7.  RECOMMENDATIONS FOR A MINNESOTA ENERGY CODE FOUNDATION RULE

Disclaimer
The contents below are not expressed in legal code language and should not be interpreted as such.  They are intended to encapsulate the physics of foundation heat and mass transfer that are necessary to ensure efficient and durable foundation insulation systems.

7.1    Introduction

The basement foundation and whole house energy performance results generated in chapters 4 and 5 have been combined with the large body of experimental data on foundation moisture transport performance developed at the Foundation Test Facility (FTF) and Cloquet Residential Research Facilities (CRRF) to develop the foundation insulation system recommendations discussed below.  It is very important to note that these recommendations are based on the publicly available, quantitative experimental data and not on anecdotal and/or experiential data comprising the aggregated knowledge-base of the Minnesota building community.  Thus, as might be expected, there are considerable differences between these recommendations and current Minnesota energy code compliant practice.  Clearly, the recommendations will be seen by some as being overly conservative and some of the details thus viewed as redundant or even unjustified¹.  Nevertheless, because the recommendations are required to be based on experimental data, they do fulfil the project mandate.

The recommendations are presented in a pseudo-code style with the listing of a set of definitions first.  The recommendations are presented as a set of performance design requirements and a criterion.  These are followed by sets of graphical prescriptive examples for each foundation configuration that show how the performance requirements and criterion are applied in practice.

Notes

1. Typical criticisms can be reduced to comments such as "I/we have built x number of houses and I/we have never had a problem"; "If it ain't broke, don't fix it"; and "Prove to us that what we are doing now doesn't work".  Developing a basis for rigorously addressing these sorts of criticisms would require the collection of a vast amount of experimental data, particularly with regard to geotechnical and interior hygrothermal boundary conditions, an enterprise completely beyond the scope of this project.

7.2   Definitions:

1. Foundation:  A foundation comprises a floor system and an exterior perimeter wall system in contact with the earth.

2. Foundation wall system:  A continuous and homogeneous vertical structural system, a portion of which must be in contact with the earth.

3. Foundation floor system: A continuous and homogeneous horizontal structural or non-structural system in contact with the earth.

4. Foundation wall system energy trade-off zone:  The portion of the wall from the base of the footing to a maximum above grade extension of 12 in.

5. Exterior:  earth and atmosphere outside the foundation.

6. Interior:  conditioned and unconditioned spaces inside the foundation.

7. Foundation air barrier system:  A material or combination of materials that has the following characteristics:

   1. It is continuous with all joints sealed.

   2. It is durable.

   3. It resists the transport of an air / water vapor mixture as a result of an exterior-interior pressure difference to the minimum extent necessary to achieve moisture durability and a given level of energy efficiency.¹

8. Water separation plane:  A single component or a system of components creating a plane that prevents liquid water capillary and hydrostatic pressure driven flows and provides a water vapor permeance of 0.1 perms or less to retard water vapor flow by diffusion.

9. Stable annual wetting drying cycle:  A water (solid, liquid and vapor) transport process operating on a foundation that produces no net accumulation of ice or water over a full calendar year and is free of adsorbed water² for at least 4 months over a full calendar year.

10. Foundation insulation layer:  A system of building materials connected in series and/or in parallel excluding surface heat transfer film coefficients that serves as thermal insulation in foundation wall and floor systems.

11. Stem wall foundation system:  A foundation wall system between the top of a frost footing and the top of a foundation floor system.

12. Unvented crawl space foundation wall system:  A foundation wall system enclosing a crawl space without insulation in the floor above and having no direct air exchange with the exterior.

Notes

1. For a particular foundation system that is tested to meet the design requirements and criterion listed below, it is possible to quantify the maximum amount of air leakage permissible for that particular system in terms of moisture durability and energy efficiency (typically specified as as an envelope infiltration rate).  From an energy perspective alone, however, it theoretically always is desirable to minimize infiltration and hence maximize the advective resistance of the air barrier.  Thus the quantitative minimum performance of the air barrier in any particular design is a function of the system and imposed boundary conditions and can be determined by standardized testing of that design.  The minimum performance can be expressed in terms of an air permeability requirement as follows:  "The foundation air barrier system shall have an air permeabilty not to exceed x ft³/min.ft² (l/s.m²) at a pressure differential of y psf (Pa) when tested in accordance with ASTM E2178."

