In a cold climate it ( reflective insulation - my insert) can negate the value. Bring your attic up to code (minimum), air-seal, and minimum ventilation. You want the solar gain in the winter...... Power fans have there own dangers.
Thank you for all the links Gary. I was only talking about radiant for hot climates and assuming the home was already built and had insulation under the attic. I was also assuming the attic was unconditioned.
For a new home which I want for myself I would have a conditioned attic. I would put 4-6 inches of polyso on the roof decking to prevent condensation over the poly I would put a reflective sheet and then lay 2 by 4 on their side to create an airspace. I would then put another decking on the 2x4s which I would cover with light colored shingles. This would kind of be like a sip roof with an airspace. I would then space dense cellulose between the rafters to exceed the code and bring the insulation value up to r 60 which I think Martin suggested in one of his blogs. I am trying to follow the guidlines of r 10 under the slab r 20 for the basement walls, r 40 for the walls above the basement, and finally r 60 for the roof with a conditioned attic. This was in his blog on too much foam and the passivhous standard which he found to be to arbitary. This is, I hope, a more reasonable standard for super insulated homes. Does this make my short post clearer? Thanks again for your very good response and the links. I realize I have a lot to learn so if I am off base please let me know. Robert Nemoyer 717-766-5799 or
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