|
CABO
|
MHCSS
|
|
U-Factor
|
| Complicated
formulas for walls, roofs and floor system U-factor.
Requirements accounting for "thermal bridging" in
metal framing. MEC provides specific design tables and
correction factors to be used in calculations for metal frame
construction.
|
Simple
requirements by zones for overall U-factor. For
single-section sized unit built to MEC requirements, the Uo is
similar to the Uo for MHCSS, especially in Zone 1.
Metal framing is not specifically called out. MHCSS
does require adjustment of Uo for framing losses, including
"thermal shorts", but does not specify adjustments.
References 1989 ASHRAE Fundamentals.
|
|
CABO
|
MHCSS |
|
Window
Construction
|
| Uses
ASTM E283 to determine window air tightness, requiring 0.5 cfm per
lineal foot of crack.
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|
Uses
ASTM E283 (referenced in AAMA 1701.2-85 1.4.2.2) to determine window
air tightness, requiring 0.5 cfm per square foot of window area.
MHCSS thus requires tighter windows than CABO for normal sizes of
windows. (ASTM 283-84 permits results reported using either
square footage of window area or lineal feet of crack length). |
| CABO |
MHCSS |
|
Airtightness
|
| Requires
treatment against air leakage at utility penetrations. Recessed
light fixtures must meet airtightness standards.
|
No
airtightness standards for recessed light fixtures; however, other
electrical, mechanical, and plumbing penetrations must limit air
infiltration. |
| CABO |
MHCSS |
|
Vapor
Retarders
|
| Requirements
for vapor retarders use wet bulb temperatures.
|
Ceiling
vapor retarders are required in thermal zones 2 and 3. Walls
require vapor retarders, permeable sheathing or cavity ventilation. |
| CABO |
MHCSS |
|
Attic
Ventilation
|
| Attic
ventilation required.
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|
Requires
attic ventilation except: 1) Single-section home with metal roof
having no sheathing and a tight ceiling. 2) Parallel membrane
roof section of a closed cell type. |
