Last winter, this house had been roughed in, but was uninsulated and unheated. Most mornings, I would see the image of the trusses on the shingle exterior as in the attached photo. I would’ve considered this to be thermal bridging, but since the building was nearly the same temp inside and out, how is this possible? Now that the house has been air sealed and super-insulated (10″ of SPF and 10+” of cellulose), I am still seeing the same condition on the outside. I’m just curious as to what this is caused by…any thoughts?
Edited 1/24/2009 10:52 am ET by shawncal
Replies
The thermal mass of the structural members will always have an effect, regardless of insulation. The areas in contact with the structure will cool more slowly, warm up more slowly, and retain solar heat longer.
Especially in situations of a light frost, a difference of a degree or less can result in a visible difference in the amount of frost accumulation.
Is the attic space inside or outside the thermal envelope now?
Edited 1/24/2009 1:28 pm by mmoogie
It is a hot roof- SPF sprayed on underside of roof deck and remaining space filled with dense-pack cellulose.Shawn
I was just curious. I'm trying to recall if I've ever seen the reverse...where the frost burns off between the rafters first. I would think that would happen if you had a thermal envelope at the attic floor, but lots of heat leakage into the attic. In that case you might think that the rafters would provide some actual insulation value vs. the open roof deck, and you would see the reverse of your picture.In your picture, where there is no heat in the building, I would think Dan's thermal mass explanation would account for it. Or possibly ground heat inside the building shell.Steve
I've definitely seen cases where there's frost over the rafters only. Not sure if it was colder there initially so the frost only formed there, or if there was frost over the entire area and it burned off everywhere except over the rafters.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith