Thermal Bridge Analysis for the PHPP

8/10/2019 Thermal Bridge Analysis for the PHPP

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Thermal Bridge Analysis for the PHPPPassive House Conference 2009

PH Consultant Session

Urbana, October 15

Revised Nov 29 2009

David White, Right Environments

[email protected]


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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 significantextra 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), dont bother to calculateit. 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 lossesand bridge linear losses.


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This workshop includes:

Key Concepts

Calculation Guidelines

Tutorial with THERM and Excel

It does not include:

Dynamic analysis methods


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Key Concepts


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Component Areas and Intersections

Key Concepts


6/33Components

roof

wall

slab center

slab perimeter

slab edge

wall

Key Concepts


7/33Intersections

roof

wall

slab center

slab perimeter

slab edge

wall

Key Concepts


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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


9/33Thermal bridging typically means that the heat gets a short cut across the envelope.

Key Concepts


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In PHPP, whether or not the heat gets a short cut, a thermal bridge coefficient can beapplied any place where heat flow cant be accurately calculated using the simplified

method, i.e. an intersection!

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

Key Concepts


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= -(W/K per meterinto the page) (W/K per meterinto the page)(W/K per meterinto the page)

Key Concepts

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.


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For component heat loss, use simpleparallel heat transfer calculation in PHPP.

Key Concepts


13/33For thermal bridge heat loss, either reference a calculation done by others...

Details for Passive Houses

Key Concepts


14/33...or do it yourself, for instance using THERM.

Key Concepts of Thermal Bridges

K C t


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Thermal bridges must be specific to

ambient, ground, or perimeter.

Key Concepts

K C t


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Ambient no ground

interactions

Ground bridge is in

contact with ground, far

from grade. Do not

include ground orexterior air film in model.

Perimeter partly

above, partly below

grade. Special!

Key Concepts

C l l ti G id li


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C l l ti G id li


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Schneiders, Protocol 16: Thermal Bridge Free Construction, PHI, January 2008

R-0.45 R-0.74

R-0.22

R-1.14

R-0.97


Surface Film Coefficients

C l l ti G id li


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Surface Film Coefficients from ASHRAE Fundamentals

PS: I used some incorrect surface coefficients during the tutorial!!!!! sorry!

IP units METRIC units

C l l ti G id li


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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)


Schneiders



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Useful hint: when applicable, put adiabatic boundary at a line of symmetry!




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Perimeter Insulation is included in ground sheet calculations and in thermal bridge

calculations (I think).




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This is how to model a perimeter thermal bridge! (from Schnieders 2008). Theindoor/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.

Outdoor temp,

e.g. 13F for NYCIndoor temp, e.g. 68F

Average of Indoor and Outdoor

temp, e.g. 40.5F

2.5m 1.0m

2.5m?

Adiabatic




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Thermal bridge analysis: not just for masochists anymore!




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Wall

Slab




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Draw detail in THERM (to save time, it is pre-drawn for the tutorial).

Assign boundary conditions and U-factor tags.




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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


adiabatic

outdoor temp with

exterior resistance

(R-0.17)

indoor temp, vertical

surface film (R-0.68)

indoor temp, downward

flow film (R-0.92)

outdoor temp withrain screen

(R-0.45)

half way between

indoor and outdoor

temp, no air film

ad

iabatic

adiabatic

indoor temp, inside corner film(R-1.14) for 8 inches or so



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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.



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Set up calculation sheet on PHPP



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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)



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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 -value.


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Thermal Bridge Analysis for the PHPP

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