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I have a classic 1964 rambler. I recently had a new roof put on and additional insulation blown in to bring the R factor to 40. The contractor recommend replacing the original venting with ridge vent. The first day the outside temperature was above 75, the entire house was cooking. Before the new roof was placed, it had to be above 80 degrees for 48 hours to create this kind of heat inside. We haven't started our summer season and if this is bad now, it will be unbearable then. They made no adjustments for additional soffit vents. Is this the reason? Shouldn't the additional insulation help keep the heat above the ceiling? Are there any steps I should take to improve the venting?

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    I would add a temperature activated fan. There are solar and 120V models and they really work well at removing the excess heat. Then your home wont feel like an oven in the evening and the AC usage will be much less. – Ed Beal May 12 '16 at 22:38
  • I doubt that a small change such as a roof vent would cause the house itself to overheat; what happens in the attic, on top of that R40, doesn't affect the rest of the house much. – Daniel Griscom May 13 '16 at 0:42
  • That might be something sort of approximating the truth if the insulation were applied perfectly and surrounded by perfect air barriers on all six sides and there were no ducts in the attic. In the real world, almost never are these three things true. And even if they were, decreasing the temperature in the attic decreases the heat flux through any level of insulation. – iLikeDirt May 13 '16 at 3:29
  • What's a "rambler" ? I started out thinking you were dealing with an automobile :-) . – Carl Witthoft May 13 '16 at 13:17
  • So: does this house have an attic/crawl space? Where was the insulation placed? Did it block soffit vents? I can guarantee that the ridge vent did not increase the heating -- that's just plain impossible. – Carl Witthoft May 13 '16 at 13:20
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This is a horrible problem to have, and yet you will encounter many who will insinuate that you are crazy for suggesting it. Hold strong: you are not crazy. The problem is real. More insulation is not always better in all situations for all climates at all times of year irrespective of ventilation. Ignore people who claim this. They have never personally experienced this phenomenon, as you and I have.

Here's what's happening: The added insulation is indeed slowing down the heat transfer, so less heat is entering your house during the day. However, the amount of diminished heat transfer may be a lot less than the sticker R-value might suggest due to a multitude of factors, and poor design or installation choices made during the work may actually have resulted in more heat gain. Here are some options for you to investigate:

  • Attic ceiling not air sealed; hot attic air can go right through the insulation and into the house
  • Uneven insulation application; heat movement barely diminished in some areas
  • Scammy contractors didn't blow/add as much insulation as you paid for; R-40 attic floor insulation is about 11" deep if it's all cellulose and 15" deep for fiberglass. If it's shallower than that, you got robbed!
  • Use of fiberglass, especially on the top layer; fiberglass is more air-permeable than cellulose, and slightly IR-translucent, which means the the infrared heat transfer from the roof decking is not getting blocked as well as you might expect
  • Potentially darker roofing material dramatically increases the amount of heat that the attic receives throughout the day
  • You said they "replac[ed] the original venting with ridge vent." What was there before? Gable vents? Turtle vents? This matters, because if you live in a windy area and/or if your roof has a low pitch, then turtle or gable exhaust vents may be more effective than ridge exhaust vents
  • Contractors might have clogged the soffit vents, and if there are only a few of them, then they may not be sufficient to provide enough air for the new ridge vent
  • The house now cools off less at night because of the insulation slowing down heat transfer out of the house once the attic cools down to below the indoor temperature; the insulation is working against you after the sun goes down!

So, what do you do? Here are some options, presented in ascending order of cost and hassle:

  • Once the sun goes down, open the windows and turn on your bath fan to cool the house
  • Ensure that soffit vents are unblocked
  • Ensure that the ridge vent was installed properly and is not blocked by anything
  • If the insulation is uneven or lumpy, smooth it out
  • Add more soffit vents
  • Add a solar-powered attic ventilator, and face the solar panel west if at all possible. Western sunlight is your enemy; use it to your advantage by having it power a fan to exhaust the hot attic air (note: only do this once you have air-sealed your ceiling/attic floor)
  • Add gable vents (if applicable)
  • Staple a radiant barrier under the rafters
  • If the top layer of insulation is fiberglass, add a few inches of cellulose insulation over the top of it
  • Lay a multi-layered radiant barrier OVER the insulation on the floor (note: must be multi-layered, such as http://insulmation.stores.yahoo.net/thcome.html (no affiliation), since the top will get dusty so there must be additional layers that dust will not settle on)
  • Add a whole-house attic fan and run it as soon as the sun goes down (note: only applicable in a dry, mountainous climate, and make sure it has insulated doors that are closed during the day)
  • Replace the roofing material with something lighter-colored and preferably made out of metal, installed over ridge-to-eave purlins to create over-sheathing ventilation channels
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  • Maybe, but you're making a lot of assumptions about just where the insulation was place. – Carl Witthoft May 13 '16 at 13:18
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    Based on the information original post, there is a nearly 100% chance that Mark's house has an attic under a pitched roof, and contractors blew cellulose or fiberglass onto the attic floor and/or over existing insulation. I said that it is possible that insulation is now clogging the soffit vents, not certain. It's simply something to investigate. – iLikeDirt May 13 '16 at 14:42

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