Originally Posted by nealtw
In extreme heat of the attic the lumber drys out and with out venting it stays in the attic air until the weather changes and all that moisture is deposited back on surface of the insulation and lowers its R value. When your in an attic at 130 degrees that hot air from out feels pretty cool. Lumber was a living thing and it will live a long time if it is treated like it has feelings.
I agree that timber gets hot, whether or not it dries is down to the relative humidity at the time.
From the time the timber leaves the kiln it starts to adapt to the average relative humidity where it is, this means increased water content during the winter/rain season and drier during the summer/dry season, obviously while the amount of water in the air varies every 24 hours between the hottest day periods and the coldest nights and the high water vapor/relative humidity levels during the night and the lows during the day there is a general drift towards drier or wetter.
I think you are writing that the moisture from the wood, stays in the attic until the weather changes with the arrival of winter or summer?
I regret to write that I disagree your comments.
This is not possible. As relative humidity levels move as day changes to night and night to day with the changes in temperature.
The way water vapor/moisture works is that it always moves towards a colder place or surface or an area of lower pressure.
The surface temperature of the insulation in the attic will be at the same temperature as the air in the attic, most of the time.
You only get condensation, when warm wet air arrives from inside the home or from outside and the timber in the attic is cold and the surface of the insulation is cold.
Cold meaning below the Dew Point of the incoming air. Then condensation or frost is likely to form.
Most of the year, when the temperature of the attic is not below the Dew Point of the incoming warm wet air, water vapor leaves the attic and moves outside, where it is almost always cooler, and if the wind is blowing it will be at a lower air pressure, as the wind always creates an area of low pressure to the lee of a building.
The problem with fiberglass and cellulose, so called insulation, is that warm wet/humid air leaves the home, rising through the insulation, and as it does so it freezes, later the ice melts, makes the insulation wet and useless, and this provides an express route out for your expensive heat, as water is 4,000 times better at conduction heat from your home than is dry air.
This is why closed cell insulation's like Styrofoam/Blueboard/polystyrene etc are so much better in real life as they do not get wet inside, they can be covered in ice and yet they still do their job of insulating your home.