Thermal insulation on basement walls

Placeholder page for the supporting reference Thermal insulation on basement walls, part of the Examitect reading list for the ExAC.

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Document at a glance

Full titlePerformance of Thermal Insulation on the Exterior of Basement Walls
PublicationConstruction Technology Update No. 36
PublisherInstitute for Research in Construction (IRC), National Research Council of Canada (NRC)
AuthorsM.C. Swinton, M.T. Bomberg, M.K. Kumaran, N. Normandin, W. Maref
YearDecember 1999
Length8 pages
LanguageEnglish (French version available through NRC Publications Archive)
Primary audienceDesigners, builders, code officials; Intern Architects studying building science and envelope assemblies for the ExAC
ExAC relevanceSupplementary resource for Section 3 building science, materials, and assemblies; also connects to Sections 1, 2, and 4
Where to accessNRC Publications Archive via DOI 10.4224/40002836

Why it matters for the ExAC

This document matters because it translates building science theory into measured field results. ExAC questions on building envelope frequently move beyond "what does insulation do" to "how does a specific assembly perform under real conditions." IRC's field data provides exactly that kind of grounded, testable knowledge.

On Examitect's ExAC study plan, this document is listed as a supplementary resource for Section 3, which covers building science, materials, assemblies, and building envelope performance. Section 3 questions often target thermal resistance, moisture management, and assembly failure modes, all of which this study addresses directly. Connections also appear in Section 1 (building envelope decisions during schematic design) and Section 2 (NBC code requirements for below-grade insulation).

The motivation for the research is itself exam-relevant. A Canadian survey of new home warranty programs found that 85% of major basement failures in 1994 and 1995 involved combined water and soil action: frost action alone contributed to 40% of failures, swelling clays to 36%, and other groundwater factors to the remaining 9%. The ExAC tests whether you understand the physics behind these failure modes and how to design against them.

ExAC sections

See the ExAC sections table below for study-plan coverage.

What this document is

Construction Technology Update No. 36 is an eight-page technical summary from Canada's Institute for Research in Construction (IRC). It reports on a multi-year field study that monitored 13 different exterior basement insulation assemblies installed on IRC's Test House No. 1 in Ottawa. The insulation systems were tracked through two full heating seasons, including a 1-in-75-year rain event in August 1996 and the Ice Storm of 1998.

The study was a collaboration between IRC and an industry consortium: the Canadian Plastics Industry Association, the Expanded Polystyrene Association of Canada, the Canadian Urethane Foam Contractors Association, Owens Corning Inc., and Roxul Inc. It investigated five insulation materials, two installation approaches, multiple board-joining techniques, drainage groove configurations, support systems for above-grade protective cladding, and two grading schemes.

The document is short but dense. Every section reports a concrete finding backed by field measurement. That makes it well-suited to ExAC questions that ask about specific outcomes rather than general principles alone.

Inside the document: key topics

The document covers seven main topics in sequence. Each maps directly to concepts you may encounter on the ExAC.

TopicKey finding
Thermal performanceAll five insulation products sustained R-values over two heating seasons with only small variation from their starting values, including during heavy rain and winter thaw events.
Two lines of defenceExterior insulation is the first line of defence; the concrete wall is the second. Grading, eave drainage, and a functioning drainpipe are prerequisites for both to work.
Drainage grooves in rigid boardsGrooves played no measurable role at this site. Water did not normally reach the concrete wall when the overall system was properly installed.
Specimens with no drainage spacesPolyethylene-wrapped specimens (smooth outer surface, no voids) still managed water at the outer face. Drainage spaces are not required to protect the basement from water ingress.
Thermal bridgingVertical Z-bar supports fastened to concrete reduced effective R-value by 13% on average. Thermally broken horizontal supports performed significantly better.
Grading durabilityA 5% positive grade sloped away from the wall became negative within one year due to soil subsidence, eliminating the primary water-diversion strategy.
Freeze-thaw protectionAt 270 mm below grade, concrete never reached freezing temperatures. The above-grade fibre-cement protective cover handles the near-grade zone where freeze-thaw cycles do occur.

Five insulation products compared

The study monitored five distinct products side by side on the same basement walls. Understanding their differences gives you a concrete mental model for ExAC questions on below-grade assemblies.

