PIR insulation
Rigid polyisocyanurate foam with conductivity around 0.022–0.025 W/(m·K), foil-faced and vapor-closed, used where thickness is strictly limited.
What is PIR insulation and how is it made?
PIR, short for polyisocyanurate, is a rigid foam insulation made by reacting polyols and isocyanates in the presence of a blowing agent. The chemical reaction generates heat which is then managed to cure the foam fully into a durable, cross-linked plastic. The result is a closed-cell board with thermal conductivity around 0.022 to 0.025 W/(m·K), among the lowest of common rigid insulation boards. Most PIR boards sold in Central Europe are foil-faced on both sides: the aluminum layer creates a vapor barrier and provides mechanical protection, but it also commits the builder to treating the insulation as part of a vapor-closed system.
How does PIR compare to other rigid insulation materials?
The table below sets out the key differences between PIR, PUR (polyurethane), EPS, and mineral wool across the properties that matter most in residential renovation and passive-house design.
| Property | PIR | PUR | EPS | Mineral wool |
|---|---|---|---|---|
| Thermal conductivity W/(m·K) | 0.022–0.025 | 0.023–0.028 | 0.032–0.040 | 0.032–0.045 |
| Thickness for U=0.15 (m) | Thin | Thin to medium | Medium | Medium to thick |
| Vapor resistance (sd) | High, 2–5 m | High, 2–5 m | Partly open, 0.3–1.5 m | Open, <0.5 m |
| Fire class EN 13501-1 | Depends on product and facing | Depends on product and facing | E | A1 |
| Cost tier (relative) | High | High | Low | Medium |
PIR and PUR are the most thermally efficient, making them ideal where thickness is strictly limited. EPS and XPS are cheaper and dominate general facades and roofs, while mineral wool is non-combustible and required by fire code on some builds. The choice in Slovakia usually balances thermal performance, fire restrictions, moisture behavior, and cost.
What are the advantages of PIR's high thermal performance per thickness?
PIR delivers high thermal resistance in minimal depth. On re-roofing projects where roof height is fixed, its thinner layer for the same U-value saves valuable headroom. The same logic applies in window and door reveals, where frame depth leaves little room: PIR lets builders fit a high-performance layer without enlarging openings. PIR boards also come in high-strength grades for load-bearing roofs or direct-fixed facades.
What does vapor-closed mean for timber roofs with PIR insulation?
Most PIR boards are factory-faced with aluminum foil to block water vapor diffusion. On timber roofs, this means the insulation layer is vapor-closed: interior moisture cannot escape through it the way it can with mineral wool or fibre boards. Instead, an interior vapor barrier below the ceiling becomes essential to prevent condensation and rot. If absent or damaged, interior humidity will accumulate in the timber. This is a design detail that must be explicit in the moisture calculation; it is not a fault of the material but a commitment to building vapor-closed from inside out.
Where is PIR insulation typically used in residential construction?
PIR's high cost limits its use to three common situations. First, over-rafter insulation when the rafter depth is fixed and headroom matters. Second, flat roofs: PIR bonds easily to concrete or OSB and can be laid directly below the waterproof membrane, creating a warm roof assembly. Third, window and door reveals where frame depth leaves little room for insulation. Attic insulation between rafters typically uses cheaper mineral wool batts or loose fill instead.
| Application | Why PIR is used | Common build-up |
|---|---|---|
| Over-rafter insulation (roof re-cover) | Minimal thickness gain, preserves headroom | PIR boards, foil-faced, over existing tiles or boards |
| Flat roof (warm-roof type) | Thin profile, direct bond to deck, vapor control | Structural deck, vapor barrier, PIR boards, waterproof membrane |
| Window and door reveals | Fills narrow gaps, high R-value per mm | PIR boards, cut to fit, sealed with foam or silicone |
| Plinth zone insulation | Resists water and freeze-thaw, closed-cell structure | PIR boards, foil-faced, below ground level or in masonry bed |
| Attic between rafters | Usually not used: mineral wool is cheaper and equally effective | Mineral wool batts (standard thickness) |
How does PIR handle moisture compared to other insulation materials?
