Terri Meyer Boake, BES, BArch, MArch, LEED AP
Professor School of Architecture University of Waterloo
email: tboake@uwaterloo.ca

Arch 173: Building Construction 2

Winter 2016: R-Value Assignment

HEAT LOSS AND THERMAL PERFORMANCE

THIS ASSIGNMENT IS WORTH 5% OF YOUR FINAL TERM GRADE.

THIS IS AN INDIVIDUAL PROJECT. NO COLLABORATION IS PERMITTED.

You are asked to do a comparative R and U value calculation for the following 2 residential building wall sections.

Wall A: Typical Brick Veneer on Wood Frame

90mm brick veneer, 25 mm air space, building paper/spunbonded olefin, 12.5 mm exterior sheathing (OSB), 38x140 wood studs with fiberglass batt insulation fill, vapour barrier, 12.5 mm gypsum board.

Wall B: Typical Wood Veneer Exterior Insulation system on Wood Frame

19mm horizontal wood siding; vertical 19x64mm wood strapping, 38mm semi rigid RSI 1.18 insulation complete with spunbonded olefin facer, 38x140mm wood studs at 400mm oc, RSI 3.52 batt insulation, vapour barrier, 12.5mm gypsum board.

a) Draw a typical portion of each of the two wall sections at 1:5 through the insulated portion of the wall AND through the stud portion for each wall type (you should have 4 building section portions)

b) List the assembly of parts, from the outside air film to the inside air film for each cross section, and then

c) Based on the following “scenario”, calculate the R-value (resistance to heat flow) and U-value (conductance) of EACH of the assemblies.

Inside air temperature 23 degrees C 40% Relative Humidity
Outside air temperature -20 degrees C 20% Relative Humidity
Inside air still
Outside air moving

d) Draw the temperature profile curve for EACH wall section indicating the DEW POINT of 7oC

e) Which wall section performs the best? Which is worst? How might a change to a stud spacing of 600mm o.c. make a difference to the overall performance?

Submit this information, neatly, on 8 1/2” x 11” paper. Please make sure your name is on the project.

Technical Assumptions:

Your project is to be done in RSI. These are the metric values for the Resistance to heat flow. R-value is the common term and can aslo imply the use of Imperial values. As Canada is a metric country, we use the RSI values. NEVER MIX UNITS.

FIRST: You need to draw sections that look like the one below, just for a portion of the wall and list all of the components and their respective R-values. Scale 1:5. Find the total R-value (resistance to heat flow). The total R-value for the wall is just the sum of all of the individual R-values. Do not forget to add the R-values for both the interior and exterior air films. Calculate the U-value (conductance).

Tricks with R values: The R-value (resistance to heat flow) and the U-value (ability to conduct heat flow) are INVERSE values. ie. R=1/U and U=1/R. BUT... in order to find the U value for the ENTIRE wall assembly, you CANNOT simply sum up all of the "little" U values for each of the components. You must first sum up the total R value for the entire wall assembly and THEN take the U value for the entire wall.

SECOND: Draw the temperature profile curve for EACH wall section, indicating the DEW POINT of 7oC - do this ON the section that you have just drawn.

For the assignment you must calculate the temperature profile through each of your building elements. This is done quite simply by taking the R value of each wall assembly and setting it as a ratio to the overall temperature difference from the outside to the inside of the wall section (ie. 23C - (-25C) = 48C)

Then take each individual R value for each building component and set it equal to your original overall resistance/temperature ratio. This will give you the temperature drop over each building element. Draw the graph. Show where the dewpoint happens.

The BIGGER the R value the more insulating the material. The smaller the R value, the less insulating the material. Conversely, since the U value (conductance value) is the inverse of the resistance value (i.e. U = 1/R), we want small conductance values as these materials are less prone to heat flow. The thermal values for windows are normally given as U values as the manufacter needs to calculate the thermal qualities of windows as they are a combination of the values at the centre of the glass, near the frame and through the frame itself.

Subsequent course (Arch 264: Building Science) will take this knowledge further. For now the learning outcome is to be an understanding of the impact of material choice on resistance to heat flow.

