1.   INTRODUCTION

The prescriptive foundation insulation rules for new construction in the Minnesota Building Code are briefly summarized and compared with those in the 2000 IECC and ASHRAE 90.1-1989 (and the 1995 Model energy code that includes ASHRAE 90.1 by reference) in the following tables grouped by foundation type.  The bulk of the discussion is devoted to basements since this is the most prevalent foundation type in Minnesota (about 88% in the Minneapolis/St. Paul metropolitan area, HUD1998).  The comparisons and subsequent critiques are made for the Minneapolis/St. Paul climate zone since this is where most of the experimental data was collected.

1.1    New Construction Heated Basement Insulation System

Table 1.1  Full basement insulation system requirements

Standard

Exterior Walls

Floor Slab

Insulation

Vapor Control System

Insulation

Vapor Control System

MN code: chapter 7672.0600

R-5 from top of wall to top of footing or slab surface

Interior insulation: “moisture barrier” between wall and insulation from floor to grade and 1 perm vapor retarder on warm side of insulation

none
(unless sub-slab heating system present)

none

ASHRAE 90.1-1989

R-12

“considered to prevent moisture from collecting within the envelope.” Use 1985 ASHRAE Handbook design practice.

none

“considered to prevent moisture from collecting within the envelope.” Use 1985 ASHRAE Handbook design practice.

2000 IECC

~R-11 (depending on window area) from top of wall the lesser of 10ft below grade or floor level

“design shall not create conditions of accelerated deterioration from moisture condensation” (default: warm-in-winter side of thermal insulation)

none

“design shall not create conditions of accelerated deterioration from moisture condensation” (default: warm-in-winter side of thermal insulation)

In terms of thermal performance, the current MN code does not yield the energy performance potential of the ASHRAE and IECC standards.  However, the elevated R-values specified in these standards are not justified by the measured thermal performance of insulated basements in Minnesota (Foundation Test Facility 1998/99 data - figure 6).  These data show that full-wall R-values in the range of 8 to 14 yield the same energy performance within the experimental error.  Thus within this experimental error, exceeding an R-value of about R-8 (in the Minneapolis/St. Paul area) yields no net measured basement envelope energy performance gains.  Thus there is no energy performance justification for requiring single thickness insulation exceeding the climate-dependent optimum.  This situation arises because foundation heat transfer is strongly 3-dimensional, so that up to the optimum R-value, the insulation effectiveness (that is, the portion of the nominal 1-d R-value effective in reducing thermal conduction) remains static or increases, and beyond the optimum it decreases so that any further increase in insulation yields a corresponding decrease in effectiveness, producing small changes in thermal performance relative to those observed up to the optimum value.  It needs to be noted that even at the optimum R-value, single thickness insulation effectiveness is less than 100% (and usually significantly lower).  Thus the thermal performance achieved using the optimum R-value can be enhanced even further by distributing the same volume of insulation in a non-uniform way (greater thickness at the top of the wall and in the corners).  However, this additional complexity is not warranted by the increased installation labor cost.

Foundation insulation, water vapor and liquid control strategies are an interdependent, coupled system and need to be designed and specified accordingly.  Failure to do so leads to the innumerable problems that have become endemic (chiefly mold and rot failures) to new and retrofit basement construction in Minnesota, most commonly with the prevalent interior insulation schemes.  None of the existing or proposed standards take a coherent, systemic specification approach to this coupled system.  For example, the requirements for vapor retarders and soil drainage are different for integral (that is, embedded in the wall structure), exterior and interior insulation.  In general, the vapor and liquid water control requirements become increasingly more stringent and more costly moving from integral to exterior to interior insulation.   This reality should be reflected by the code that in turn would provide an incentive in terms of real cost savings for avoiding problematic interior insulation altogether.