2. The degree of dryness (or gravimetric moisture content) required to achieve a stable annual wetting drying cycle for the materials in a particular design is a function of the specific materials used, the system and the boundary conditions.  For example, above-grade testing at the Cloquet Residential Research Facility (CRRF) has shown that a stable annual wetting drying cycle can be realized with gravimetric moisture contents for particular materials sampled in June as follows:

   • cellulose:  12.3 - 15.7%

   • blown-in blanket system (BIBS): 1.3% - 2.6%

   • fiberglass batt: 2.0 - 3.0%

   • wood stud: 6.0 - 10.3%

   Thus provided a particular material does not exceed its bound water carrying capacity (much higher for hygrophilic materials such as cellulose, much less for hygrophobic materials such as BIBS) and so has no adsorped (or "surface-attached") water for at least 4 months, it is sufficiently dry for achieving annual wetting/drying cycle stability.

7.3   Design Requirements

A building foundation shall be designed to meet all the following requirements:

1. Have a continuous water separation plane between the exterior and interior¹.

2. Not allow external liquid water intrusion (including capillary flow) across the water separation plane after the foundation is backfilled.

3. Have a foundation wall system insulating layer with a minimum equivalent continuous uniform² thermal resistance of R-10³ without any thermal breaks⁴ within the foundation wall system energy trade-off zone⁵.  Interior insulation shall extend from at least the top of the foundation floor system to the top of the foundation wall system.  Exterior and integral insulation shall extend from at least top of the footing to the top of the foundation wall system.

4. Have a foundation air barrier system between the interior and the exterior.

5. Meet all the requirements of R403.1.4.1 for footing frost protection⁶.

Notes

1. This requirement applies to both the foundation wall and floor systems.  However, foundation floor systems built with vapor retarders according to R506.2.3 (with the proposed Minnesota amendment) do not constitute a continuous water separation plane. In other words the advection of liquid water by hydrostatic pressure is not prevented by a cracked 3.5" concrete slab underlain with either perforated 6-mil. polyethylene (typical in actual construction) or 1 in. thick extruded polystyrene with edge gaps.  The latter option also does not provide a permeance of 0.1 perm or less.

2. The equivalent continuous uniform R-value of discontinuous foundation insulation layers (such as fiberglass batts in wood framing) needs to be calculated.  A standard method for performing such a calculation is described in chapter 23.8 of the 2001 ASHRAE Handbook of Fundamentals.  However, this basic, 1-dimensional heat flow methodology does not produce an accurate R-value for installed framed wall foundation insulation layers in particular and tends to overestimate the R-value.  Currently, the best method of calculating an accurate R-value is to use 3-dimensional finite element analysis or equivalent with temperature dependent material properties.  Also note that in terms of the definition of a foundation insulation layer, surface heat transfer coefficients are excluded from contributing towards the equivalent continuous R-value.  This is because such film coefficients, the effect of the surrounding earth, etc, already have been factored into the calculations for the stipulated insulation layer R-value.

3. There is a desire to encourage the use of exterior foundation insulation by allowing the use of less than optimum levels of foundation insulation, specifically, exterior R-5 insulation.  Further, typical current practice is to use interior slab-on-grade insulation systems with a thermal break at the slab edge.  Because there is no physical basis for these practices, they cannot be recommended.  Nevertheless, energy trade-off methodologies and design examples have been developed for both these scenarios and are reported in section 7.4 below.

4. In this context, a thermal break is a direct heat flow path between the interior and exterior that bypasses the insulating layer.  Hence the uninsulated slab support lip at the top of the foundation stem wall for slab-on-grade construction with interior insulation (the common practice) is a thermal break, but a stud that is part of the insulating layer in a traditional cavity insulation framing system is not.

