June 09 Foam insulation article
bedfordfred
| Posted in Energy, Heating & Insulation on
One thing that stood out in this article was the graph and statement that “As the thickness of the insulation increases for both open-cell and closed-cell foam, the insulating value of each diminishes drastically”.
The article goes on to say how a R-40 foam wall reduces conductive heat flow through a wall by only an additional 2% over a R-20 foam wall.
Is this performance particular to foam, or all insulation products in general?
Replies
It is not just foam. All insulation works the same. You are not looking at R value per inch you are looking at R value and heat loss.
Someone will be along with a link to a curve that shows R value and heat loss. The first few R produce the greatest return and the figures you cite may be correct.
The savings is 2% of what? Hey, would you give me 0.001% of the national debt? Heck, it's only 0.001%, so that isn't so much, is it?
The R40 wall will lose half the heat of the R20 wall. It's a matter of how big the half saved is, in absolute terms.
R, the resistance to heat transfer, is 1/U, where U is the heat transfer coefficient. In English units, that would be BTU/ft2-hr-F. Converting the R to U, the R40 is U= 0.025, and the R20 is U= 0.05. The difference is 0.025 BTU/ft2-hr-F. How big is that in real terms?
Suppose you have a total of 2000 ft2 of exterior wall area, you keep 70 F inside, and on a cold winter night it's zero outside. Multiplying out: 0.025 x 2000 x 70 = 3500 BTU/hr for the R40 wall, twice that for the R20 wall. The difference equates to a bit over a kilowatt if you were heating with electric resistance heating.
Now, one could argue that an R40 wall probably is a double-wall, with a real R value of 40, while the "R20" wall might be your typically leaky 2x6 wall with a poorly installed FG batt, which might perform at only R13 under those conditions. If the R20 wall is a foam job in 2x6 studs, then the thermal bridging reduces the whole wall to about R16.
Edited 5/26/2009 1:49 pm ET by DickRussell
Dick, your explanation makes perfect sense. Are you disagreeing with the statement in the article?I'm in a learning mode right now, and have been for the past two years. In a couple years, I plan to owner-build a Cape-style house. My goal is to use materials and techniques that will result in an extremely energy efficient home.I trying to learn the points of diminishing returns. My inclination is/was to aim for a R30-40 wall, and R50-60 roof. I'm considering 2x6 mooney with 2" exterior polyiso, or 2x8 mooney without exterior foam, or a 10" double wall, all with cellulose insulation. I was also considering a 2x6 mooney with spray foam. This article confused me though. It suggests that anything past R20 is a waste of money.
Fred, I haven't seen the article. My issue just arrived; I saw the title, figure it will be a good read at least. My own project, up in Moultonborough, I hope to get under way later this year, or next spring at the latest. I've been looking at all sorts of ways of getting to essentially "superinsulated" level. My targets are much like yours in the general sense: R40 wall, R60 attic floor. Done from the start, those aren't all that difficult to do.I've often seen that argument about going past R20 not being worth it, because 95% of the heat has already been saved. Never do you hear the writer state how big the other 5% is. Whenever I hear the word "percent," I automatically think "of what?"Until I read the article, I can't speak about the background of the author, but I am speculating that he is in the spray foam business. Understand that building codes typically limit the amount of "plastic foam" that can be in a wall to 4", or 6" in a ceiling below the attic, unless a fire test report shows that more can be used and pass the tests. The E84 test apparatus, as I understand it, more or less limits the test to a 4" thick layer of the material. There is a more expensive test that can be done, but I don't know how often it is done.I admit to not knowing just where and under what codes applying spray foam insulation in greater thicknesses (total after multiple lifts) is done and approved. I would like to know this.In any case, it does seem that reaching superinsulation levels with spray foam insulation is difficult to do. SIP panels can get you there with prefab foam. The 4" of thickness, done with CC foam, at aged R6 or so per inch, will give [edited:] R24 at the center of insulation, or R 17.5 when allowing for thermal bridging of studs @16"oc, R=1.2/inch. I wouldn't call that superinsulation. Neither would those advocating a longer look at the future for housing in cold climates.If R20 is all that can be achieved in a wall with spray foam insulation under present codes, it would make sense for the vendors to argue that going for more than that is not cost effective. If your only tool is a hammer, then obviously nails are the only fasteners you really need.