Submit this information, neatly, on 8 1/2” x 11” paper.

REFLECTION....

Exterior walls are very bad places to run pipes as they can tend to freeze in the winter months. Look at the temperature profile of your wall section to see where the temperature crosses the freezing line. If this happens within the insulated cavity - the one that would be used to run plumbing - it means that you cannot place sinks, tubs or toilets along that wall as the pipes will burst.

This is a broken copper pipe. Broken as it was located in an exterior wall of some student occupied housing in Cambridge. Did not survive the extreme cold of the winter of 2015.

RESOURCES:

Degree days are the measure of the "hotness" or "coldness" of a climate. Degree days are the sum total of the deviation above (for cooling) or below (for heating) 18degreesC for the entire year based on the mean temperature.

Minimum RSI Values for Various Locations by Degree Days

Degree Days for Various Locations in Canada

Simplified Psycrhometric Chart

understanding the Psychrometric Chart

When looking up the RSI Values for airspaces and air surface films, please note that they are different depending on whether the airspace is oriented vertically or horizontally as heat flows differently in these directions.

RSI Values for Air Spaces and Air Surface Films

When calculating the R values for your individual materials, note that some of the values on the charts below are based on the value per mm (in which case you simply multiply the number by the thickness of your material) or for the actual thickness of the particular unit (as in concrete blocks).

RSI Values for Materials Page 1

RSI Values for Materials Page 2

RSI Values for Materials Page 3

R values for a wide variety of Insulation Types (these are all in Imperial and must be converted to RSI)

Questions? Email me and I will post the answers here: tboake@uwaterloo.ca

Last updated Monday, February 8, 2016 10:52 AM

Q. What is meant by beadfoam insulation and what is its RSI value?

A. This is a type of water resistant board insulation that has a higher insulation value than batt type insulation. It need not be as "hard" for a wall application as for a roof because it is not (normally) walked on! There are values here on this CMHC web site. Use the one for Type III or IV Polystyrene. We usually see the pink type on residential walls. This chart is also interesting as the right hand column comments on the environmental advantages/disadvantages of each type.

Q. I cannot seem to find the R value for our strapping.

A. It is the same as for wood studs. Both are made from softwood lumber.

Q. Are we using clay brick or sand/lime brick?

A. Normally in this part of the country we use clay brick.

Q. I cannot seem to find the R value for building paper/tyvek.

A. It is 0.060

Q. I cannot seem to find the R value for our wood studs.

A. It is "other softwood logs and lumber". Take the per mm amount and multiply it out.

Q. If it says 12mm or greater for the Airspace value, and our airspace is 25mm, do we multiply the R-value by 2?

A. No. The value applies for all airspaces from 12mm min to a max of 75mm. Over 75mm you can get conductive air currents and there is no R-value applied.

Q. For the stucco wall, does the strapping go vertically or horizontally?

A. If you put the strapping in the same orientation as the studs you would reduce the overall R value of the wall by making a very thick wood element. They are put horizontally and either drilled or stagger spaced to allow the air pressure to equalize within the space.

Q. What is the difference between 90mm brick and 100mm brick. I can only find an R value for 100mm brick? What do I do?

A. To be accurate you should take the value for the 100 mm brick and multiply by 9/10 to scale the value to that for a 90mm brick. You will find though, that the difference is minimal.

Q. Does our stucco have a reflective backing?

A. I have not specified anything in the assembly that will give you a reflective side to the airspace. The reflective coatings add cost to the materials, and tests on buildings that have aged show significant tarnishing happening to the reflective surface, that cuts down on its efficiency. So I don't bother installing these.

Q. For the wood strapping and airspace, is the airspace located in the same plane as the strapping, or is one in front of each other in the wall?

A. The strapping and airspace are in the same plane. We only have a totally void airspace in brick veneer type construction, where the brick ties hold the layers of the wall apart to create a clear airspace. When you are separating thin elements like stucco or wood veneer from the Tyvek and stud part of the assembly, it is the strappiing that physically ties the front and back of the wall together.

Q. I am looking for an R-value for OSB. I can't seem to find one.

A. It will be reasonably close to the value for plywood.