Thanks so much for the lesson--I need it, it seems! What you shared puts it into much better perspective, and given all we've spend on the renovation, and the fact we've tried to make it as green as possible (w/in reason cost-wise), I think I'll just go w/the R-38 to save the additional cost, since we're not done yet!

I live in zone 5, outside of Chicago, and the building code in our town requires R19 in ceilings. I see you are in zone 7. How extreme do your temps get?

There is not a place in Illinois that the Department of Energy, International Codes Council, nor any other serious energy policy body suggests should have anything less than an R38 in ceilings and at minimum an R49 in others.

That Illinois has yet to adopt national standards only speaks of Illinois' lack of seriousness as far as energy conservation as well as building code standards goes, and Illinois should be seen for the exception it is rather than as any rule or canon by which to measure the rest of the US.

That said, what kudzu9 said is essentially true: more energy can be lost through a windows than through ceilings... but the caveat is that you must still comply with the energy code established by your jurisdication or energy compliance "paths" allowed.

What this means is that you are legally obligated to do certain things depending upon your jurisdiction and the path selected. DC area may require as little as R30 for ceilings and as much as R49.

The ultimate and final authoritative answer will only come from your local building code enforecement office....not the builder and from anyone in Illinois.

Energy Codes are adopted statewide and modified locally just as are building codes.

So check yours for complianace paths and alternatives.

GardenWeb/ThatHomeSite "zones" are completely irrelevant to the discussion.

I wasn't really trying to compare windows to ceilings...just giving an idea of relative effects of R-factors. But manhattan42's comments made me realize that a better comparison, obviously, would be an uninsulated ceiling to an insulated one. I did a little more hunting and found the R-factor for an uninsulated ceiling, which is about 5. So a ceiling with R-38 insulation will have a total R-value of about 43 (i.e., 38 + 5), and will let through about 5/43 (or about 12%) of the amount that an uninsulated one will; a ceiling insulated with R-49 insulation will have an R-factor of about 54 (i.e., 49 + 5) and let through 5/54 of the energy (or about 9%). So you would get an additional energy savings -- for the roof -- of about 3% with the higher R-factor insulation. How much this will translate into actual energy savings depends on what your other losses are (walls, floors, windows, fireplaces, cracks and infiltration). And I also agree that this is a code question. Hope this helps you refine your decision.

I bet the cost difference is not that great. I say more is betterer, and don't forget, blown-in will lose its efficiency over time as it compacts.

Of course the GC's 'taking the easy way' as they are often wont to do.

Checking the code requirement is probably the first step.

Having said that, I'm getting my roof redone on monday, and ideally I'd like a better underlay but I'm not working and just on my wife's salary we're strapped for cash - and we're going with 'how it gets done round here' too.

Thank you, Manhattan42, for such a definitive and authoritative response to the OP's question. It's so reassuring to know that New Yorker's know exactly how building codes are applied throughout the rest of the US.

I'm not a New Yorker but I am a nationally certified ICC code official....including soon to be Residential Energy Inspector.

And since the ICC Codes are the law of the land in 47 of 50 US states and most territories, I DO have something defintive and authoritiative to add to the discussion.

That said, one loses more energy through windows than through ceilings, and decreasing the U value of windows packs a greater energy savings punch than does increasing the R value of ceiling insulation.

You can also increase the energy efficiency of your home for far less by simply stopping air infiltration with caulks and sealers than by increasing attic insulation.

Energy studies indicate that between 30-50% of a home's heat loss/heat gain is from air infiltration and not from lack of insulation.

Again, the original poster needs to check with his/her local building code office for legally allowed energy compliance paths.

Manhattan42-
Good perspective and good points. Another way to think of the big drain that windows allow is to compare even a double pane window with a well-insulated attic. Since a standard thermopane window (R-2) will have about 20 times the heat flow (per square foot) as an R-40 ceiling, a 10 square foot window will lose as much heat as 200 square feet of well-insulated attic.

kudzu9 are you sure it's that simple? Because hot air rises, much of the window isn't in the hottest part of the room/building. So lack of insulation in a ceiling would presumably make much more difference than lack of insulation in the lower part of a window (assuming no infiltration)????

I'm reading your R-38 thread with much interest. I'm working on a very tight roof condition and want the thinnest possible R-38 insulation. Any ideas?

"I'm reading your R-38 thread with much interest. I'm working on a very tight roof condition and want the thinnest possible R-38 insulation. Any ideas?'

Closed cell spray foam.
Ron