Edited 5/27/2009 10:16 am ET by DickRussell
I read the article last night. I understand now that the author (an associate editor) is not a spray foamer, so I am assuming he got graph of insulation "efficiency" vs R or thickness from someone in the business.I think that graph is misleading, since efficiency hasn't really been defined in this case. If R20 is said to have saved 95% of "the heat," then the basis for comparison must be an R1 wall. That would be basically an open stud wall, with just sheathing and siding between the folks inside and the great frozen outdoors.Suppose we define the reference point as a thin sheet of copper over the studs, and no siding, so that the total R is, say, 0.1 with a whopping heat loss. By adding a half inch of open cell foam, we now have saved 95% of "the heat" and anything more than that is a waste of money, right?
I haven't read the article yet but i have been shown the same type of graph described. It was based on the difference in surface temperature, -20 on one side +70 on the other. The point is there is a diminishing return as you add insulation. The question is always: how much is enough?
I found some good comments about this article here:
http://www.greenbuildingadvisor.com/blogs/dept/musings/passivhaus-beginners
Try doing a heatloss calc on your proposed house. It will tell you everything you need to know. Windows are everything. Walls mean little. 1/2 the heat goes out the windows, half the remaining will go out the roof. Put the money there.
Huhhh....................????????????????????
in ref to the guy wanting r30 wall.....
Not always so.
Heat loss happens where the insulation is the worst. if there is no insulation in the walls and plenty in the roof, that is where the heat loss occours.
We see that all the time in older homes where somebody heard the myth you just repeated, and they insulated the attic because it was easy, but leave the walls un-insulated.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Even on a new home, it depends.
Take a 1500 sq ft house, slab on grade, 30 x 50, 10 foot walls. R20/R40
1500 sq ft of ceiling, 1500 sq ft of floor, 1600 sq ft of wall, minus doors and floors.
Can't see half going through the roof.
But there is validity to the heat rises theory. In my case my ceiling has 12 inches of foam (closed cell) my walls only have six inches. Well except they also have 4+ inches of stone, or black walnut timbers.. and 6 inches of white oak timbers..
Even with 107 windows my house retains heat extremely well during the winter and keeps the house really cool during the summer.. It's 78 degrees outside right now, and the inside temp is only 69
"there is validity to the heat rises theory."no there is not.warm air rises compared to cooler air, but radiant heat loss is the same in all directionsA combiunation of convective loop in the heated space and conductive heat transfer at the ceiling can add to some added loss of BTUs at the ceiling plane compared to the floor, but in a tight louse with little convective heat loss, that is negligible.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
You've touched on something I've often wondered about. Does regular insulation (FG, cel, spray foam) do anything to stop radiant heat loss, or just conductive?Pete
Pete,
I think that Martin's report about R-value gives a nice description.
http://www.greenbuildingadvisor.com/blogs/dept/musings/understanding-r-value
Piffin,
I agree...heat loss/gain happens where there is a weakness....I think that many people confuse what is cost effective and what needs to be done to slow down heat transfer.
Why is it that building codes often mandate 2 or more times the r-value for ceiling/roofs than walls?....Because it is generally easier to add R-value at the ceiling/roof....Especially in Existing construction.
I think that the Poor old wall is being neglected.
Especially if you are building a new Low Energy Home....A right sized home (not-so-big) with a low Surface Area to Floor Area ratio may be a 1-1/2 or 2 story house with exterior wall area equal to or EXCEEDING roof area.
The ceiling/roof may be R-40 to R-60...Windows will be decent and hopefully not too many square feet.... The home will (hopefully) be reasonably airtight...The sun may be considered and allowed for.
