A middle-unit townhome should not have the same load as an identical end unit.
Why?
The middle unit shares more surfaces with conditioned neighboring space.
That is exactly the kind of distinction Manual J is built to model.
Manual J covers more than detached houses
ACCA describes Manual J as applicable to residential buildings including:
- Single-family detached homes
- Small multi-unit structures
- Condominiums
- Townhouses
- Manufactured homes
The modeling challenge is identifying which surfaces are actually exposed to outdoors or unconditioned space.
Shared walls are not exterior walls
A demising wall between two conditioned dwelling units generally does not behave like an exterior wall.
If both sides are maintained near indoor conditions, the temperature difference is much smaller than across an outdoor wall.
Do not model every perimeter wall as exterior just because it appears on the unit floor plan.
End units have more exposure
An end-unit townhome may have:
- One extra exterior side wall
- More windows
- More solar gain
- More infiltration exposure
That can create a larger heating and cooling load than the identical middle unit.
Using one tonnage for an entire development can miss those differences.
Top-floor condos have roof exposure
A middle-floor condo may have conditioned units above and below.
A top-floor condo may have:
- Roof/attic exposure
- Higher solar load
- Ducts in attic
- More exterior envelope
That can materially change cooling load.
Ground-floor units have different floor/foundation conditions
A ground-floor unit may sit over:
- Slab
- Crawlspace
- Parking garage
- Unconditioned basement
- Conditioned common space
A second-floor unit may have conditioned space below.
Model the actual adjacent condition.
Garages and parking decks matter
Condos and townhomes often have:
- Integral garages
- Tuck-under parking
- Shared parking decks
A floor over an unconditioned garage is not the same as a floor over another conditioned apartment.
Air sealing and pressure boundaries can also matter.
Shared corridors are not always conditioned the same way
An apartment wall adjacent to a conditioned corridor may see a smaller temperature difference than an exterior wall.
But corridor conditions depend on the building.
Do not assume every common area is conditioned to the same setpoint as the dwelling.
Use the actual project design and accepted Manual J modeling approach.
Unit orientation changes cooling load
A repeated floor plan can face different directions across a development.
One unit may have the large living-room glass facing east.
Another may face west.
If solar exposure is significant, the cooling loads can differ even when the unit geometry is identical.
Duplexes can have asymmetrical loads
A side-by-side duplex may have one shared wall.
The exposed exterior surfaces still depend on:
- Orientation
- Windows
- Roof
- Foundation
- Garage
- Unit layout
Do not simply split a whole-building load in half.
Run each dwelling or system zone based on its actual exposure.
System boundaries need to match the HVAC design
If each unit has its own HVAC system, each unit needs its own load.
If a dwelling has multiple systems, room assignments still need to be separated.
If a common system serves multiple spaces, project scope and applicable standards may become more complex.
Manual J is residential; larger multifamily/common-system projects can move into different load-calculation territory.
Verify the appropriate standard for the building and system type.
Infiltration in attached housing
Attached units have less exterior envelope than detached houses.
That can affect infiltration.
But leakage can also occur to:
- Garages
- Attics
- Corridors
- Shafts
- Adjacent units
Blower-door methods and program requirements can differ for attached dwelling units.
If measured leakage data exists, make sure the test boundary and units are understood before using it.
Why small efficient units are easy to oversize
A middle-unit condo may have a very small design load.
Available residential equipment may not come in tiny increments.
That makes:
- Minimum compressor capacity
- Variable-capacity equipment
- Ductless systems
- Zoning
- Latent performance
especially important.
Do not add capacity because the calculated load "looks too small."
The unit may genuinely have little exposed envelope.
Room-by-room still matters
Even a small condo can have uneven room loads.
Examples:
- Corner bedroom with two exterior walls
- Interior bedroom with one small window
- West-facing living room
- Top-floor room under roof
Room loads help with airflow and comfort.
Common attached-housing mistakes
- Modeling shared walls as outdoors
- Ignoring roof exposure on top units
- Ignoring garage exposure below
- Using one repeated load for all orientations
- Dividing whole-building tonnage by unit count
- Oversizing small middle units because the load looks unfamiliar
Bottom line
Townhomes, condos, and duplexes often have less exterior exposure than detached homes—but the exposure varies by unit position.
Model:
- Shared walls
- Roof/floor adjacency
- Garage/common-space boundaries
- Windows and orientation
- The actual HVAC system boundary
That is how repeated floor plans turn into defensible unit-by-unit loads.