ProductFormWater strategyNotable characteristic
EPS Type 1 Rigid board, lower density Manages water at outer face; exclusion approach Now permitted below grade in contact with soil (NBC code change, 1998); compressive strength sufficient for typical soil pressures
EPS Type 2 Rigid board, higher density Manages water at outer face; exclusion approach Performance essentially identical to Type 1 in field conditions; slightly higher compressive strength
SPF (spray-polyurethane foam) Sprayed in place; fully adhered Continuous sealed layer around projections and footing Only product with no evidence of water at the footing; may eliminate need for dampproofing at the lowest wall level
Mineral fibre board Dense semi-rigid board Fibrous voids facilitate active drainage Substantial water movement documented at outer face; none reached the concrete wall; outer fibres do the work
Glass fibre board Less dense semi-rigid board Fibrous voids facilitate active drainage Compresses under soil load; manufacturer pre-inflates the stated R-value to compensate; field performance confirmed this strategy works

The key takeaway: products use different water strategies (exclusion vs. active drainage), but all five delivered sustained in-situ R-values. System-level variables, including thermal bridging, grading, and board joint details, affected performance more than product choice alone.

Key terms every ExAC candidate should know

TermWhat it means for the ExAC
Exterior basement insulationInsulation placed on the outside face of the foundation wall. Acts as a thermal barrier and as the first line of defence against water ingress.
Two lines of defenceThe water-management model where exterior insulation is the first line and the concrete wall is the second. Both must function if either is to remain effective.
In-situ R-valueThe measured thermal resistance of an insulation in its installed location, reflecting real moisture and temperature conditions rather than controlled laboratory values.
EPS (expanded polystyrene)A rigid board insulation in two density types. Type 1 is now permitted for below-grade soil contact under the NBC following a 1998 code change.
SPF (spray-polyurethane foam)A sprayed-in-place insulation that bonds continuously to the substrate, covering projections and the footing joint. Achieves higher R-value per unit thickness than board products.
Thermal bridgeA path of higher conductivity through an insulation layer that reduces effective R-value. Vertical metal Z-bars fastened to concrete reduced assembly R-value by 13% in the IRC study.
DampproofingA coating applied to a concrete wall to resist moisture movement under non-hydrostatic conditions. SPF's continuous coverage may make dampproofing unnecessary at the lowest wall level.
Frost actionDamage caused by the expansion of water as it freezes in soil or assemblies. A contributing factor in 40% of major new home basement failures surveyed in 1994-1995.
Soil subsidenceSettlement of backfill after construction. Can reverse a 5% positive grade into a negative grade (sloping toward the wall) within one year.
Hydrostatic pressureWater pressure exerted by a standing column of water. Builds against the foundation wall when drainage fails and soil becomes saturated.

How this compares to other ExAC references

ReferenceWhat it adds on basement insulation
This document (CTU No. 36)Field-measured R-value data, specific water-management findings, and the 13% thermal bridging penalty number. The empirical foundation for ExAC building science questions on this topic.
NBC 2020Code requirements for below-grade insulation, vapour control, and drainage in Part 9 (houses) and Part 5 (environmental separation). Sets the regulatory floor that this research helped inform.
CHINGVisual cross-sections of basement wall assemblies, footing details, and drainage configurations. Use CHING to visualize what CTU No. 36 measures in numbers.
Building Envelope Thermal Bridging GuideFull analytical framework for thermal bridge calculation across assembly types. CTU No. 36 gives one concrete example; the Thermal Bridging Guide covers the broader method.
Canadian Wood-Frame House ConstructionBroader below-grade wall construction guidance for wood-frame residential buildings. Complements CTU No. 36's concrete-wall focus.
NECB 2020Energy performance requirements for below-grade assemblies in non-residential buildings, including minimum effective RSI values that field research like this study helps set.

