6.5.1 Wall Components
Framing must be included. For walls, vertical and horizontal framing must be included. Typical assemblies are built out in 3D.
Actual spacing of vertical framing should be used to define the ‘width’ of the typical assembly created, i.e. 16” o.c., and framing added in as horizontal subdivisions.
The ‘height’ of the assembly created should be defined by a typical floor to floor height. The top and bottom plates must be added into the assembly as vertical subdivisions.
Some insulation is temperature-dependent; use the R-value at 75 F for the energy model. Some insulation is thickness-dependent; use the R-value at the appropriate thickness for the assembly.
In WUFI Passive, please note that default conductivity values are used from the existing database.
- Conductivity or thermal resistance values should be updated in the project file to match the insulation specification.
- This can be done under the Edit Assembly screen by clicking into the “Conductivity” input, for the specific assembly layer, selecting the [R/in] button, and inputting the accurate R per inch.
Surface / Radiation Balances / Solar Absorption/Emission
The radiation balances need to be input for every opaque surface above grade. This includes values for: emissivity, absorptivity, and shading. Rough values are acceptable.
6.5.2 Window Components
PHIUS has verified data for a number of windows and manufacturers; these are listed on the Find & Compare Windows page of the PHIUS website.
If the project is using windows included in the PHIUS Verified Data for Window Performance Program, PHIUS’ data must be used.
Window Components in 3D visualization: In the energy model, each individual ‘lite’ or individual ‘piece of glazing’ should be its own clickable component. This ensures that the proper solar gain and frame to glazing ratio is accounted for.
Window Groups in 3D visualization: In WUFI Passive, windows could be grouped/ungrouped and through the 'Assign Data' option give them common properties.
- To add 'Basic Data' and 'Frame Data' and ‘reveal’ shading, they can be grouped by "Window Type".
- To add 'General Parameters' for site shading (fractions of solar exposure), you could group them by 'orientation' or by 'floor level'.
- WUFI Mean Monthly shading factors are calculated per window, regardless of grouping.
Window Sizes: The windows are entered into the building energy model in their actual size and configuration (every lite), with the performance of the framing members and the glass broken out separately (as opposed to using whole-window properties at a standard size.)
Window Performance Entries: Specification data is needed for:
- Center-of-glass U-value
- Solar Heat Gain Coefficient (SHGC)
- Frame width
- Frame U-value
- Linear heat loss coefficient at the edge of the glass (“psi-spacer”)
PHIUS’ window program calculates all these. But the project certification program is fairly flexible as to the source of that data, that is, it is not “hostage” to the window rating program. Usually, center-of-glass properties can be obtained from the glazing system manufacturer. For the frame, rating data per CEN standards or NFRC's CMAST can be used, or a conservative default based on frame material type can be used. The linear coefficient at the edge is the hardest to come by, but a conservative default can be used for that (0.023 BTU/hr.ft.F). Similarly, a default value can be used for the linear heat loss coefficient at the outer edge of the frame (“psi-install”).
Psi-Installation / Frame-to-wall psi-value
The frame-to-wall psi-installation value is dependent on how the window is installed within the wall. Please do not use values from manufacturers’ data sheets unless they have completed a project specific calculation for you.
- A default value of 0.030 BTU/hr.ft.F can be used for most installation details.
- Improved detailed can use the values below:
- As low as 0.020 (Btu/hr.ft.F) for a mid-wall mounted window that is not overinsulated.
- As low as 0.015 (Btu/hr.ft.F) for a mid-wall mounted window that is overinsulated.
- If a THERM calculation is provided, psi-installation values below these thresholds are acceptable.
6.5.3 Solar Protection/Shading
126.96.36.199 Reveal Shading
Often windows are not installed flush with the exterior of the building façade which causes reveal shading to the left and right sides of the window from the window in-set in the wall.
- Depth of window reveal: Measured from the outside of the leading shading edge to the glazing.
- Distance from edge of glazing to reveal: Measured from the edge of glazing to the shading edge.
Reveal shading must be input for all windows.
- In WUFI Passive, this can be done quickly with the [Assign Data] function.
- The 3D geometry will visualize the window in-set into the wall when entries are adjusted.
- All reveal entries are calculated as if on both sides of the window.
- If the reveal depths are significantly different on the left and right, due to a vertical shading fin or similar, it is recommended that the irregular shading device is drawn into the 3D geometry rather than input numerically.
- See ‘Shading due to building geometry’ below.
188.8.131.52 Shading Devices
Sunscreen/shading devices in WUFI Passive are assumed to be used only during the cooling season. If year-round blinds are used, please input as an “other shading fraction of solar exposure”.
External blinds offer better thermal protection than internal blinds because the solar radiation is partially absorbed by the fabric before reaching the glazing and reflected outwards.