5. Outside the wall system energy trade-off zone, the heat flow is no longer influenced by below-grade effects and the required insulation levels theoretically should conform to those of above-grade envelope components that are significantly larger than the optimum below-grade insulation levels.  However, current practice is to use the same level of insulation over the entire foundation wall system regardless of exposure.  In order to accommodate this practice without arbitrarily dividing the foundation wall system into above- and below-grade components, additional energy savings must be realized elsewhere in order to achieve the same level of conservation for the foundation envelope that would be achieved if the entire wall system were in the energy trade-off zone, regardless of actual exposure.  This does introduce a bias into the overall conservation schema by nominally encouraging the use of foundation wall structures for above-grade usage in basements.  However, this is mitigated by the higher cost of this practice, and so it is unlikely to be used much in practice.  The trade-off methodologies developed are reported in section 7.4 below.

6. In the principal investigators' opinion and as illustrated by the simulation results presented in section 5.2, Minnesota frost footing depths of 42 and 60 in. are inadequate for unconditionally protecting slab-on-grade / stem wall footings against frost heave by temperature control alone.  There are three approaches to resolve this problem, (a) increase the required frost footing depth, (b) require a functional drainage system experimentally verified to prevent frost from penetrating beneath the footing, or (c) a foundation drainage system minimally conforming to the specification of R405.1 and an increased frost footing depth.  Developing a basis for recommending an increased footing depth is beyond the scope of this project and specific experimental data for the frost prevention efficacy of frost footing drainage systems in Minnesota are not available.  Hence, by default, a drainage system conforming to R405.1 has been used in all the relevant stem wall slab-on-grade and unvented crawl space foundation system examples.

7.4    Design Criterion¹

On the interior side of the water separation plane, a building foundation system shall be designed to have :

1. A stable annual wetting/drying cycle.

2. No visible or olfactory fungal or other biotic activity².

3. No bulk water movement.

   Exceptions:

   Bulk water movement (such as condensate run-down) is allowed on the interior side of the water separation plane under the following conditions:

   1. There is no accumulation of free water on the interior side of the water separation plane³ and the above-grade portion of the foundation wall system does not exceed 17% of the foundation wall system height, or,

   2. There are components of the foundation wall system inside the water separation plane designed specifically to absorb and store moisture so that there is no accumulation of free water on the interior side of the water separation plane³ when at least 63% of the foundation wall system height is above-grade.

Notes

1. Criterion is used here in terms of its strict definition as a "principle taken as a standard in judging".  Because of the complexity of heat and mass transfer in foundation envelopes, it is not practical to cast the desired moisture performance as a requirement because no matter how sophisticated the design analysis, ultimately, the only credible test for compliance is the long-term experimental evaluation of full-scale prototypes.  However, when used as a criterion, the desired moisture performance described in the criterion has proved to be very effective in yielding designs that have displayed the desired physics without exception upon being tested.  After the systems have been tested and proved successful, then specific quantitative moisture performance requirements based on standard test procedures for that particular system can be developed and stipulated as design requirements.

2. Currently, the consensus amongst the fungal testing community is that there are no efficacious standard test procedures currently available for determining the a-priori fungal performance of a foundation system.  There are guidelines for fungal sampling of built building components but these are invasive and are based on culturing retrieved samples.  Further, these tests also are subjective to a degree since they involve establishing a cultured species "colony forming units/unit area" (or equivalent) count pass/fail criterion that is related to human macro detection based on sight and smell.  Thus, despite its subjectivity, a "no see, no smell" fungal activity evaluation has proved to be a practical and effective means for establishing the fungal performance of foundation systems and has been used as a standard in developing successful foundation insulation systems at the Foundation Test Facility to the satisfaction of the research sponsors.

3. Note that in terms of design requirement 1, this applies to both the foundation wall and floor systems.

7.5    Energy Trade-Offs

Two trade-off options have been evaluated.  The first is to increase the heating plant AFUE in the presence of continuous exhaust-only ventilation as specified in rule N1104 above the current required minimum of 78%.  The second option is to replace exhaust only ventilation with an Energy or Heat Recovery Ventilator (ERV/HRV).  This option (with exceptions under maximum trade-off conditions) provides significantly greater energy savings than needed simply for foundation system insulation purposes.  Thus these savings result in a nominally lower AFUE than the required 78% minimum so that the difference in AFUE is available for energy trade-off purposes elsewhere in the envelope.

