Thermal Bridge Analysis for the PHPP
Thermal Bridge Analysis for the PHPP Passive House Conference 2009 PH Consultant Session Urbana, October 15 Revised Nov
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Thermal Bridge Analysis for the PHPP Passive House Conference 2009 PH Consultant Session Urbana, October 15
Revised Nov 29 2009
David White, Right Environments [email protected]
Overview: Overall Method for Envelope Analysis 1. Analyze the envelope component areas & U-values using outside dimensions. Areas go in the “Areas” tab and U-values go in the “U-Values” tab. This is called “the simplified method.” 2. Examine the thermal bridges at the intersections between component areas. Based on best judgment, decide whether or not the bridge is a significant extra heat loss (or may win back significant heat relative to the simplified method in step 1). 3. If the bridge is not significant (less than 0.01 W/mK), don’t bother to calculate it. If it is significant, you can account for it by taking a value from a catalog of thermal bridges or doing your own 2D calculation. Input the thermal bridges in the “areas” tab of the PHPP. 4. PHPP calculates total envelope loss as the sum of component area losses and bridge linear losses.
This workshop includes: • • •
Key Concepts Calculation Guidelines Tutorial with THERM and Excel
It does not include: •
Dynamic analysis methods
Key Concepts
Key Concepts
Component Areas and Intersections
Key Concepts
roof wall
wall
slab edge
slab center slab perimeter
Components
Key Concepts
roof wall
wall
slab edge
slab center slab perimeter
Intersections
Key Concepts
Intersections Note: taking windows from NFRC to PHPP has two problems: 1) physics of ISO vs. NFRC and 2) NFRC has one combined value for glass, frame, and spacer. Check my website for window inputs calculation method – coming in time.
Key Concepts
Thermal bridging typically means that the heat gets a short cut across the envelope.
Key Concepts
Thermal Bridges • wall to slab (can be a big one!) • wall to roof • wall to wall • glass to frame (“spacer” in WinType) • frame to wall (“installation” in WinType) • etc
In PHPP, whether or not the heat gets a short cut, a “thermal bridge coefficient” can be applied any place where heat flow can’t be accurately calculated using the simplified method, i.e. an intersection!
Key Concepts
(W/K per meter into the page)
=ψ (W/K per meter into the page)
(W/K per meter into the page)
Key understanding: the thermal bridge is the extra heat loss associated with the intersection. This means that the thermal bridge loss is the total loss from 2D analysis minus the loss calculated for that same section using the simplified method.
Key Concepts
For component heat loss, use simple parallel heat transfer calculation in PHPP.
Key Concepts
“Details for Passive Houses”
For thermal bridge heat loss, either reference a calculation done by others...
Key Concepts of Thermal Bridges
...or do it yourself, for instance using THERM.
Key Concepts
Thermal bridges must be specific to ambient, ground, or “perimeter.”
Key Concepts
Ambient – no ground interactions
Ground – bridge is in contact with ground, far from grade. Do not include ground or exterior air film in model.
Perimeter – partly above, partly below grade. Special!
Calculation Guidelines
Calculation Guidelines
R-0.45
R-0.74
R-1.14
R-0.22
R-0.97
Schneiders, “Protocol 16: Thermal Bridge Free Construction,” PHI, January 2008
Surface Film Coefficients
Calculation Guidelines
IP units
METRIC units
Surface Film Coefficients from ASHRAE Fundamentals – PS: I used some incorrect surface coefficients during the tutorial!!!!! sorry!
Calculation Guidelines
Schneiders
• EN 10211-1 recommends straight sections extend 1m “clear” (consistent construction) • Rule of thumb: 4x wall thickness (although for PH this can be 6 feet!) • Beware: adiabatic boundary will force isotherms to be parallel! Red herring! • Above example is for a simple detail – higher fluxes may need longer straight sections • Too long can cause inaccuracy through rounding errors (?) • When in doubt, test it at various lengths (as above)
Calculation Guidelines
Useful hint: when applicable, put adiabatic boundary at a line of symmetry!
Calculation Guidelines
“Perimeter Insulation” is included in ground sheet calculations and in thermal bridge calculations (I think).
Calculation Guidelines
Outdoor temp, e.g. 13°F for NYC
Indoor temp, e.g. 68°F
Adiabatic
2.5m?
Average of Indoor and Outdoor temp, e.g. 40.5°F
2.5m
1.0m
This is how to model a perimeter thermal bridge! (from Schnieders 2008). The indoor/outdoor temps are arbitrary in terms of of calculating psi. The specific temps used here are useful because they also tell us something about condensation risk.
Tutorial with THERM and Excel
Tutorial with THERM and Excel
Thermal bridge analysis: not just for masochists anymore!
Wall
Tutorial with THERM and Excel
Slab
Tutorial with THERM and Excel
Draw detail in THERM (to save time, it is pre-drawn for the tutorial). Assign boundary conditions and U-factor tags.
Tutorial with THERM and Excel adiabatic
outdoor temp with outdoor temp with rain screen exterior resistance (R-0.45) (R-0.17)
indoor temp, vertical surface film (R-0.68) indoor temp, inside corner film (R-1.14) for 8 inches or so indoor temp, downward flow film (R-0.92)
adiabatic
adiabatic
half way between indoor and outdoor temp, no air film
Note: U-factor tags are drawn in red along interior surface. The U-factor tags can go anywhere as long as they mark one “gate” through which all heat flow passes so THERM can measure the flow. Note: German and US air film values differ slightly, so there are a few discrepancies w/ slide #18
Tutorial with THERM and Excel
Set error tolerances and max iterations Then run it. Manual (7.3.2) warns of accumulated rounding errors below 5% Maximum Error Energy Norm. Software author says 5% is ok.
Tutorial with THERM and Excel
Set up calculation sheet on PHPP
Tutorial with THERM and Excel
Calculate heat loss of straight sections by one of the following methods: • simulate straight section on THERM (more accurate, but how much more?) • use PHPP calculation (faster)
Tutorial with THERM and Excel
Subtract 1D losses from 2D losses. Be careful to assign the correct temperature difference to each component. Divide the net loss by the deltaT to ambient (not ground) because PHPP asks for perimeter thermal bridges with respect to outdoor temperature. The result is the ψ-value.
Thank you for your kind attention