We have a project with 2-unit and 3-unit townhouses. Is a separate REScheck required for each unit and how do we treat the common walls between units?
First a review of a definition from the IRC (International Residential Code) is useful in the discussion:
TOWNHOUSE. A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on not less than two sides.
Technically, two attached residential units are considered twin homes or two-family dwelling. In any case, we are talking about single family dwellings (SFD), built out the IRC. A townhouse may be described as single-family dwelling with a zero-lot line between units. As multiple units are literally touching each other, there are fire-resistant construction requirements defined in IRC 302: however, this discussion is limited to the Energy Code requirements for these unique structures. For this discussion, refer to the attached partial floor plan section from the plan submitted with this question. Please consider:
- Please remember REScheck is one of the options for showing compliance with the residential energy code. See the training videos for discussions on all five options allowed by the Utah amended Residential IECC.
- A REScheck Compliance Certificate requires identification of each component of the thermal envelope, i.e., components separating conditioned from unconditioned space.
- For townhouse units, an individual REScheck must be complete for each unit, even though the units may appear to be identical. Why? The end units each have an extra exterior end wall, which are common walls on those units sharing a common wall.
- The common wall area highlighted with green separates two conditioned areas. Leave it off the REScheck as you also will on a Manual J load calculation. There is typically little if any temperature difference between the two space; hence, not heat transfer between the spaces.
- The yellow highlighted wall area separates conditioned interior space from the unconditioned outside environment. Insulation for these wall areas complying with the code must be listed on the plans, the REScheck, and the Manual J load calculations. R-values for the wall insulation must match on all construction documents.
- The blue highlighted wall areas separate conditioned interior space from the unconditioned garage and require complying insulation, also identified on all construction documents.
- There is a unique condition for the wall areas identified with red highlights. These walls separate the entry for the right-hand next-door unit from the unconditioned garage for the left-hand unit. It must be insulated with R-values identified on the plans, RESCheck, and Manual J load calculations.
- As these units are slab-on-grade, slab edge insulation must be included on all documents, with the insulation extending to the top of slab (see below). This can be challenging where the entry, hall, and kitchen floor slabs (blue) must be isolated from the garage slab. The insulation must extend to the top of the heated space slab, which typically is higher than the garage slab.
- If REScheck is the chosen option for energy code compliance, an individual REScheck must be calculated for each unit.
- Walls and other components separating two conditioned spaces may be consider interior walls/components and should not be included on the REScheck.
- And the corollary, any wall or other component of the building separating conditioned from unconditioned space must be included on the REScheck and all other construction documents, with R-values, U-factors, SHGC’s, any thermal performance specification accurately identified.
- Slab edge insulation must extend in an approved manner to the top of slab.