PIR and EPS both resist water absorption because they are closed-cell foams: neither the foam nor the foil facing allows significant diffusion. Mineral wool, by contrast, is hygroscopic and can absorb and slowly release moisture, making it breathable when not faced. In moist climates like Slovakia, this difference matters greatly. A mineral wool build-up can tolerate occasional condensation because the moisture slowly diffuses back out through the material. A foil-faced PIR or EPS assembly cannot: it depends entirely on vapor barriers and proper ventilation to prevent moisture accumulation. The choice comes down to how much diffusion you want. If building vapor-open for drying capacity, mineral wool excels. If committing to vapor-closed design with careful detailing, PIR offers equal protection with less thickness.
How are joints and seams detailed in PIR roof assemblies?
In over-rafter and flat-roof applications, PIR board joints require careful sealing to maintain the vapor-closed envelope. The aluminum foil facing on adjacent boards is typically taped with compatible sealing tape to create a continuous foil layer, blocking water vapor diffusion across the seams. On sloped roofs, staggered board layouts break the continuous joint line and reduce the risk of water finding a seam path. In flat-roof assemblies where boards are laid tight and flat, seams between boards are sealed with tape, and the perimeter edges are sealed with foam closure pieces or mineral wool infill where the boards meet the building edge. These details are critical: even small gaps in a roof assembly can allow interior moisture to reach the rafters and insulation void, negating the vapor-closed design. The builder must specify and inspect these seals during installation.
What fire safety properties does PIR have?
PIR boards are classified for reaction to fire under EN 13501-1; the specific class depends on the product and its facing and must be checked on the declaration of performance. A complete external insulation system (ETICS) with PIR and mineral render is usually classified as B-s1,d0 to C-s2,d1. Fire-safety design under the STN 92 0201 series decides where combustible insulation may be used. For low-rise residential, PIR is common; where non-combustible insulation is required, mineral wool is the standard answer.
PIR is a premium material best suited where thermal resistance per millimeter is the limiting factor. Its cost and vapor-closed nature make it unsuitable for general use, but in over-rafter insulation, flat roofs, and tight reveals it offers the most practical solution in Slovakia.
Frequently asked questions
- What is the difference between PIR and PUR insulation?
- PIR (polyisocyanurate) and PUR (polyurethane) are both rigid foams, but PIR starts as a different chemical precursor and is heated during or after foaming to cure fully. This makes PIR slightly more stable at higher temperatures and gives it a small conductivity advantage, roughly 0.022–0.025 W/(m·K) versus 0.023–0.028 for PUR. In practice, for residential building in Slovakia, both are comparable; the choice often comes down to availability and cost.
- Why is PIR insulation thinner than mineral wool for the same U-value?
- PIR has one of the lowest thermal conductivities among common rigid boards, around 0.022–0.025 W/(m·K). Mineral wool sits around 0.032–0.045 W/(m·K). For the same thermal resistance, PIR needs significantly less thickness. This matters greatly on over-rafter insulation during a roof re-cover, where adding thickness is expensive or impossible, and in window reveals where space is tight.
- What does foil-facing do to a timber roof build-up?
- Foil-faced PIR boards are deliberately vapor-closed because the aluminum foil blocks water vapor diffusion. On a timber rafter, this means the build-up cannot expel interior moisture slowly into the insulation layer. The timber roof becomes dependent on an interior vapor barrier to stop condensation; without one, moisture can accumulate in the timber and cause rot. Designers must verify this explicitly in the moisture calculation.
- Can PIR insulation get wet, and does it dry out?
- PIR boards do not absorb water into the foam itself because they are closed-cell. If water sits on the surface or between boards, it sits there; it does not soak in and does not escape through the foam. This makes PIR excellent for plinth zones and exposed horizontal surfaces. However, if the board is damaged or joints leak, water trapped inside the cavity can persist, so workmanship and drainage design are critical.
- Is PIR insulation a fire risk on a residential facade?
- PIR boards are classified for reaction to fire under EN 13501-1; the specific class depends on the product and its facing and must be checked on the declaration of performance. A facade system built with PIR is normally classified as B-s1,d0 to C-s2,d1 depending on the specific build-up. Fire-safety design under the STN 92 0201 series decides where combustible insulation may be used; for sites where fire code rules out combustible material, mineral wool or non-combustible boards are the required answer. For low-rise residential in Slovakia, PIR is commonly used.
- Where does PIR insulation perform best in residential renovation?
- PIR excels in three situations: over-rafter insulation when re-roofing (where roof height is fixed and thin boards save headroom), flat roofs (where the compact thickness reduces load on the structure), and window and door reveals where existing wall thickness does not leave room for thicker insulation. In attic insulation between rafters or between joists, thicker and cheaper mineral wool or fibre batts are typically a better choice.