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My attic currently has batt insulation between the joists, and a second layer of batt insulation on top of that. In the summer, the second floor is quite a bit warmer than the first floor. I'm thinking about adding more insulation to the attic, but I'm not sure what the best approach is.

Originally I was considering simply throwing another layer of batt insulation on the floor. Now I'm thinking about insulating the roof instead, by adding batt insulation between the rafters.

Obviously I'd have to extend the baffles, to allow air to flow up to the ridge vent.

Would this approach cause any moisture, or any other unforeseen problems? Is there any benefit to this approach, over simply adding an additional layer of batts to the floor? Should I use faced or unfaced batts?

  • In addition to suggestions below, make sure wire penetrations, wall top plates, etc are air sealed. If they are not no amount of insulation will fix it. – user20127 Jul 28 '15 at 1:01
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Remember, heat rises. In the summer the second floor will be hotter than the first for several reasons. I found that after reaching a reasonable amount of insulation in the attic which helps more in the winter season, removing the heat from the attic is the best remedy. I installed a power vent running off a thermostat. You need to make sure you have enough vents around the eve of the house and that they are not blocked for this to work effectively. The cooler outside air will replace the hot attic air and as hot air rises, this happens quickly. It worked for me when I lived in New York, but now that I am in Florida it is even more important to get the heat out of the attic. Most Florida homes have wind turbine vents and several of them to make this effect work. Newer homes here have high ceilings eliminating attics.

  • The building science consensus on power venting is that it can work, but you have to weigh the cost of running the fan (may be on 24/7) against the AC savings (might only reduce its daily runtime by 30 minutes or less). But if you're looking for comfort and are willing for the net cost of running a power vent vs AC to be neutral or negative, then go for it. – iLikeDirt Jul 28 '15 at 22:05
  • Also, your attic floor needs to be more or less airtight, or else it can actually suck air out of the house, defeating the advantage completely. – iLikeDirt Jul 28 '15 at 22:06
  • 25 Amp A/C at 240 volts over 30 minutes is 3000 watt hours. That can run a 4 amp 120 volt fan for over 6 hours. Just a thought. – Damon Dec 2 '15 at 9:18
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As you've discovered, an insulated attic doesn't really provide much comfort, even if it does reduce heat flow by some amount. In order to get true second-floor comfort with insulation, you need to really, really improve the amount to R-60 or greater, and you need to use insulation that's opaque to infrared radiation, which fiberglass is not. I suggest adding 10 to 20 inches of loose-blown cellulose on top of the layers of batts.

Insulating your attic at the roofline presents many problems. To build an unvented roofline insulation assembly virtually requires spray foam, which is incredibly expensive in the amounts needed to actually produce comfort. To do this without spray foam requires ventilation channels, reducing the amount of space available between the rafters for insulation unless you fur out the rafters to get more depth or fasten several inches of foam or mineral wool boards underneath them. Finally, you need an airtight layer under that, which usually means drywall, which is a waste if you're not planning to use the space as finished area anyway.

There is an experimental method of doing a much cheaper unvented roof without spray foam and drywall using 4 lb/sf dense-packed cellulose between furred-out rafters, held in place with a vapor retarder (not barrier) membrane like Intello or MemBrain (no affiliation) but, again, this is experimental.

The physics behind what's happening in your attic is that insulation doesn't do anything to reduce the heat flow, it just slows it down. If you don't have enough insulation to slow the heat flow down so much the heat fails to make it through the entire insulation layer during a daily cycle, the heat gets through your ceiling and you feel uncomfortable. Even if it's not a lot of heat, it's enough to raise the temperature of your ceiling plaster or drywall, and that heat radiates down onto your head, and you feel much less comfortable than the air temperature alone would suggest. Having less than the critically large amount of insulation necessary to slow the flow enough will reduce the heat flow and reduce your AC bills, but not enough to prevent comfort problems anyway.

This suggests an alternative approach to adding like two more feet of insulation: actually reduce the heat flow itself with radiant barriers. Don't put it at the roofline; the best place is above the roof sheathing itself, but this may be hard if you don't plan to re-roof soon. The next-best place is above the existing insulation. To prevent dust from degrading its performance, use a type with multiple staggered layers, such as http://www.savenrg.com/upton.htm (No affiliation)

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I agree that proper ventilation will likely go a lot further in keeping your 2nd story cooler than added insulation. I'd try that first and then see if you still want to add insulation.

We recently replaced the roof on our two story home and left the insulation in both attics alone/as it was. We didn't add any insulation or change anything else in either attic. However, we did have the roofers install a continuous ridge vent on the first and second floor and that has kept the temp in both attics much cooler than the existing venting we had before (at the eaves and at each end of the roof) the new roof. The two bedrooms upstairs still feel warmer to us in the summer than the first floor does but it's not as pronounced as it was before the additional venting in the roof.

  • I currently have ridge vents in the roof. – Tester101 Jul 28 '15 at 22:01

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