Insulated Roof Decking
I live in the Houston (area 2B) area and am in need of a new roof. It is a 2-story Cape Cod style home situated N-S. The upstairs portion is approximately 1,080 sq ft with cathedral ceilings (R-19) and numerous sky lights (110 sq ft-double paned, low E) and 2 small double-paned windows. The area is cooled by a 4-ton unit, which has trouble keeping it cooler than 79 degrees during our 100 degree days (unit has been serviced annually and duct work is in good condition).
Question 1: Would I be better off using Atlas Nailable Cross Vent/Rb roof decking that is vented with a radiant barrier and a insulation value of R-25 or ACH Foam Nailable roof decking that is not vented and has no radiant barrier but has a R value up to R-43?
Question 2: Are there any other options/ideas for decreasing my energy cost for this area?
Question 3: Are the “cool” roof’s worth invesigation?
Thanks,
Steve Wardlow
GBA Detail Library
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Replies
If your concern is reducing cooling load, then a light-colored roofing over a radiant barrier vented insulated nailbase would give you the best results - but you're going to either have to eliminate the skylights or install radiant barrier shades.
Steve,
The R-value of the panels you install on top of your existing roof will give you a good idea of their thermal performance. The R-43 panels will clearly perform better than the R-25 panels.
Your proposed project will be expensive, of course, because all of the skylights will need to be raised. Robert's suggestion -- to eliminate some of the skylights -- is a good one. Skylights facing south or west are the most problematic for overheating.
Considering the expense you are about to take on, it's always possible to vent your new roof on top of unvented insulated panels. First put down the nailbase, and then install eave-to-ridge 2x4s to create ventilation channels, and then install another layer of roof sheathing. While this is always possible, I doubt whether the added expense makes sense in a climate like yours.
This is so overly-simplistic to be meaningless.
If the only or primary mechanism of summer heat gain was conduction of heat from the outside air, then simple R-value would be the appropriate measure.
But most heat gain through the roof in hot, sunny climates is from solar radiant energy. Depending on the color and reflectivity of the roofing material, a roof can be up to 100° hotter than ambient air when the sun shines.
There are three approaches to reducing peak roofing temperatures (and hence heat flux to the interior and consequent cooling load): light colored reflective roofing, a radiant barrier under the roof deck, and venting (in the order of relative importance).
Installing a vented nailbase radiant barrier insulation (like the Atlas CrossVent RB) will reduce summer cooling loads. Unless very reflective roofing is used, a vented radiant barrier may be more important than mere R-value.
Increased R-value will prorportionately reduce conductive heat flux, but if the surface is 50°-100° hotter, then the delta-T between the roof and the indoor environment can more than offset the increased resistance to conductive transfer.
So, however, it's accomplished, it's more important in a cooling-dominated climate to reduce peak roofing temperatures. But no roofing strategy will help if heat gain through the skylights is not controlled.