What is the ideal R value for walls and roofs?
Last Post 14 Mar 2012 11:21 AM by Dana1. 16 Replies.
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SCIP PanelUser is Offline
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03 Jan 2012 02:49 AM

What is the ideal R value for walls and roofs?

For starters my location is Denver Colorado.

I have heard arguments for R30, R40 and some say R50 is where the tipping point of cost starts to over shadow value.

I have been told I am building an over insulated home with walls R40 and roof R80.

Having turned off the heat a few days ago with the interior morning temperature in the low 60s and afternoon highs 60s to low 70s I like over insulated if that’s what it is.

Exterior temperatures have ranged from teens to thirty’s and the home is still under construction with workers going in and out.

AltonUser is Offline
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03 Jan 2012 08:12 AM
Since this will be your home for some time, build in all the creature comfort that you can afford.  The longer you live there the more you will enjoy a super-insulated home.  I would not consider it OVER-insulated as long as it is built tight and ventilated right.

We buy a lot of creature comfort in cars without any concern about return on investment - why not homes.  During the boom days when home prices were going up as the bids were being opened, builders had a tendency to offer a lot of eye candy but not much substance.  Now that home prices are not apprecating very fast, I think a person can still have creature comfort but will have to be more careful about overpaying.  In other words, get more bids and consider different ways to build.
Residential Designer & Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu, 334 826-3979
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03 Jan 2012 09:01 AM
Builders offer what people want to buy. It is unlikely that folks will start wearing their fuel bills around their necks, but most of those retiring now are beyond the BMW, fur coat and diamonds. Of
course there carbon foot print would be lower regardless of taste as the kids are gone along with the energy to consume.
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Dana1User is Offline
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03 Jan 2012 12:29 PM
The $/R-foot number is radically diferent depending on the methods used, and the net-present-value of the R depends on actual future energy costs, so there's no "ideal" number. Even at relatively modest R values air tightness is more important than ever higher R too.

Using lower cost methods of insulating (stick built, cellulose, rigid-foam sheathing) and assuming heating & cooling energy costs no more than the current cost of grid-tied photovoltaic solar running heat pumps with an average whole system coefficient of performance (COP) of 2.5-3.5 it's possible to make a financial argument for R30 walls and R75 roofs in Denver (US climate zone 5), and probably even R40/R80. At R values that high the air tightness and the quality/type/orientation of the windows will drive the thermal performance of the house. (Going to R40 whole wall vlalues with closed cell polyurethane at $0.17/R/ft probably wouldn't cut it though.) For a discussion on the economics & lifecycle costs of high-R houses, see:

http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones

See table 2, p10, and note that those are whole-assembly values, with all thermal bridging factored in.
jonrUser is Offline
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03 Jan 2012 12:35 PM
A major factor is how long you intend to live there.
Dana1User is Offline
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03 Jan 2012 03:12 PM
I think it's fair to hazard that nobody is going to build an R40-walled house with an R80 roof an flip it in short years.

From purely a net-present-value point of view there isn't an argument for even code-min values if you're only sticking it out for 5-6 years. But from a comfort point of view 1.5-2x code-min can be "worth it" on day you move in (if move-in day happens to be the coldest or hottest day of the year.) Code-min for Denver is R19 cavity or R13+5 for stick-built, which works out to ~ R15 after thermal bridging. Taking that up to R30 whole-wall is a difference you can feel when outdoor temps are at the temperature extremes, but only if the rest of the place is similarly upgraded.

Air sealing to under 2ACH/50 might make as-big or bigger difference than going 2x code min on R values from a heating energy use POV though. "Typical" new construction these days runs ~ 5ACH/50, with many examples at 10+, but there will also be examples in the 1.5-3 range, if the builder paid any attention. Code max per IRC2009 is a wimpy 7ACH/50, which would normally be easy to retrofit should it actually fail (maybe it would be a good idea to shut the windows? :-) ) If there's such a thing as an "over insulated" house, it's one that's little more than a well-insulated wind-tunnel.
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03 Jan 2012 03:37 PM
Unless the floors/ceilings are radiated, in which case, the comfort angle is weaker. Having both is the bomb! Hehehehee
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Lee DodgeUser is Offline
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08 Jan 2012 11:22 PM
SCIP Panel atarted this thread with:
"What is the ideal R value for walls and roofs? For starters my location is Denver Colorado. I have heard arguments for R30, R40 and some say R50 is where the tipping point of cost starts to over shadow value.
I have been told I am building an over insulated home with walls R40 and roof R80."

The folks at National Renewable Energy Lab. that wrote BEOpt suggested the following general answer to this question. We all know that increasing insulation, say in the attic, costs the same for each inch, but it saves less and less energy for each added inch. At some point, your long-term cost will be greater than the amount of money saved in utility bills. Further, if your goal is to reach zero net energy, and you have a location suitable for solar photovoltatics, then at some point for a given additional investment, you can generate more energy from the solar PV system than you can save energy with more insulation.

Using BEOpt, I ran some cases for Colorado Springs, and it looks like R49 cellulose in the attic is about that limit before it is better to invest additional money in PV. (Denver would likely be the same.) For the walls, I assumed a stud frame house with studs on 24" centers, and looked at R19 walls plus zero, 1" or 2" of foam. The model indicated that the 2" of foam would be "cost effective" by the criteria above, but I did not run cases with thicker amounts of foam to see what the optimum might be. Using this approach, the optimum R-value for the roof will not be the same as for the walls, since it is usually more expensive to add insulation to walls than to a flat ceiling (assuming a vented attic).
Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
TexasICFUser is Offline
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18 Jan 2012 06:06 PM
Lee, Very interesting. It seems to me you have two important decision points:

1) When adding an additional inch of insulation gives you only a minor savings that isn't big enough to warrant paying for the additional inch.