How to study this document for the ExAC

  • Read it once, straight through. At eight pages, this is one of the shortest documents on Examitect's ExAC study plan. Read it cover to cover before doing anything else. The argument builds sequentially, and the conclusions land harder if you've followed the field data.
  • Memorize the two-lines-of-defence model. Sketch the basement wall cross-section from memory: exterior insulation as first line, concrete wall as second. Know what compromises each line and what happens downstream when it fails.
  • Build a five-product comparison table. List EPS Type 1, EPS Type 2, SPF, mineral fibre, and glass fibre side by side. Note each product's water strategy, compressive behaviour, and any distinctive field finding. This table is the kind of organized knowledge the ExAC rewards.
  • Commit the 13% thermal bridging figure. The ExAC is known to test specific quantitative results from building science research. Vertical Z-bars fastened to concrete reducing effective R-value by 13% is precisely that kind of testable number.
  • Understand the grading warning. A 5% positive grade is not enough. Soil subsidence erased it within one year. The ExAC may ask you to identify this as a design error or to specify a corrective measure.
  • Connect to the NBC and NECB. The 1998 code change permitting EPS Type 1 below grade ties this research to NBC 2020 requirements. Knowing why a code rule exists strengthens your ability to apply it correctly under exam conditions.

ExAC sections this document supports

ExAC SectionHow this document shows upRole on Examitect's study plan
Section 1: Design and analysis Below-grade thermal strategy and building envelope design decisions in schematic and design development phases Supplementary
Section 2: Codes NBC 2020 requirements for below-grade insulation, vapour control, and the 1998 EPS Type 1 code change Supplementary
Section 3: Sustainability and final project Building science and assemblies: thermal performance, moisture management, below-grade assembly failure modes, and envelope detailing Supplementary (primary focus)
Section 4: Construction and practice Installation detailing for board joints, Z-bar selection, and grading maintenance during construction administration Supplementary

Tips for Intern Architects reading this document

Tip 1, read the conclusions and summary first. The document's final page lists eight bullet-point conclusions. Read these before you start the body. They prime your attention so the field data lands in context, not as isolated numbers.

Tip 2, sketch Figure 1 from memory. The cross-section showing the two lines of defence, the drainage path, and the footing drainpipe is the conceptual core of the document. Being able to draw it from memory makes ExAC scenario questions on basement failures much easier to parse quickly.

Tip 3, separate product performance from system performance. All five products sustained thermal performance. The differences in the study came from system-level choices: Z-bar type, grading scheme, and board joint details. The ExAC often hinges on this product-vs.-system distinction.

Tip 4, connect the grading finding to your IAP site work. During your Internship in Architecture Program (IAP), you may have issued site review reports without flagging near-grade grading conditions. This document quantifies why that matters. Soil subsidence reversed a 5% slope in under a year. Use this to sharpen your site observation habits now.

Tip 5, note what SPF can replace. Spray-polyurethane foam is the only product that continuously bonds to the footing. The study found it was the only product with no evidence of water at footing level, suggesting dampproofing may not be required when SPF covers the footing. ExAC questions on footing protection may test exactly this distinction.

Tip 6, pair this with the Building Envelope Thermal Bridging Guide. This document gives you one concrete thermal bridging example: Z-bars, 13% R-value penalty. The Thermal Bridging Guide covers the full analytical method. Together they give you both the number and the framework to apply it elsewhere.

Common ExAC scenarios where this document is the answer

  • A scenario asks why an exterior basement insulation system failed despite positive grading at the time of construction. You apply the soil subsidence finding: a 5% positive grade can reverse direction within one year.
  • A question asks which of five insulation products continuously protects the footing joint. You select spray-polyurethane foam, the only product that bonds to the footing and showed no evidence of water in that area.
  • A scenario describes an assembly where vertical metal Z-bars connect the exterior protective cladding to the concrete wall. You identify this as an unbroken thermal bridge and note it can reduce effective R-value by around 13%.
  • A question asks whether drainage grooves in rigid insulation boards improve water management in a properly installed basement system. Based on the IRC findings, the answer is that they play no significant role when the primary system is functioning.
  • A scenario notes that EPS Type 1 is specified for an exterior basement application in direct contact with soil. You confirm this is permitted under the NBC following the 1998 code change.
  • A question asks what the first and second lines of defence are in a basement envelope system. You describe exterior insulation as the first line and the foundation wall as the second, with proper grading and a functioning drainpipe as prerequisites for both.
  • A scenario asks about the leading cause category for major basement failures in new Canadian homes in the mid-1990s. You recall that 85% involved combined water and soil action, with frost action alone responsible for 40%.