If you are very progressive you may have an HRV.
So now Where is the greatest Weakness???
THE WALL including all of it's common thermal bridges.
I think that R-30 and Greater walls can make good sense in many climates...perhaps a longer "payback"...but good sense.
I think that the FHB article was very poor !
John Brooks
A heat loss calculation, like I did on my house, where you can see the contribution of each wall, window, etc can be very enlighting.For example the MBR has a vaulted ceiling with only R-22 in it. But the losses through it are so small that it would take a 100 years to payback just for the cost of 2" of polyiso and probably a 1000 to get install it and and finish it.But about 30% of my loses are through the slab, even with a small amount of edge insulation.And 25% is infiltration..
William the Geezer, the sequel to Billy the Kid - Shoe
Just for giggles, ignoring a lot of important stuff, including infiltration, a 10 year old 24x40 ranch will have:1000 feet of roof, 1000 feet of wallwe will say 200 feet of windows/doorsroof at r 40
walls at r20
windows r 2delta T 501250btu thru roof
2500 thru walls
5000 thru windowsThe roof is lower because we have already insulated it better
also ignoring the warmer air at the ceiling moves more heat out the roof than the walls. the windows account for more than the rest combinedWere I building from scratch, I wold use sips on the order of 4 inch walls and 6 inch roof and the best windows available, quad pane, whatever it takes. no one is talking about uninsulated walls, just that there is no payback on the huge cost of overbuilding a wall. Ceilings are usually
cheap to add insulation, either now or later
Yes. That was the strong part of the guys statement before he went to repeating old myths. He recommended getting the study done.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
As a bit of a follow up to gusfhb’s post, and looking strictly at window U-value performance comparisons:<!----><!----><!---->
Assuming three different houses, identical except for window performance; and using the 200ft2 of total window area that gusfhb used; and assuming windows in the three houses with U value performance of U .50, U .30, and U .15 or R 2.0, R 3.3, and R 6.7 respectively.
If delta T = 10°:
then (200ft2 / R 2.0) = 100 * 10° = 1000 Btu/hrthen (200ft2 / R 3.3) = 60.6 * 10° = 606 Btu/hr <!----><!---->
then (200ft2 / R 6.7) = 29.9 * 10° = 299 Btu/hr<!----><!---->
Considering the more robust delta T of 50° that gusfab used:then (200ft2 / R 2.0) = 100 * 50° = 5000 Btu/hrthen (200ft2 / R 3.3) = 60.6 * 50° = 3030 Btu/hr <!----><!---->
then (200ft2 / R 6.7) = 29.9 * 50° = 1495 Btu/hr<!----><!---->
And a mind-numbing delta T of 100°:
then (200ft2 / R 2.0) = 100 * 100° = 10,000 Btu/hrthen (200ft2 / R 3.3) = 60.6 * 100° = 6060 Btu/hr <!----><!---->
then (200ft2 / R 6.7) = 29.9 * 100° = 2990 Btu/hr<!----><!---->
<!----> A good argument for paying attention to window performance.
<!---->
Edited 6/26/2009 11:29 pm ET by Oberon
The only window performance my wife is concerned about is if the window opens. Gotta have fresh air even with a delta T of 110.
Mine is just the opposite. She is a huge advocate of air conditioning.
Gotta have fresh air even with a delta T of 110.
Agreed .. but opening a window when the Delta T is 110 would not be very wise.
You should really think about getting an HRV ;-)
or a space suit
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Oberon... I agree with you about windows..Very Important
I think that many people that visit this forum are interested in building Low Energy Homes either for others or as their own personal home.
"Bang for Buck" thinking and "5 or 10 year payback" is just not relevant.
We should expect our homes to last at least 50 years and MORE.
The price of Energy WILL rise.
Even if we live in our home for only 10 years and then sell it ....
the Low Energy Home will have More "Value" after 10 years than a "built to code" or even Energy Star home.
Saving Energy is better than producing Energy.
