Sizing HVAC for Your SIP Home

The following question was in my Inbox this morning.

Does SIPA have any contacts in the “HVAC world” regarding proper sizing of HVAC equipment for SIP homes? 

At the 2015 Annual Meeting the Structural Insulated Panel Association announced that SIPA had entered into a buying agreement with the folks at NORESCO, LLC to offer discounts and training on REM/Design software. It’s the same program used by HERs raters to perform energy modelling on any structure, not just those built with SIPs. Technically the registered “raters” use REM/Rate to generate reports pursuant to a HERs score. However, the input, training, and modelling for REM/Rate and REM/Design are exactly the same.

The benefit of using this software is multifaceted. To aid with equipment sizing, REM/Design will calculate the BTUs required to heat and cool not just the entire structure, but room by room based on orientation, windows and doors, and specific envelope characteristics. Another benefit is the program’s ability to show compliance with code, and in most jurisdictions this report will suffice as a building inspector’s assurance that the home is being built to meet the applicable building code. In addition, the program’s output can graphically show where energy weaknesses exist in a structure. This valuable information helps to cost-effectively add performance where it is needed and trim dollars where it’s allowed.

In my experience, the real benefit of using modelling software like REM/Design is to provide a third party specification of equipment sizing. The notion that all HVAC contractors use a manual J calculation to properly size the equipment package they propose is beyond laughable. It is commonplace for HVAC contractors to use rules of thumb and size the equipment based solely on square footage, resulting in multiple bids with the tonnage of A/C and the BTU output of the heating system ranging as wildly as the bid price. If a specification of size is generated by a third party (like a HERs rater) using modelling software (Like Rem/Design), the result is all your HVAC contractors bidding to a spec as opposed to their unique rule of thumb.

Many SIPA members have seen the value of offering energy modelling to their clients. I encourage you to consider adding that service to your company’s list of value-added services.

Or, get your client to call a HERs rater and they will get the benefit of:

    • Equipment sizing (with REM/Rate)
    • Code compliance verification
    • Quality assurance via site inspections and blower door testing
    • The Green addendum leading to:
      • Better appraised values
      • Better mortgage rate

 I encourage you to ask your SIP provider if they offer energy modelling. If they don’t, the easiest and quickest path is to contact a  HERs rater.

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