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Community and Q&A

R-values of ceiling and walls

Tony Tibbar | Posted in General Questions on

Zone 5a (on the border of zone 4)
Existing uninsulated concrete slab.
No basement. 2×4 frame.
A bunch of newbie questions.

http://www.homepower.com/insulation-zones
Why is the insulation recommendation for an attic so much higher than for an cathedral ceiling?
Both are the shell of the building but the attic has an ceiling under it that at least adds some insulation value. I would expect the recommendations to be closer or even reversed.

Many sites stress the importance of a high R-value to save energy.
More insulation saves energy; but that seems only part of the equation if I look at the recommended R-values.

The fact that floor and ceiling/attic have higher insulation recommendation than walls shows those area’s leak much more energy.
But when checking calculation on some sites the energy loss is a simple relation between inside temp, outside temp and R-value. If it was that simple I would expect the insulation values the same for the whole house.

Attic R-38 vs wall R15.5 ==> Can I conclude that an attic leaks 2.5 more energy by the same R-value?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tony,
    1. The tables published on the Home Power website are obsolete.

    2. A more relevant table is the one from the 2012 International Residential Code (see image below). In many U.S. jurisdictions, the values in this IRC table represent the law of the land. These are minimum prescriptive R-values; some code compliance paths (performance paths) allow lower values if the building is modeled using energy modeling software and otherwise complies with the intent of the code.

    3. Section N1102.2.2 of the 2012 IRC notes, "Where Secton N1102.1.1 would require insulation levels above R-30 and the design of the roof/ceiling assembly does not allow sufficient space for the required insulation, the minimum required insulation for such roof/ceiling assemblies shall be R-30. This reduction of insulation from the requirements of Secton N1102.1.1 shall be limited to 500 square feet (46 m2) or 20 percent of the total insulated ceiling area, whichever is less. This reduction shall not apply to the U-factor alternative approach in Secton N1102.1.3 and the total UA alternative in Secton N1102.1.4."

    In other words, you can sometimes get away with an R-30 cathedral ceiling -- but only if the rafters are too shallow for more insulation, and only if the cathedral ceiling area is either (a) 500 square feet or less, or (b) unless the cathedral ceiling represents no more than 20% of the total insulated ceiling area. Note that if 20% of the ceiling area is 350 square feet, you can't install R-30 insulation in a 500-square-foot cathedral ceiling -- you are limited by the 20% rule. Similarly, if 20% of the ceiling area is 600 square feet, you can't install R-30 insulation in a 600-square-foot cathedral ceiling -- you are limited by the 500-square-foot rule.

    4. GBA has never advocated in favor of the R-30 exception for cathedral ceilings. For a variety of reasons, including the prevention of ice dams, we have always advised builders to include the prescriptive minimum R-values shown in Table R402.1.1.

    .

  2. GBA Editor
    Martin Holladay | | #2

    Tony,
    Historically, the main reason that code-minimum R-values for walls have been lower than code-minimum R-values for attics is that it is much easier (and cheaper) to install thick insulation on an attic floor than on or in a wall. Practicality trumps building science in this case. While an R-20 wall may be code-legal in Zone 5, an R-30 wall would, of course, perform better than an R-20 wall. It's just expensive to build compared to an R-20 wall.

  3. Tony Tibbar | | #3

    Martin, thanks for your lengthy answer.

    Seems a case of back to the drawing board, because right now my plans exceed both the 20% and 500sqft rule.

    Everything is still in the design phase, so I likely can meet the code for the ceiling. R39 was already planned. Does exceeding the minimum mean I'm allowed a larger area and/or percentage? For example 40%.

    Are there similar relaxed codes for floors?
    The floor is a simple concrete slab without any insulation.
    Besides the problem of heat leaking away like with a low R ceiling the floor willl cause comfort problems.
    From that point of view I do need good floor insulation.
    I guess that means at least 6" of floor insulation.
    Is it allowed to have no slab edge insulation if the insulation of the floor has a (much) higher R value?

    In reply to your second answer. So, if a new cheap R50 foam board is invented the next update of the wall would required to be R50 too?
    How wrong I was.... I always assumed the ceiling got a high R because heat rises and slab insulation is higher because contact with soil leeches more heat than with air :-)

    Is there an article on GBA that gives hints and tips to construct a high R cathedral roof? Even if insulation was dirt cheap there is still the problem (for me) to nail down those many inches of insulation.

  4. GBA Editor
    Martin Holladay | | #4

    Tony,
    Here is a link to an article about insulating cathedral ceilings: How to Build an Insulated Cathedral Ceiling.

    I'm just guessing -- but your project may benefit from the help of a designer or architect. It sounds like some basic principles are new to you.

  5. GBA Editor
    Martin Holladay | | #5

    Tony,
    One more point: If you are trying to improve the insulation details on an existing house, you probably don't need to meet minimum code requirements. Talk to your local code official to determine what is required in your jurisdiction.

  6. Charlie Sullivan | | #6

    On the hypothetical question, "So, if a new cheap R50 foam board is invented the next update of the wall would required to be R50 too?"

    The way building codes get established is quite complicated and there are political aspects as well as technical aspects to the process. And the cost of insulation is not just the cost of the material, but also the cost of the installation, which can include the impact on the cost of other things, such as window installation, that can be affected by how thick the insulation is and how it gets installed.

    But if we set aside the political aspects--perhaps the new technology is non-proprietary, so it's available from multiple manufacturers--and we assume that the new insulation board is thin, easy to install, non-toxic, and durable, I think the answer is yes, the building code would be updated to require R50 or something around there.

  7. Expert Member
    Dana Dorsett | | #7

    R50 would probably not be required, unless there were an economic case for it. The economic case for current code-minimums is fairly conservative on a lifecycle basis- there is usually a lifecycle case for better than code minimums, but that isn't without limit.

