Extra insulation and/or radiant barrier, for an attic
I want to make my house cooler in summer afternoons and if possible a little warmer in the winter. I've pursued the question via a thread asking about extra attic insulation, and the idea of using a radiant barrier was brought up.
My house's axis is east-west, with the faces of the steep gable roof (metal-clad) being toward the north and toward the south. The walls are about R22 (fiberglass insulation plus thick wood siding) and about R20 in the second-floor ceiling (vermiculite between the ceiling joists in an unused attic space). The attic space is well ventilated. The house is fairly energy efficient - neighbors tell us we're using less winter heating fuel than they often are, even with smaller homes.
I'd like the house to stay cooler in mid-summer weather, and if the strategy can make the house retain heat better in winter, so much the better.
Originally, I thought I'd add another R20 (6-inch) layer of fiberglass insulation above the ceiling joists in the attic. I could do this, or I could use a radiant barrier behind the southside roof rafters. I could also use blown-in cellulose above the ceiling joists, but I'm not sure about the cost of the cellulose and particularly getting the equipment and personnel here to do the application (our land is semi-remote).
I suppose I could use both a radiant barrier as described above, plus more ceiling insulation. I haven't costed that out yet.
Seems to me a radiant barrier facing outward to repel heat back out of the south-facing roof will not have much effect to increase winter heat-holding efficiency.
I invite your thoughts about a cost-effective strategy in the case described. Thanks.