In the principle investigators' opinion, the experimental evidence is unambiguous that the current prevailing practice for interior insulation and vapor retarder placement (as prescribed by Minnesota code) is susceptible to mold and rot failure as a result of excessive moisture build-up in Minnesota climate conditions.  More specifically, such vapor retarder configurations have an unstable annual wetting/drying cycle that leads to significant annual moisture accumulation within the insulation cavities (GA2001).  This situation is exacerbated by ineffective or partially effective soil drainage systems that produce inherently wet walls.  Further the rule language is unclear, since a “moisture barrier” is undefined.

The ASHRAE procedures are not of much practical help since they are not prescriptive in nature and neither do they address the complex moisture transport and phase change physics experienced in Minnesota, particularly in the southern half of the state (where moisture boundary conditions range from mimicking those of Florida in the summer to very dry conditions in mid-winter).  The default 2000 IECC vapor control system is specified as being a 1 perm vapor retarder on the warm-in-winter side of the insulation.  This specification produces moisture failures of even greater severity than the current Minnesota code requirement owing to the absence of any constraint on soil water vapor transport into the insulation cavity.  The 2000 IECC also permits a performance-based option as well (“design shall not create conditions of accelerated deterioration from moisture condensation”).  However, no guidance for achieving this design is provided.

Sub-slab basement insulation can produce energy savings of about 5-6% more than that possible with wall insulation alone in a single-thickness configuration and can be significant in reducing or eliminating slab surface condensation.  This might be an attractive savings option in a performance-based standard (together with lower vapor permeability, dry and “warm” floors) and could be offered as an option in a comprehensive code rule.

Experimental data has shown that a slab vapor retarder integrated with the wall retarder system reduces the foundation envelope vapor diffusion by 20-30%.  This can make a significant difference in reducing latent loads and thus yield additional energy savings.  It also should be included in the code standard as a viable technique for controlling interior humidity.

1.2    New Construction Slab-on-grade

Table 1.2  Slab-on-grade insulation system requirements

Standard

Stem Walls

Floor Slab

Insulation

Vapor Control System

Insulation

Vapor Control System

MN code: chapter 7672.0600

R-5 vertical only from top of slab to design frost line or top of footing.  NB Frost footings are required structurally.

none

None (unless sub-slab heating system present)

none

ASHRAE 90.1-1989

Unheated frost footing: R-8 vertically for 24” or top of footing

Unheated shallow: R-8 to top of footing plus 24” wide internal or external R-16

Heated: As above with R-values increased by 2

“considered to prevent moisture from collecting within the envelope.” Use 1985 ASHRAE Handbook design practice.

None (excepting internal wing insulation as required)

“considered to prevent moisture from collecting within the envelope.” Use 1985 ASHRAE Handbook design practice.

2000 IECC

R-8 to a depth of 4 ft

”design shall not create conditions of accelerated deterioration from moisture condensation” (default: warm-in-winter side of thermal insulation)

None

”design shall not create conditions of accelerated deterioration from moisture condensation” (default: warm-in-winter side of thermal insulation)

The Minnesota and 2000 IECC standards presume a frost footing required at 42 in. below grade in southern Minnesota and 60 in. below grade in the northern half (thus the IECC requirement would protrude beyond the footing in southern MN which is not desirable or cost effective).  The three-dimensional heat transfer issues affecting full basement foundations also apply to slab-on-grade foundations so that there also is an optimum R-value beyond which additional insulation is not cost-effective.  Vapor control is important at the top of the stem wall as well as at the interface between the stem wall and the slab where typically there is a structural break.  Vapor transport through these regions combined with thermal bypasses can lead to condensation and consequent mold on nutrient above-grade components (framing, carpeting, etc).

