How can I calculate the effective R or U value of a ceiling knowing the under and top side temperatures?
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| Posted in Mechanicals on
I have taken IR thermo gun readings at:
– indoor ceiling
– top of the insulation in the attic volume
– ambient temperature of the attic volume
– underside of the roof
– outside air temperature
All measurements taken after dark to eliminate solar gain
The existing ceiling insulation is compromised by gaps and penetrations in the vapor barrier. The end result is to find out what gain the addition of R12 spray foam on the underside of the roof and gable ends and sealing the attic volume from infiltration will produce. The project is defined, I am trying to estimate the insulation improvement the project will yield.
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Replies
John,
The easiest way to determine the R-value of your ceiling is to measure the insulation thickness. Old fiberglass insulation or cellulose has an R-value of about 3.5 per inch.
The "whole-ceiling R-value" will be less than the R-value of the insulation, due to the thermal bridging of the ceiling joists. The thicker the insulation — especially if the insulation is much deeper than the joists — the less the thermal bridging.
Air leaks do not diminish the R-value of the insulation, but they seriously compromise the thermal performance of your ceiling.
Actually, air leaks through the loose fill or batt insulation does diminish the effective R-value, which relies on dead air. Convection of air particularly reduces the effective R-value of fiberglass which has almost no ability to restrict air movement.
John,
You cannot determine R-value by temperature surface measurements. Since R-value is the inverse of thermal conduction, the only way to measure it is by measuring actual thermal conduction from a constant heat source to a heat sink on the other side in a controlled environment.
If the problem is penetrations in the air/vapor barrier at the ceiling, then your best return will be from sealing those penetrations and adding additional insulation depth (ideally air-resisting cellulose) over the existing insulation. Adding R-12 foam to the roof sheathing and gable walls may only trap moisture and create a condensation plane.
Robert,
The phrase "effective R-value," although often used in advertising, has no technical meaning.
To understand the thermal performance of a wall assembly, it's important to know several factors. Knowing the air leakage characteristics of the assembly is, of course, essential. However, the R-value of the insulation is what it is.
The term "effective R-value" is right there with "sound science", you know a slight of hand is in the works.
Martin,
You can call it what you want, but the tested R-value of insulation is in a convectionless environment. That's not necessarily a real world environment, particularly with fiberglass.
There are two effects of exfiltration/infiltration. One is the sensible and latent heat loss or gain from the mass transfer of air. The other is the reduction in thermal resistance (effective, or in-service R-value) which alters the thermal conductivity through the insulating material.
Glass, whether in window panes or fibrous insulation, has about the same insulating quality as brick or mortar. It's the captured "dead" air in the glass fiber matrix that creates the thermal resistance. If that air is not "dead" but moving, then the fiberglass has little measureable R-value.
Because the scientifically precise term "effective R-value" has been misused by advertisers doesn't make it either imprecise or useless. The difficulty is in determining what the effective R-value is for any installed fibrous insulation because of the near impossibility of differentiating between diffuse flow, orifice flow and channel flow of air through a thermal envelope.
I am studying the Whole Wall R Factors of a building. And am using the ORNL Whole Wall R Factor Calculator for the walls. I am now looking for information on the most current practices in ceiling and roof design. To know what is most often used as the ceiling joist (2"x6' etc) and what insulation is most often used? From that I can calculate my whole ceiling r factors. ~ a.k.a. the effective r factor.
I am analyzing the entire home energy requirements for a building about 48' x 36, with a whole building R Factor of within a target of R 19. From this I want to compute the number of solar space heating panels I need, and how much renewable electrical energy I might need.
Green WorkShop,
It's possible to fram a 36'x48' building many ways. The ceiling might be framed with 2x4s acting as the bottom chords of roof trusses, or with 2x12 joists or rafters, or with a wide variety of other materials.
In other words, there is no single answer to your question, "What size are the ceiling joists in a 36'x48' building?"
Green Work Shop,
There is no standard or average ceiling joist or insulation type in today's housing market. You have to specify a particular configuration, which should at least meet the current IECC insulation and air-sealing standards for your climate zone, and calculate on that basis.
This is what I was told when I asked the same question on the Snell Group's IR forum. One of their engineers sent me a paper on the topic, and it's very technical. It is absolutely not something you're going to do using IR to inspect a building in the field.
R value. Again this thread highlights why R value should be dropped and btus/Sqft/deltaT be implemented. Codes, plans, discussions about each assembly and how it performs becomes straight forward.
Apples to apples... not R this 16" frame and R 24" frame and changing cold or placement performance that is overlooked by marketing and clients and builders and codes....
If we dropped R value from use this site would have 100% less insulation confusion to debate, no?
AJ,
No. R-value is a useful metric. It measures something real.
Air leakage measurements are also useful; they also measure something real.
Banning a metric used by engineers doesn't make any sense. If you don't want to know the R-value of a product you are buying, that's fine. You can ignore it. Many people, however -- including me -- want to know the R-value of a product we intend to buy.
I don't understand Martin how you miss my points so often. I never said ban R values. My post is trying to say that we should use R values to then determine energy consumption per square foot for a particular assembly and home or whatever and then work with good info. R value is definitely part of the equation but is meaningless till we assemble the R rated product into an assembly and then into a structure. At that point we can talk meaningful numbers and compare apples to apples.
