Construction Design-The Code-Minimum House-An Introduction

I am currently involved in a new home build working with my friend and general contractor, Strommer Home Improvements.  My involvement in the build is that of the designer, tester, and laborer.  Yep, I’m back swinging a hammer and enjoying every minute of it.  I’m calling this build the code-minimum house because the budget is such that we have very little extra funds for upgrades.  No custom showers, no granite countertops, and no exterior insulation…but that doesn’t mean some low cost performance can’t be designed into the home.

Much like last year’s concrete-less slab on grade home, this year’s code-minimum house will have several blogs dedicated to different parts of the build.  This posting will be about the design and the direction I hope to steer the build.

The homeowner wants a low maintenance, single level home.  Three bedrooms, two baths with an attached heated garage.  The property is located within the city limits of a small town with no code enforcement.  We have two inspections, electric and sewer.  The owner had a couple basic floor plans picked out before I became involved, the first was too far outside his budget.  I was given the second and asked to make a set of drawings simplifying the build as much as possible.  Here is, for the most part, the final design.

To keep building costs low, the decision was made to use a frost protected shallow foundation.  In early discussions with the concrete contractor, I learned of a concrete form system called Mono Slab EZ Form which we have decided to use.

The walls are simple 2 x 6 construction and OSB sheathing with no exterior insulation.  (Notice the double bottom plate in the SketchUp drawing above, more on that in a future blog.)  Although Minnesota has adopted the 2018 IRC code, the state rewrites the energy code and has decided to eliminate (for now) the exterior insulation requirement (a topic for another post).  R-21 fiberglass insulation will be used in the walls.

A vented attic with at least R-49 of blown fiberglass is planned.  A manufactured truss has been designed and a portion of the truss system will have a plenum truss, or a way to keep ducts and other utilities inside the conditioned space but still tucked into the ceiling.  The home will have hot water heat, the only ductwork is that of the heat recovery ventilator (HRV), dryer and kitchen range hood.  There will also be a vent for the natural gas combination hot water boiler/domestic water heater.  The plenum truss chase will simplify the ductwork design and install.

As I said, the home’s main heat source is hot water in-floor heat supplied by a combination boiler.  The issue with a system like this is it can over-heat a home during the shoulder seasons when just a little heat is needed.  Another problem is no ductwork for air conditioning.  Even though this home is being built in northern Minnesota, AC is standard in almost all new construction.  (Us cold climate people start melting when the temp hits the low 80’s!)  To address spring and fall heating needs along with AC, we are planning a cold weather air source heat pump (mini-split system).  The homeowner also requested a fireplace.  The charm of a wood burning fireplace can’t be beat, but cost, difficulties with the air sealing details, and the fact that the homeowner wanted a low maintenance property, the decision was made to install a thermostatically controlled gas unit.  The fireplace being installed has a variable BTU output and is large enough to meet the heating needs of the entire home, a nice backup system.

We had a third party heat loss calculation performed on the home which indicated a total heating load of 30,000 BTU for the home and an additional 13,000 BTU for the attached garage.  Not quite as low of a heating load as last years Concrete-less Slab on Grade build, which was 23,000 BTU for the home, but still lower than most I’m seeing built today.

The windows selected are an above code performing PVC manufactured by a local company that I have used several times on lower cost builds, including my own home.  Stay tuned for a post dedicated to these windows and their installation.

I performed some modeling on this design to determine if the additional air sealing (and possibly additional insulation) is worth the effort.  I used BEopt, a free modeling software through the Department of Energy.  The results showed energy cost savings of around 6% by moving from a standard air sealing result of 3 ACH50 to 1 ACH50.  Though not a major reduction in heating and cooling costs, we are gaining comfort and an improvement in indoor air quality.  The energy cost savings could have also been improved an additional 7% by  adding R-15 exterior insulation, but at a substantial increase to the cost of the build.  The increased air sealing, with it’s low price tag, was a no brainer, the extra insulation added too much cost.

So, with this being such a simple home, what makes this build a blog worthy subject?  The air sealing details!  I recently had an article published on Green Building Advisor’s (GBA) web site, Straight Forward Air Sealing Strategies.  I will be practicing many of the strategies discussed in that article in hopes to achieve a sub 1 ACH50 blower door test, and also a possible second article for GBA showing the details in use.

Stay tuned for future posts on this build, starting with the foundation.

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