Some components of a Low Energy Home are going to cost more than others.
Does that mean that we should ignore the costly strategies and only do the easy cheap stuff?
The major weak links that I see with most Energy Star Homes are still
Air Infiltration
Windows
Walls
Just for giggles check out this calculator
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm
Look at the weak points in the example home.
Double all of the r-values and you are near or better than Energy Star
Look at the weakest points again
Air Infiltration
Windows
Walls
Does adding r-value to Walls beyond R-20 continue to save more Energy or not?
Spray Foam .. What do you Really Know? the article
http://www.taunton.com/finehomebuilding/how-to/articles/choosing-spray-foam-insulation-what-you-need-to-know.aspx?ac=fp
In addition to the mis-leading conclusions about R-value....
The Illustrations for the R-22 roof and the R-45 roof illustrate POOR "Thermal Bridging" practice.
The top plate should have been "protected" from Thermal Bridging.
Not only is there a Heat transfer weakness .. but also an increased potential for condensation.
Again, another simple comparison of window performance, using the same 1000ft2 wall with 200ft2 of windows, and while adding and comparing wall insulation as well:
Total wall = 1000ft2 <!----><!----><!---->
Windows = 200ft2<!----><!---->
Wall – window = 800ft2<!----><!---->
<!----> <!---->
Total wall insulation value (excluding windows) = R10 or U.1<!----><!---->
Window a = U.5<!----><!---->
Window b = U.3<!----><!---->
Window c = U.15<!----><!---->
<!----> <!---->
Wall = 800ft2 x U.1 = 80<!----><!---->
<!----> <!---->
Window a = 200ft2 x U.5 = 100<!----><!---->
80 + 100 = 180/1000ft2 = U.18 or R 5.5<!----><!---->
<!----> <!---->
Window b = 200ft2 x U.3 = 60<!----><!---->
80 + 60 = 140/1000ft2 = U.14 or R 7.14<!----><!---->
<!----> <!---->
Window c = 200ft2 x U.15 = 30<!----><!---->
80 + 30 = 110/1000ft2 = U.11 or R 9.1<!----><!---->
<!----> <!---->
So in this scenario upgrading windows from U.5 (typical clear glass dual pane performance) to really good LowE triple panes at U.15 improves overall wall R-value performance by 65%.
<!----><!---->
<!----> <!---->
Increasing total wall insulation value to R-20, again excluding windows, then<!----><!---->
<!----> <!---->
Wall = 800ft2 x U.05 = 40<!----><!---->
<!----> <!---->
Window a = 200ft2 x U.5 = 100<!----><!---->
40 + 100 = 140/1000ft2 = U.14 or R 7.14<!----><!---->
<!----> <!---->
Window b = 200ft2 x U.3 = 60<!----><!---->
40 + 60 = 100/1000ft2 = U.10 or R 10.0<!----><!---->
<!----> <!---->
Window c = 200ft2 x U.15 = 30<!----><!---->
40 + 30 = 70/1000ft2 = U.07 or R 14.3<!----><!---->
<!----> <!---->
Doubling wall insulating value, and still using windows with U-value of .5, results in an overall wall R-value improvement from R 5.5 to R 7.14. <!----><!---->
<!----> <!---->
Interestingly, in this scenario, improving window performance from U.5 to U.3 (dual pane clear glass to dual pane LowE and argon) in the R10 wall results in the same improvement in overall wall R-value performance as does increasing wall insulation from R10 to R20 while using windows with a U.3 in both walls. <!----><!---->
<!----> <!---->
In the R10 wall, improving window performance from U.50 to U.15 resulted in an overall wall improvement of 65%. <!----><!---->
<!----> <!---->
In the R20 wall, the same window upgrade results in a 100% improvement in overall wall value performance.<!----><!---->
<!----> <!---->
Again doubling the wall insulation, this time up to R40, and yet again excluding windows, then<!----><!---->
<!----> <!---->
Wall = 800ft2 x U.025 = 20<!----><!---->
<!----> <!---->
Window a still = 200ft2 x U.5 = 100<!----><!---->
20 + 100 = 120/1000ft2 = U.12 or R 8.33<!----><!---->
<!----> <!---->
Window b = 200ft2 x U.3 = 60<!----><!---->
20 + 60 = 80/1000ft2 = U.08 or R 12.5<!----><!---->
<!----> <!---->
Window c = 200ft2 x U.15 = 30<!----><!---->
20 + 30 = 50/1000ft2 = U.05 or R 20<!----><!---->
<!----> <!---->
