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We have a house in Seattle that is 60 years old with 6-8" of blown in cellulose insulation in the attic with an inch of rock wool under that. There is basic ventilation in the attic. The trouble we are facing it seems the insulation is holding the summer heat during days over 80 degrees and in the evening the house is warmer than outside. Is it possible we need to replace the old insulation??

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Um, you understand the POINT of insulation is to prevent heat from moving between spaces, right? – The Evil Greebo Jul 14 '14 at 16:24
Are you saying the heat from the insulation is being released back into the house, or that the air in the house is not cooling off as fast as the air outside? If the latter, I'd say the insulation is working as designed (i.e. preventing the transfer of heat through the ceiling). – Tester101 Jul 14 '14 at 16:25
Replacing the old insulation won't fix the problem, more attic ventilation to remove hot air and radiant barriers to keep the Infrared from the underside of the roof sheeting from penetrating into the insulation will. As also will a change in reflectivity/color of the roofing material to prevent heat penetration in the first place. – Fiasco Labs Jul 14 '14 at 17:43
What actually happens is that the house warms and cools slower than the outside - when the point in the day that the outside is cooler than the inside comes, open the windows and turn on some fans. Contrariwise, when its cooler in the house as the day starts to heat outside in the morning, close the windows to delay the warmup inside for as long as possible. And yes, get more insulation - and possibly increase the "basic" ventilation in the attic, too. – Ecnerwal Jul 15 '14 at 0:42

UPDATE: This answer is too simplistic and lacks understanding of the true issue. See a superior, more complete answer here: Ridge vent conversion creating heat problem

The insulation is not holding heat (i.e. acting as a thermal mass). The likelier problem is that your attic itself is what's holding heat. Attic heat gain during the summer is primarily caused by sunshine. The sun hits your shingles and heats them up, and that heat radiates down into the attic. The darker the shingles, the more pronounced the effect.

As a result, an attic during the summer can easily reach temperatures 20-40 degrees higher than the ambient temperature due to this solar gain, even in your relatively cloudy climate. You can see for yourself by going into the attic around noon or 2 PM. It should be punishingly hot.

It's your insulation's job to prevent all that heat from getting through the ceiling and heating up the house. 6-8 inches of cellulose (R-3.5/inch) + 1 inch of rock wool (R-4/inch) works out to R-25 - R-32, which are numbers that are decent but not great for an attic. You need more! Blow more cellulose in there; 10 or 12 inches is a good start.

Another approach involves changing the color or material of your roof. The next time you have to redo it, if you choose light-colored shingles, or even light-colored metal, that will help tremendously to reflect the sun's rays, preventing a lot of that heat from even making it into the attic to begin with. But by the same token, this will hurt you in the winter, when that heat is actually helping to keep your attic warmer than the outside temperature. The degree of the penalty depends on how much of a heating load you have compared to your cooling load. Adding more insulation will help with both and is the recommended approach.

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