adequately ventilated to prevent moisture buildup (and heat buildup in summer). –  MGOwen Sep 29 '10 at 4:16

You don't want to plug the space between the outdoors and the attic. As Joe mentioned, that will cut off airflow in the attic and cause ice dams in the winter. It will also keep your attic VERY hot in the summer, and raise your AC costs.

Ideally you want a "cold" attic - lots of insulation between the ceiling and the attic, and no insulation between the attic and the roof, along with plenty of soffit and ridge vents to keep the air circulating up there.

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Some poor bastards have bathroom and kitchen exhausts which vent into the attic, which makes moisture buildup another concern. Soffet + ridge ftw. –  Will Jul 21 '10 at 20:16
    
@Will - yup, myself among them :) –  Eric Petroelje Jul 21 '10 at 20:30
    
Appears to be the same here from what I can tell. What's worse is that I don't have an access to the attic, so if it really is dumping into the attic it's going to remain that way for awhile. Sigh. –  Brian Knoblauch Sep 22 '10 at 13:12

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