Insulation under Tar and Gravel

Hello Again
Actually it is very important to insulate with 3" or more on your new roof (some municipalities require r-19 or better, and I've had a roofing consultant suggest 4"). I believe that 3" of solid insulation is equivalent to just about R-19, and 2" is equivalent to R-14. To give you a better idea of how this helps, exterior 2X4 walls are typically insulated with R-13 (most original Eichlers are insulated with R-11); now, if you have ever experienced the temperature of an unheated non-insulated structure, say a well sealed garage, vs. a non-heated insulated structure (say a home that one returns to after a long vacation),
the difference is noticeable. The comfort index in this climate has the most impact during summer months, when during the days that it climbs above 85 degrees, I and many of my clients have experienced a 10-15 degree change inside a well insulated house vs. outside. Further, since the roof takes all the sun all day long, the temperature of the roof is often hotter than the recorded ambient temperature.
Last of all, a radiant heat sub-contractor with whon I'm currently working, Glen of Anderson Radiant Heat, told me last Thursday that the temerature of the water in the lines in the slab can be 40 or more degrees less in a well insulated house with a high efficiency boiler(i.e. walls, roof, and glass) but every component helps (update one project at a time.......). It is important to weigh the benefits; your roof is the first and last line of defense against sun and rain, and since you'll be only have one chance to insulate properly in the next 25-30 years (you are allowed to build up 2 tar and gravel roofs in most areas, 3 in some; check with your local building dept.). Also, when you do get a roofer at your house, you might ask him or her if you can avoid the tear-off and overlay; it will save maybe $2k-$3k, but it will make all the information that I just posted about insulation somewhat academic.

We replaced our leaky T&G roof with foam 5 years ago and it made a HUGE difference, especially in the summer. I would put the difference at between 5 and 10 degrees which, on a hot day can feel significant.

I cannot comment on the recommended thickness of the foam in your situation where it is used under T&G as insulation, I will just say that we chose to have the T&G torn off completely and replaced with foam to get the weight off of the house.

Cathye

15 yrs ago we had 2" foamboard insulation installed when we reroofed with tar & gravel. It has made a HUGE difference. The installation was done in July and I could tell which part of the roof they had finished just by walking under it - those rooms were, and are, much cooler... Now, for some problems: in the last 4 yrs we've had roof cracking along the edge of the insulation where it stops. The roofer says they have a better membrane now to put all along that interface which SHOULD prevent this problem. The tarpaper layer is showing major signs of wear - I think too much for 15 yrs, but maybe not - it gets full sun most of the day and has dried out, which encourages the cracking. We're beginning the process of looking into re-roofing with the same, or different, material. The job is complicated by the water lines which had to be run over the roof 3 years ago. Now for a question? What kind of roof lasts longest?

I cannot comment on how our rigid insulation has helped regarding temperature because we replaced it before we moved into the house. However, we did have a 4" thickness installed. It raised the roofline a little 1-2", and the flashing was tight and very clean looking. However, our model has a shingled pitch in the middle which didn't have insulation, so there it looks to be about 1-2" thinner. It's negligible and no one has ever commented on it. Just wanted to give you some perspective from an aesthetic point.