Technical Foundation
What Is a Insulation Board?
Insulation board is rigid, dimensionally stable insulation manufactured in flat sheets rather than batts or loose fill, used where a stable substrate is needed under roofing, siding, or as a standalone thermal barrier on foundations and mechanical equipment. Polyisocyanurate (polyiso) offers the highest R-value per inch of the common rigid foams, making it the standard choice for low-slope roofing where thickness (and therefore roof height and detailing) is constrained.
Extruded polystyrene (XPS) and expanded polystyrene (EPS) trade some R-value per inch for lower cost and, in XPS's case, higher compressive strength and closed-cell water resistance — a common reason XPS is specified below grade on foundation walls where EPS's higher water absorption would be a liability. Mineral wool board, made from spun basalt or slag fiber, offers non-combustible fire resistance that foam boards cannot match, along with sound absorption benefits, at a lower R-value per inch than the foam options.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Material Type | Polyiso, XPS, EPS, mineral wool | Each trades R-value, cost, moisture resistance and fire performance differently |
| R-Value per Inch | Polyiso ~6.0, XPS ~5.0, EPS ~3.6-4.2, mineral wool ~4.0-4.3 | Determines board thickness needed to hit a target total R-value for the assembly |
| Compressive Strength | Rated in psi, varies by density/type | Below-slab and roofing applications require adequate compressive strength for the load |
| Moisture Resistance | Closed-cell (XPS, polyiso) vs. open structure | Below-grade and roofing applications need low water absorption to maintain R-value over time |
| Fire Performance | Combustible foam vs. non-combustible mineral wool | Mineral wool specified where code or design requires non-combustible insulation |
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Applications
Where Insulation Boards Get Used
Insulation boards are specified anywhere a stable, rigid insulation substrate is needed — roofing, foundations, continuous wall insulation, or mechanical equipment.
Low-Slope Commercial Roofing
Polyiso board providing high R-value per inch under membrane roofing systems where roof height is constrained.
Foundation & Below-Grade Insulation
XPS board's low water absorption and compressive strength specified for foundation wall and under-slab insulation.
Exterior Wall Continuous Insulation
EPS and polyiso board reducing thermal bridging as continuous insulation over wall sheathing.
Fire-Rated Wall & Mechanical Insulation
Mineral wool board specified where non-combustible insulation is required by code or design.
Mechanical & Industrial Equipment Insulation
Rigid board insulation on tanks, ductwork and equipment housings requiring a stable, cuttable insulation form.
FAQ
Insulation Boards Questions, Answered
What's the difference between XPS and EPS foam board?
XPS (extruded polystyrene) has a closed-cell structure that gives it lower water absorption and higher compressive strength than EPS (expanded polystyrene), which is made of fused foam beads with more void space between them. XPS is generally preferred below grade and in wet-exposure applications, while EPS costs less and is widely used above grade where its higher water absorption is less of a concern.
Why does polyiso have a higher R-value per inch than XPS or EPS?
Polyiso's closed-cell foam is manufactured with a different blowing agent and cell gas than XPS or EPS, giving it a higher initial thermal resistance per inch of thickness. This is why polyiso is often the choice for low-slope roofing, where minimizing insulation thickness while meeting a target R-value matters for roof detailing and height.
When should I use mineral wool board instead of foam insulation?
Mineral wool board is non-combustible, unlike foam insulation boards, so it's specified where fire code or design requirements call for non-combustible insulation — common in certain wall assemblies, fire-rated partitions, and around penetrations. It also provides better sound absorption than rigid foam, a secondary reason it's sometimes chosen even where fire rating isn't the driver.
Does the R-value of foam board insulation change over time?
Some foam insulation types, particularly certain polyiso and XPS formulations, can experience a gradual decline in R-value over the years as the blowing agent gas within the cells slowly diffuses and is replaced by air — published long-term thermal resistance (LTTR) values account for this aging effect and are the more accurate figure for design calculations than initial R-value.
Can insulation board be used below grade on foundation walls?
Yes, but material choice matters — XPS is the common choice for below-grade use due to its low water absorption and adequate compressive strength under soil loads, while EPS and polyiso are less commonly specified below grade unless a specific product is rated and detailed for that exposure.
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