1.3    Objectives

The principle objective is to bring the MN energy code for foundations into compliance with the 2000 IECC (supplanted with the 2003 IECC after the start of the project) in terms of its foundation thermal performance for full basement foundations, unvented crawl space foundations as well as stem wall, slab-on-grade foundations.   The optimization of the insulation thermal resistance is to be based on an experimentally determined optimum value for full basements that yields the greatest energy savings / thermal resistance ratio.  These optimum R-values are to be included in a set of recommendations that can be used as the physics basis for developing a new MN foundation energy code rule that takes a systemic approach to foundation insulation including energy and moisture transport as well as structural considerations.  The recommendations are to include both prescriptive and performance options that focus on realizing optimum energy savings in a durable foundation envelope system.  More specific objectives are as follows:

  1. Develop a standard Minnesota test house model that can be used for assessing the foundation and whole house energy conservation performance.

  2. Determine the minimal set of climate zones that characterize Minnesota from an envelope heat transport perspective and use these zones to simulate the foundation insulation and whole house energy performance.

  3. Determine the full basement foundation insulation wall location (interior, integral or exterior) that yields the worst energy performance and use this location for evaluating the optimum insulation thermal resistance.

  4. Optimize the foundation wall insulation using a uniform thermal resistance over the wall surface for each of the climates determined in b. above for full basements and stem wall slab-on-grade foundations.  Use these bounding values to interpolate the intermediate insulation optimum for unvented crawl spaces (thermally equivalent to a full basement with a reduced wall height).

  5. Include the optimum foundation insulation into the whole house energy performance model and evaluate the following whole house trade-off strategies for all the Minnesota climate zones that yield the same whole-house energy performance:

    1. Reducing the exterior insulation R-value to R-5 by increasing the furnace AFUE.

    2. Reducing the exterior insulation R-value to R-5 by replacing exhaust-only ventilation with an ERV/HRV (energy/heat recovery ventilator).

    3. For full basements, increasing the above-grade wall exposure beyond 12 in. by increasing the furnace AFUE or by replacing exhaust-only ventilation with an ERV/HRV.

  6. Develop a performance-based foundation energy code rule for designing and installing durable foundation systems in Minnesota encompassing heat and moisture transport as well as structural issues arising from foundation insulation, specifically frost heave and adfreeze effects.

  7. Develop a set of prescriptive rules that reflect that are fully in compliance with the performance rules and express these rules as a set of detailed schematics for full basement, unvented crawl space and stem wall slab-on-grade foundations.  These prescriptive schematics should embrace all the major categories of foundation systems currently included in the 2003 IECC.

  8. Assist the State of Minnesota in translating the recommendations into statutory rule language.  This embraces the following:

    1. Attendance by at least one of the principal investigators at all Minnesota Building Codes and Standards Energy Advisory Committee (EAC) meetings at which the foundation rules are discussed.

    2. Taking the recommendations of the EAC and helping the State translate them into draft rule language and/or schematics on an ongoing basis.

    3. Develop a web site on which the draft rule can be published and continually updated to promote review and publicization of the rule language as it develops.

    4. Modify the project objectives and adjust the course of the research as necessary to address the issues raised by the EAC with a view to developing a final rule that can be statutorily implemented.

  9. Prepare a final report and publish it on the world wide web on a separate web site.  This will permit a clear distinction between the foundation energy code recommendations of objectives f. and g. and the draft rule that emerges out of objective h.

1.4    Methodology

The energy performance evaluation will be performed using computer simulation.  The foundation energy performance will be evaluated using a proprietary three-dimensional heat transfer program (BUilding Foundation Energy Transport Simulation (BUFETS)) while the whole house energy performance will be determined using  the EnergyPlus simulation developed by the U.S. Department of Energy.  A methodology based on previous research at the University of Minnesota will de devised to couple these two simulation programs so that the three-dimensional foundation energy performance can be included in the whole house energy performance evaluation.

The moisture transport recommendations will be based on experimental data gathered at the University of Minnesota's Foundation Test Facility and Cloquet Residential Research Facility.  Where necessary, these experimental data will be augmented by hygrothermal simulation using the WUFI version 2.1 two-dimensional simulation.