I.E.- Passive house... great idea... in the way they go about energy consumption/insulation/Rvalue. The codes should use a similar approach as well as our discussions here. Would be nice if there was an open source movement in the low to no energy use world. Robert should get a copy of the passive house spread sheet and take that along with his own spreadsheet and start the first open source energy calculator.
R value alone is very very misleading. That is my point. Do you understand me better now?
And please don't tell me there are ways for me to do the calculation, what I am saying is... we should be doing something all of us when we work with energy use that is different than... "spec the walls at R 40 and the roof at R 60." Really... this last statement I just made has no real world meaning that could possibly be converted into actual useful numbers and facts for any particular person to use for any particular situation. There is TOO MUCH MISSING INFO! True or false?
AJ (unverified),
It's you who fails to understand the importance of R-value and it's precisely for people like you that the prescriptive code uses the simplistic language of R-value requirements.
The singular purpose of R-value is for an objective comparison of the relative conductive insulation value of different types and brands of insulation products. It is enforced by the Federal Trade Commission in order to prevent misleading advertising, labeling and claims among manufacturers, merchants and installers.
But, once a scientifically objective method of determining R-value was universally enforced, building codes could use R-value language to prescribe minimum insulation requirements. Current energy codes also take into account the different common ways that insulation is installed in a building assembly, including thermal bridging vs continuous insulation and dynamic thermal mass value. Air tightness is specified separately in code.
The energy code also allows a performance-based approach to compliance for those who are sophisticated and knowledgeable enough to do their own energy modelling, which is what you're proposing that we use exclusively.
The reason that most builders prefer a prescriptive code is that, like you, they don't understand the complexities of energy modeling and just want to be told "This is OK" and "That is not OK".
And most homeowners and clients need a language that is simple enough to understand, both for comparison shopping at Home Depot and for talking to builders about thermal envelope options. For those purposes and the purpose of a prescriptive code, R-value is the best language to use.
"The reason that most builders prefer a prescriptive code is that, like you, they don't understand the complexities of energy modeling and just want to be told "This is OK" and "That is not OK".
And most homeowners and clients need a language that is simple enough to understand, both for comparison shopping at Home Depot and for talking to builders about thermal envelope options. For those purposes and the purpose of a prescriptive code, R-value is the best language to use."
Exactly opposite my point Robert. I would like to see us post assembly Energy use at Home Depot and here not the dumbed down approach of R value which by itself is NOT A COMPLETE SOLUTION/ANSWER.
Got it yet?
If you don't then let's just leave it at that.
He doesn't appear to be the only one AJ.
Martin and Robert have both been pretty clear - but your obstinacy is getting in the way.
Since no one is buying a building assembly at Home Depot, it makes as much sense to advertise the value of how a customer MIGHT use the insulation material as it does to advertise the nutritional value of food on the basis of how the food might get cooked and eaten once it's taken home.
Supermarkets are required to label their foods on the basis of price per quantity for fair and accurate comparisons, even if necessarily based on the simplistic method of ounce for ounce.
Similarly building supply stores are required to label their insulation materials in a way that allows an objective comparison of their potential value.
And building codes are required to use objective standards for prescribing insulation options, giving builders credit for better installation methods, such as continuous or combined with thermal mass.
And discussions of insulation systems also need an objective basis for comparison, which makes R-value the only useful metric for such discussions. If you are uncomfortable with R-value, then perhaps you could come up with an AJ-value that makes you feel better, but it will still leave you wondering why no one understands you.
"Similarly building supply stores are required to label their insulation materials in a way that allows an objective comparison of their potential value."
Exactly my point Robert. The R value on the package does NOT state that fiberglass batts perform poorly when subjected to very cold conditions, that they do not perform well if the well if used as another layer added to attic insulation like the BIG BOXES ADVERTISE FOR CUSTOMERS TO DO!@!!!
I would think you would be in total agreement with this aspect Robert as I could go back over dozens of posts where you have illuminated this very aspect of fiberglass batts!
Ranting negatively against my posts even when I am agreeing with prior posts of yours is ridiculous oh wise one.... Obi-Wan Kenobi
For once in your life semi agree with someone you feel such angst with 24/7. I do often agree with you even though I too feel yaa could be a bit more pleasant to most who post here. OK.. I admit you do have 1 or 2 that love you that never say a word back to your posts and you lovingly oblige likewise. Classic bully syndrome.
And, as I patiently explained to you before, the revised FTC R-value testing procedures require all fibrous insulation materials to be tested at a 50° delta-T. That offers a real-life model for performance and a fair basis for comparison.
If you don't like the procedures that ASTM has developed for testing insulation, then you'd do better to complain to them, rather than accusing people here of misunderestimating you.
[By the way, how many of your voting aliases did it require for you to get a +2 rating for this last, thoroughly useless and completely inappropriate post?]
I stand by my statement that insulation R value is by itself not a very useful bit of information. Whole assembly ratings are.
We should ski this winter together. If you don't downhill ski then you picked the wrong state to live in!
I guess the answer then to my ceiling fabrication question would then be one to my own design, to which I would seek advice from my architect. Something with an Whole Wall R Factor near 19 would be the target so architect might suggest something.