So, improving 800ft2 of wall space from R10 to R40, and using windows with a U-value of .50 in both walls, results in an overall wall improvement from R5.5 to R8.33 or 51%. <!----><!---->
<!----> <!---->
And yet again (again), assuming my math is correct, in this scenario 800ft2 of wall with R10 insulation, but with 200ft2 of U.15 windows, actually has better overall wall R-value performance numbers than does a wall with R40 insulation and U.50 windows installed. <!----><!---->
<!----> <!---->
Food for thought...<!----><!---->
Oberon,
I still agree...Windows are Very, Very important...right after air infiltration
We need to use better windows.
The only thing that I question about your example is the Ratio of Window area to overall wall area.
20 percent window area to total wall area is TOO much for a small Low Energy Home.
The good news is that by reducing the window area ... you could more easily afford to upgrade the windows to your suggested standards.
Reducing window area and increasing wall area will yield an even better Overall wall R-value.
And just think how much more comfortable we would be with those improved surface temperatures.
We do not have to choose between A (infiltration), B(windows) or C(wall R-value)..... we need to attack all weakness....there are many more also.
I agree that 20% window to wall ratio is a bit high; I was continuing the numbers that gusfhb had used in his post and that I had used in my previous post.
I very much agree that window size and location should be carefully considered in the construction of a home.
While the ratio might be high, I was just pulling out of the air, and I do not think it is out of line with the example of an existing house.Also the ratio is a bit of a red herring, the point is the numbers are grossly in favor of high efficiency windows above all other issues. Once you have moved your insulation losses well below your infiltration losses, you are really down in the noise. If you move infiltration down, you need to start ventilating purposefully, and we are no longer talking about insulationMy original house of 1100 sq ft, with r13 walls and ~r30 roof and ~r40 floor[on piers] and oh, um 192 sq ft of windows/doors[honest] burned ~275 gallons of K1 a year with a monitor heater[~90%] in Eastern Mass, pretty much always, sometimes I setback, sometimes not. 68-70 days. tell me the money is not going out the windowsI don't wonder if the issue of thermal mass comes in at some point but that is a different conversation.......
And while I thought 20% was a bit high, as you said that isn't really unrealistic and it did work well for illustrating the differences in both insulation and window performance.
Bill, how does one do the calc? Does it involve specialized gear?Thx,Scott.
You could do it with just a table of r-values and some forumulas and knowledge of the local conditions.But I used this software.http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=121553.1Great for doing "what ifs" for common new construction and remodels (including window replacement) and additions.However, options are not detailed enough to see the small changes on a super insulation house and for more uncommon types of construction..
William the Geezer, the sequel to Billy the Kid - Shoe
while the "R20" wall might be your typically leaky 2x6 wall with a poorly installed FG batt, which might perform at only R13 under those conditions.
Hey, careful, you are inserting reality into dicussion that often have very little of that product used.
The insulating and regulating world likes to gloss over things like that. Like how "Rnn" insulation does not create a wall that is insulated to that value. All it means is that the empty void spaces in the wall have some form of fill in them. This is where the silliness of overstuffing comes in. That, and ignoring things like how many penetrations are in a wall. "Oh, look, my wall is R38, I must be good"-except it's 50% R4 glazing?
Oops, fell into rant, sorry.Occupational hazard of my occupation not being around (sorry Bubba)
And forgive us our rants, as we forgive those who rant unto us.