How Examitect reinforces this topic

Examitect's practice questions on building science and envelope assemblies are written to match the format and difficulty of the ExAC. Questions tied to thermal insulation on basement walls ask you to apply findings, not just recall them. You'll encounter scenario prompts that describe a real assembly condition and ask you to diagnose the failure mode, identify the thermal bridge, or select the appropriate design response.

Each question explanation points you to the specific finding in the source document. When you get a question wrong, you'll know exactly which section of CTU No. 36 to return to. That feedback loop makes each wrong answer more useful than a passive reread.

If you're working through Examitect's ExAC study plan, you'll encounter this document in the Section 3 building science sequence alongside the Building Envelope Thermal Bridging Guide and Canadian Wood-Frame House Construction. Covering these together builds a connected picture of below-grade envelope performance.

Try a free practice question to see how Examitect frames building science scenarios, or see plans to access the full question bank.

Thermal insulation on basement walls: ExAC FAQ

Construction Technology Update No. 36 is an eight-page research summary published in December 1999 by the Institute for Research in Construction (IRC) at the National Research Council of Canada. It reports findings from a multi-year field study that monitored the thermal and water-management performance of five exterior basement insulation products over two heating seasons at IRC's Test House No. 1 in Ottawa.

The five products were: moulded expanded polystyrene (EPS) Type 1, moulded expanded polystyrene (EPS) Type 2, medium-density spray-polyurethane foam (SPF), semi-rigid mineral fibre board, and semi-rigid glass fibre board. All five sustained thermal performance over two heating seasons. Their water-management strategies differed, but field results were similar across the board.

The first line of defence is the exterior surface of the insulation, which manages water from grade level down to the footing drainpipe. The second line is the concrete or masonry foundation wall, which handles any incidental water that bypasses the insulation. Proper grading, eave drainage, and a functioning perimeter drainpipe are prerequisites for both lines to work as designed.

The IRC study found that drainage grooves in rigid boards played no measurable role at the Ottawa test site. Water did not normally reach the concrete wall behind properly installed insulation with functioning above-grade drainage and a drainpipe. The grooves are at best a minor enhancement to the second line of defence, relevant only when all other water-management strategies have already failed.

Even partial contact between metal fasteners and thermally conductive concrete short-circuits the insulation layer. The study measured that vertical metal Z-bar supports fastened to concrete reduced effective thermal resistance by 13% on average compared to a thermally broken horizontal support system. The bridge effect extended well below the bottom of the Z-bars because the concrete itself conducts heat along its full height.

Yes. Prior to 1998, the National Building Code of Canada restricted EPS Type 1 from direct contact with soil in houses and small buildings. In October 1998, the Canadian Commission on Building and Fire Codes removed that restriction. The IRC study's field results supported that decision by demonstrating that EPS Type 1 sustained thermal performance and managed water effectively under the test conditions.

The IRC study found that a 5% positive grade sloped away from the wall became a negative grade sloping toward the wall within one year due to soil subsidence. Once that primary water-diversion strategy reversed, both test walls experienced similar water exposure in the second heating season. Designers must specify steeper initial grades, use durable surface drainage measures, or address soil settlement during construction administration.

SPF was the only product that continuously bonded to the concrete wall and footing, forming an unbroken protective layer around projections and joints. It was the only product with no evidence of water at footing level. The researchers suggested that when SPF fully protects the footing, dampproofing of the concrete may not be required, even at the lowest level of the wall.

Other ExAC reference books

The six primary references on Examitect's ExAC study plan, plus the full coverage map.

CHING

Building Construction Illustrated: visual assemblies, details, and building systems for ExAC candidates.

CHOP

Canadian Handbook of Practice: the RAIC's guide to architectural practice, primary for Sections 1, 3, and 4.

NBC 2020

National Building Code of Canada: the code reference for ExAC Section 2 and Part 9 building envelope requirements.

NECB 2020

National Energy Code for Buildings: energy performance requirements including minimum RSI values for below-grade assemblies.

RSMeans

Construction cost data used in ExAC cost management and Section 1 estimating questions.

Yardsticks

Yardsticks for Costing: Canadian construction cost reference for ExAC Section 1 budget and cost management.

All ExAC resources

See the full coverage map of primary and supporting references on Examitect's ExAC study plan.