I am looking for some kind of chart or data that shows relative heat loss vs R-value. My brother and I bought on old 1968-vintage A-frame. Cathedral roof with 3/4″ decking, but no insulaton. Needs a new roof, but budget is very tight. Am looking for something that will help be complete the statements:
“R19 will reduce heat loss by xx% compared to just roof decking”,
and “R30 will reduce heat loss by an additional yy%”
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Replies
R value is related to heat loss per square foot as an inverse value.
So an R-19 means the heat loss is 0.0526 BTU/sq.ft./hr. R values for each component of a wall or roof system are available in charts, as well a u-values (heat loss).
You also need to know your design temperatures and the number of heating and cooling degree days for your area. Unless you have all that information even a good system engineer would just be taking a SWAG at what you want to know.
With your tight budget, it sounds like you want to get the most bang for your buck. Experience has tought me that the payback period for a belt and suspender approach is acctually longer than making a larger up front investment.
Thanks,
that does help. which brings up a different question. if you have 2 different materials stacked on top of each other, do you calculate an equivalent R-value by adding up the numbers? For example, you have some foam that is R-19, and the wood decking that is, say, R-4. So the effective R value wold be 19+4 = R-23?
What is the R-value of wood per inch of thickness, anyway?
The Federal government has already put together insulation guidelines for different areas of the country. My State - Cali - has broken it down even further as we have so many microclimates. For me, I'm in Federal zone 3 (Cali Zone 7 I think), which calls for an R30 roof.
I'm in the process of replacing and insulating at the roof deck at the moment. What I'm doing is furring up portions of my roof from the deck out, and insulating that new space OVER the deck. You could easily do the same as well, using Larsen trusses, blown cells, and a covering of foam... covered with a new deck and shingling over that. Ta Da - you get all the insulation you need without disturbing the inside of the A frame.
Generate a simple spreadsheet of heat loss vs. thickness/R-value. Graph the result ... it will quickly show the classic concept of 'diminishing returns' on insulation. The reason codes require e.g. R-19 walls and R-38 ceiling has more to do with the economics of the construction. Attics are easy and inexpensive to insulate to high levels; walls are more difficult.
R-19 will reduce heat loss through conduction by about 95%. R-30 will reduce it by about 97%. There are other factors that affect heat loss aside from conduction. Air infiltration is the biggest.
Well put. Other things being equal, though (e.g. insulation material and installation quality), the absolute amount of air leakage doesn't necessarily change w/ R-value (although the ratio/percentage heat loss may).
what about the end walls?
The problem is more complicated than just the R value added to the roof surfaces. What about the end walls? Are they mostly glass? Are you planning on upgrading their insulation levels by a similar amount?
I presume so and that your question was specifically about how to insulate the roof. As it stands one cannot estimate your xx% etc.
Prehaps check the local building code for what they are currently requiring for cathedral roofs in your area. That would be my starting point.
John