  8. Tony Tibbar | | #8

    If the R50 is cheap there likely is an economical case for it.
    The way I understand Martin's answer (#2) is that the only/main reason that code isn't R49 for the whole house, is because it's very impractical or expensive.

    While it's expensive and impractical it's possible to build R50 everything right now except.... windows.
    Windows make me wonder about the use of high R values in the rest of the outer shell.
    I like to play around with the numbers and for that reason made a simple spreadsheet to do some calculations. An R30 wall with R1.8 window. When I add an additional R99 to the wall the total R-value hardly changes. But if I add just 0.1 to the window the difference is quite big.

    For that reason I'm wondering if building according to code-minimum and using the money saved to buy better windows is the most energy efficient way.

  9. GBA Editor
    Martin Holladay | | #9

    Tony,
    It sounds like you understand how to model buildings, and you are asking the right questions.

    As long as your energy modeling program or spreadsheet is a good one, and as long as it accounts for air leakage, you're on the right track. Keep playing around with different options. That's the way to determine the best envelope for your climate and budget.

  10. Charlie Sullivan | | #10

    Tony, if you want to compare the effect of improving windows vs. improving the wall, you'll need to make some assumption about the window area vs. the wall area. Your result--that an improvement from R-1.6 to 1.7 in the windows makes more difference than improving the wall from R30 to R99--would only be true for very large window area--something like equal wall and window area. It's possible that you didn't take the area into account at all in your calculation, and thus implicitly assumed they were equal area.

    One way to do the calculation is R_effective = (Total area)/(A_wall/R_wall + A_winding/R_window)

  11. Stephen E | | #11

    The problem lies in the use of R Value to determine how efficient house is. R value is based off of U value.

    Cheat sheet:

    R5. 80% Effecient
    R10 90%
    R20 95%

    Whole wall values. The deminishing returns on R value are steep. Number one heat loss is from air leaks, not insulation.

  12. GBA Editor
    Martin Holladay | | #12

    Stephen,
    There is no correlation between R-value and "efficiency." In fact, it's entirely unclear what you mean by "efficiency" in this context.

    As you correctly point out, there is a correlation between R-value and U-factor.
    R=1/U
    U=1/R

    Every time you double the R-value, you cut the rate of heat flow in half.

  13. Tony Tibbar | | #13

    Charlie,

    I think I must clarify post #10
    It was an exaggeration. Obviously adding R69 to the wall has a positive effect, but the money to do so may be better spend on other things to get more Rs for a buck :-)

  14. Buzz Burger | | #14

    Here's another example that I think illustrates Charlie's point. I did some calculations for a hypothetical wall that was 80% wall and 20% window. If the solid wall part had an effective average R-value of 20 and the 20% window part was R3 (U .33) the average R-value for the total wall is only R 9.37 (you can't average R but you can average U). As the percentage of window increases the results become even more dramatic. A 30% window/70% solid wall with same R 20 R 3 factors would yield an average of R 7.4.

    Back to the 80/20 wall: If you increase the solid wall R from 20 to 60 and keep the windows the same at R 3, the new average for the wall is R 12.5. Or, you could leave the walls the same (R 20) and install triple pane R 5 windows and the average is also R 12.5. So tripling the wall R value gives you the same improvement as increasing the windows from R 3 to R 5. A window with a U factor of .2 is probably going to be expensive so I'm not sure which change would be the more cost effective. But either way it's a little discouraging to see how difficult and/or expensive it is to get this hypothetical wall from R 9.37 to R 12.5. And if you upgrade both (R60/R5) the new average is R 18.75.

  15. Ben Rush | | #15

    I am REALLY trying to avoid work!
    Back to Tony's original question about the difference in required R-values for roofs/ceilings vs. walls...
    Warm air rises, so there would be bigger inside-to-outside temperature difference at the roof/ceiling than at the walls, right?
    Also (and maybe this is only a bad thing in cooing season), the roof and walls are exposed to the outside air (convection) equally- namely, all the time- but with most buildings the roof is exposed to more sunlight (radiant energy) than the walls are, since W walls are shaded in the morning and E walls are shaded in the afternoon.
    Are either of these part of the story?
    Thanks,
    Ben

  16. Expert Member
    Dana Dorsett | | #16

    It's not about the convection or hot air rising so much as it is the differences in average radiant temperatures of what roofs and walls are facing.

    The roof is angled toward the sky, and sees substantial night time radiational cooling that brings it's temperature 10s of degrees below the ambient air temperature when the air is clear. This is more pronounced in winter than in the summer due to higher humidity air in summer trapping some of that radiated heat, but it's generally true year-round. Roofs get down to the outdoor air dew point often and can be visibly wet for the first hour or so after dawn. Walls face the landscape, and hew fairly close to the air temperatures overnight, accumulating dew only rarely. This is by far the biggest mechanism for seeing bigger average roof temperature differences in roofs than walls in winter, but it is happening year-round.

    Roofs are also angled toward the sun, and hit higher peak daytime temperatures than even sun-exposed walls, which is another major difference that shows up primarily (but not exclusively) in summer.

    Bottom line, the diurnal temperature swings of roofs are much larger than the swings in air temperature, and bigger than the swings in wall temperature (on any side of the house.)

  17. Ben Rush | | #17

    Dana,
    You totally blew my mind with this sentence:
    "The roof is angled toward the sky, and sees substantial night time radiational cooling that brings it's temperature 10s of degrees below the ambient air temperature when the air is clear."
    You're saying that if I measure my roof temperature before sunrise, it will be colder than the overnight low air temperature? Wow. Just... wow.
    Now I'm wondering what else I don't know...

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