Expert: Daniel Humphrey - 1/14/2012

Question
Dan,
I have an exposed deck on beam constructed home in Southern New Jersey and I would like to insulate my roof. I cannot maintain a warm home in the winter or a cool home in the summer. I feel that insulating the underside of the roof would help. Given your experience with timber frame construction I thought you would be the best one to ask. Are there any methods, details that work with this type construction?

Answer
Dear Bill,

The proper way to do it is to insulate above and re-roof. Infilling between the beams is difficult, doesn't look as good as the original deck, and the insulation value is not very efficient. If you leave the beams exposed, that thermal bridging is at most about R-10. Painting and trimming around the beams is tedious.

When you first think about it, it seems like a lot more work and materials to build up the roof, but it usually turns out faster and a better roof, for just a bit more money. Basically, you'r investing into a superior roof, rather than an interior patch job. If the roof shape is not complex, the insulating and roofing is very easy. Three persons with roofing experience should be able to do an average house in a week. You entirely avoid disturbing the inside of the house and all the interior finish that infilling would require.


The easiest way to build up the roof is to apply 2x4 purlins on edge onto the existing roof deck, infilled with 3-1/2" foam panel insulation. On top of that, perpendicular, screw another series of 2x4s, like rafters. Infill this with 2" foam panel and leave 1-1/2" air gap. Above that goes plywood and then whatever you want to roof it with. Install new fascia, and include screened vents at the eaves and the peak to keep the roof assembly dry and help avoid ice dams and heat damage.

So it's two layers of rigid foam, with very little thermal bridging, totaling about R-30, depending on the foam you choose. The roof can also be thicker, but the 7" dimension works well. I use either extruded polystyrene (XPS) or polyisocyanurate (Poly Iso). XPS is easier to work with, but it has a lower R-value per inch.

Hope this helps.

Daniel