2) When targeting net-zero and that additional inch of insulation, although not saving much energy at all, still saves enough to eliminate PV panels.

Do you agree? Or are you saying these two points are the same?

--- Sometimes PV decisions are made too early --- Buying solar powered bilge pumps for a boat with leaks comes to mind. Regards.
Lee DodgeUser is Offline
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18 Jan 2012 06:45 PM
TexasICF-

Yes, I think you have stated it correctly. Let me say the same thing with different words.

Case 1: Minimum lifetime cost: Start with no insulation (or code minimum) and add insulation and compute the heat loss. Compute the cost of the house with insulation over a 30-year mortgage, or whatever lifetime you desire. At some point, you will pay $100 for the additional inch of insulation, but including fuel savings and cost plus interest over 30 years, the savings will be only $90. So that additional inch is not justified for minimum lifetime cost, and you should stop at the previous insulation thickness.

Case 2: Net zero source energy design: Start at the house design that resulted from Case 1. Determine the slope of the line that represents the cost/unit energy to reach net zero using solar PV. This varies with the solar insolation at your location, but the BEOpt model computes this. You add additional insulation (ceiling, wall, basement, foundation) until the cost/unit energy savings for the last inch of insulation has a higher slope (greater cost) than just generating more energy with solar PV.

I like this philosphy, but it was stated by the DOE guys that wrote BEOpt. Their explanation is clearer since they use graphs. See Figure 3 at http://www.nrel.gov/applying_technologies/pdfs/nrel_be_opt_tool.pdf.
Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
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22 Jan 2012 04:52 PM
Thanks Lee - extremely interesting. Regards.
LbearUser is Offline
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22 Jan 2012 07:59 PM
Then as a general rule of thumb, would you say that a true R-25 wall and a R-40 attic would be sufficient and the breaking point in most homes?
Lee DodgeUser is Offline
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22 Jan 2012 09:10 PM
Lbear-

No, not at all. I mentioned those values that I had already computed for Colorado Springs since the original poster said that he was from Denver. Denver and the Springs are pretty close, so something in the ballpark of R-25 walls and R-50 ceiling would be a rough guess for that part of Colorado, according to BEOpt which relies on DOE2.

What is more important is the philosophy of targeting either: (1) a minimum investment of mortgage plus fuel costs over the lifetime of the improvements, or (2) if the goal is a net-zero energy house, then additional insulation levels are not justified if the same amount of money could be used to buy solar PV additions that would produce more energy than would be saved with the additional insulation. Now this philosophy by the DOE folks that wrote BEOpt, puts saving energy and generating renewable energy on the same level of desirability, and passive house folks may have a different philosophy. My personal philosophy is the same as the one used in BEOpt.

The R-25 walls/R-50 ceiling might be different for folks in Alaska or Houston, since the climate is dramatically different from Colorado. However, BEOpt can be used to account for those differences, and compute insulation levels for minimum cost and/or the net-zero energy option.
Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
Dana1User is Offline
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23 Jan 2012 11:26 AM
It IS fair to say that R25 whole wall and R40 whole-roof values are higher than current code in most of the lower 48 of the US, and WAY higher than the national average on existing stock, as well as pretty comfortable anywhere (even in Arctic zones).

Whether it's a breaking point from a long-term present value point of view depends on the climate and how those R levels were achieved. (What Lee said.) A roof/attic insulated with cellulose at 3 cents/R/square-foot will have a different breaking point than one insulated with closed cell spray polyurethane at 17 cents/R/square-foot, or EPS at 10 cents/R/square-foot.
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12 Mar 2012 05:57 AM
These answers vary dramatically based on what insulation and construction methods you use. I will attach a couple figures I am preparing for the book I am writing on building a net-zero home at a cost no greater than a custom house.

Figure one shows different insulations and there relative cost to achieve R-100.

Figure two shows a comparison of the cost/R/square foot of above grade construction methods and a general comparison. It is part of a discussion of why we build with the systems we do. This is a list of systems we have used and analysed. The Top system delivers R-value for at 20% of the cost of SIPs, ICFs or SCIPs. This is why we moved on. Sorry for the blurry image it is the largest the system would allow.

Brian
Lee DodgeUser is Offline
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13 Mar 2012 08:24 PM
Posted By zehboss on 12 Mar 2012 05:57 AM
These answers vary dramatically based on what insulation and construction methods you use. I will attach a couple figures I am preparing for the book I am writing on building a net-zero home at a cost no greater than a custom house. the system would allow. (Figures and text clipped)

Brian
Brian and Dana-

Please realize that the BEopt model includes a wide variety of construction types and insulation types.  There are costs, R-values (and for windows, solar heat gains) etc. for each option.  Therefore, the R-values that I quoted above were those that resulted when the model and I sorted through various insulation options and some standard construction options, and came up with results for the low-cost options with the best thermal performance out of those available.  If you insisted on choosing a more expensive insulation option, say spray foam rather than loose cellulose for an attic, then the model would happily arrive at a somewhat lower optimum R-value based on the higher cost.  But if you give it a range of different insulation types, it will pick out the lowest cost/highest performance option within the constraints of the system (such as wall thickness).  Of course, costs may vary locally and in time, so the user must adjust the costs used in the model for more precise local calculations, but the model allows for that.   

Lee Dodge, Residential Energy Laboratory, in a net-zero source energy modified production house
Dana1User is Offline
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14 Mar 2012 11:21 AM
Lee: I'm a fan of BeOpt, but have yet to fully explore it. ( IIRC it was only released for freebie download last year.) I'm sure I'll be using it often going forward.
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