The effectiveness of window blinds needs to be de-rated if they are manually operated, to account for occupant behavior. If the shading reduction factor for a blind in the closed position is “z”, and “Z effective” is in the input in WUFI Passive then:
For exterior blinds use:
Z effective = 0.3 + 0.7 * z
Example: If blinds allow 46% solar access (solar transmittance, Ts) when closed, use that for z, and z effective turns out to be 62%.
Z effective = 0.3 + (0.7*0.46) = 0.622
For interior blinds use:
Z effective = 1- (1-z) * (1-0.6)
Example: If blinds allow 46% solar access (solar transmittance, Ts) when closed, use that for z, and z effective turns out to be 78%.
Z effective = 1 – (1-0.46) * (1-0.6) = 0.784
Blind Ratings: Should be provided by manufacturers. Solar transmittance is used in the equations above. Solar radiation is always partially transmitted through, absorbed or reflected by the fabric.
Ts = Solar Transmittance – Proportion of solar energy transmitted through the fabric. Low percentage means the fabric performs well at reducing solar energy.
Rs = Solar Reflectance – Proportion of solar radiation reflected by the fabric. High percentage means the fabric performs well at reflecting solar energy.
As = Solar Absorptance – Proportion of solar radiation absorbed by the fabric. Low percentage means the fabric absorbs little solar energy.
Ts + Rs + As = 100% of solar energy
OF = Openness Factor - Percentage of blind fabric/material that is open (between the threads)
Insulated Interior Blinds
Unless the blind is air sealed to the window opening and is airtight, much of the insulating value of the blind can be bypassed. Because of this, we are giving credit for essentially, the R-value of the two air films on either side of the blind and not the blind itself. We generally round this to R-2 in IP units. We also assume that user behavior impacts the use of the blinds and that somewhere around half the time, the blinds are not in the right spot, or are removed. Therefore, we currently allow an R-1 adjustment to be made directly to the glass R-value and the Frame R-value. One word of caution is that if they do work as intended/advertised, there is a significant risk of condensation on the window in cold climates, which over time could lead to problems and durability issues. This must be checked and managed.
Overhangs may be included in the 3D geometry or entered numerically into WUFI Passive.
If including in 3D geometry, angled or complex overhangs may be modeled.
If entering numerically, an overhang parallel to the window head will be simulated. Three parameters must be defined:
- Depth of overhang: Measured horizontally out from the glazing to the shading edge.
- Vertical distance from top of glazing to bottom of overhang: Measured vertically from edge of glazing to shading edge.
- Side spacing: The default side spacing is ‘0’, which would simulate an overhang that matches the width of the window it is assigned to.
- If the overhang extends longer than the width of the window, ‘side spacing’ may be input to extend the overhang on each side of the window.
- At this time, the extension must be uniform on each side of the window. The numerically entered overhangs will appear in the 3D geometry.
- If an overhang is both included in the 3D visualization and entered numerically, that is OK, it will not double count the shading if the two overlap in the 3D geometry.
Please note in WUFI Passive, if no overhangs are assigned to a window, the reveal depth dimensions are automatically set as the overhang. If this inaccurately represents the shading condition, please assign an overhang to the specific window.
184.108.40.206 Mulled Windows and Divided Lites
If the connector for the frame mullion is similar in material to the frame, adding a bit of thickness to the frame may work. If the connector is substantially more conductive than the frame, a thermal bridge calculation may be necessary.
There are some different ways to deal with true divided lites that bridge the gas cavity. The SHGC could be adjusted to account for the blocked light, and linear thermal bridges added to account for the extra conduction loss.
Simulated Divided Lites
If these are a kind of divided lites such that the gas cavity is not actually bridged, we will still reduce the SHGC, but no additional thermal bridging need be accounted for.
220.127.116.11 WUFI Mean Month Shading Factors
Shading due to building geometry
Shading from building geometry such as overhangs, bump-outs, complicated geometry, surrounding buildings etc. will be considered.
- Any elements that should be considered for shading can be included in the 3D geometry imported into WUFI Passive. This includes overhangs, surrounding buildings, etc.
- These elements should be assigned as ‘opaque’ and ‘Outer Air’ should be assigned on both the Inner Side and Outer Side of the components.
- The ‘Calculate WUFI shading’ button should be used.
- If an entry changes that affects the shading calculation, a warning will appear stating “Shading factors are not up-to-date…”. When this appears, the shading calculation must be re-run.
- Monthly shading factors are calculated and shown on the ‘WUFI Mean Month Shading Factors’ tab.
- Correction/reduction factor per month can be applied in the row above the results, or
- Default correction factor input can be used to apply a uniform reduction factor for all months.
- Results are calculated per window, even when windows are grouped.
- Note: Unique shading properties such as reveals, numerically entered overhangs, blinds, etc. will not be retained per window if grouping and ungrouping. When windows are grouped, they are given uniform numerical entries based on the first window selected when grouping.
- More information can be found in this presentation from NAPHC2018.
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