7.5.1    Full Basement Foundation Wall System

 

Table 7.1  Option 1: minimum furnace AFUE requirements with exhaust only ventilation

Above-grade foundation wall exposure (%)a	Southern Zone		Northern Zone
	R-10 insulation (%)	R-5 exterior insulation only (%)	R-10 insulation (%)	R-5 exterior insulation only (%)
≤ 10b	78e	81	78e	81
> 10 & ≤ 47c	80	84	80	84
> 47 & ≤ 87d	83	88	82	87

Table 7.2  Option 2: minimum furnace AFUE requirements with ERV/HRV ventilation^f

Above-grade foundation wall exposure (%)a	Southern Zone		Northern Zone
	R-10 insulation (%)	R-5 exterior insulation only (%)	R-10 insulation (%)	R-5 exterior insulation only (%)
≤ 10b	70	72	70	72
> 10 & ≤ 47c	72	75	72	75
> 47 & ≤ 87d	74	79g	74	79g

Tables 7.1 and 7.2 notes

1. Calculated as the above-grade area of the foundation wall system divided by the total area of the foundation wall system above the interior surface of the foundation floor system.
2. Corresponds to the entire foundation wall system being within the energy trade-off zone for the MN-CALC house that has an attached 2-car garage.
3. Corresponds to the portion of the foundation wall system beneath the attached 2-car garage plus 50% of the balance of the  foundation wall system being below grade.
4. Corresponds to the entire foundation wall system being above grade with the exception of the portion beneath the attached 2-car garage.
5. Minimum baseline furnace AFUE.
6. HRV/ERV design sensible effectiveness = 82%.
7. In these cases, there is no potential for trade-offs elsewhere.

7.5.2  Stem Wall Slab-on-Grade Foundation System

 

Table 7.3  Minimum furnace AFUE requirements with exhaust only and ERV/HRV ventilation^b

Zone	R-10		R-5 interior insulation only
	Exhaust Only Ventilation (%)	ERV Ventilation (%)	Exhaust Only Ventilation (%)	ERV Ventilation (%)
Southern	78a	67	80	69
Northern	R-10		R-10 interior insulation only
	78	68	79	69

Table 7.3 notes

1. Minimum baseline furnace AFUE.
2. HRV/ERV design sensible effectiveness = 82%.

7.5.3  Unvented Crawl Space Foundation Wall System

 

Table 7.4  Minimum furnace AFUE requirements with exhaust only and ERV/HRV ventilation^b

Zone	R-10 insulation		R-5 exterior insulation only
	Exhaust Only Ventilation (%)	ERV Ventilationc (%)	Exhaust Only Ventilation (%)	ERV Ventilation (%)
Southern	78a	68	80	70
Northern	78	69	80	71

Table 7.4 notes

1. Minimum baseline furnace AFUE.
2. HRV/ERV design sensible effectiveness = 82%.

7.6   Compliant Examples

In order to exemplify the application of the requirements and criterion of sections 7.3 and 7.4, a set of example foundation systems based on experimental data has been developed and is presented in this section.  These examples are not intended and should not be considered to be a full set of all possible systems meeting the above requirements and criterion.  In order to meet their specified performance, the example foundation systems need to be built as shown without omitting any details.  ALL THE EXAMPLES INCLUDE A FOUNDATION FLOOR SYSTEM WATER SEPARATION PLANE that is configured to enable radon mitigation using sub-slab depressurization.  The following notes apply to all the drawings:

1. In cases where a vapor retarder and/or capillary break is required between the footing and the wall, structural coupling between the footing and the wall is required and is shown in the drawings regardless of whether it is necessary for structural compliance with the building code.
2. Exterior drain tile drainage systems are specified as being "to" R405.1 rather than as "per" R405.1 in order to emphasize that the drainage systems as shown in some cases have required features in excess of the minimum stipulated in R405.1.
3. The R-values given are always equivalent uniform values.

7.6.2    Stem Wall Slab-on-Grade Foundation Systems
 

7.6.3    Unvented Crawl Space Foundation Wall Systems with the Interior Grade at the Footing Top
 

7.6.4    Unvented Crawl Space Foundation Wall Systems with the Interior Grade above the Footing