The foundation slab is one of the most critical components in a Passive House building. Unlike conventional construction, where slab insulation is often an afterthought, Passive House standards demand a superinsulated slab assembly that achieves R-values of 40 to 60 or higher. This level of insulation dramatically reduces heat loss through the floor, eliminates cold floors in winter, and contributes to the overall ultra-low energy performance that defines the Passive House standard. For a comprehensive overview of below-slab insulation strategies, see our guide to insulating beneath concrete slabs.
This article details the materials, design principles, and installation procedures for constructing a superinsulated slab that meets Passive House requirements. Whether you are building a certified Passive House or simply aiming for the highest possible energy performance in your next project, the techniques described here will help you achieve a durable, thermally efficient foundation.
Passive House Slab Design Principles
The superinsulated slab in a Passive House serves multiple functions beyond simple thermal resistance. It must provide a continuous air barrier, control moisture migration, resist structural loads, and maintain its thermal performance for the life of the building.
Target R-Values and Thermal Performance
Passive House standards require slab insulation levels far beyond typical building code minimums. The specific R-value target depends on climate zone and the building’s overall energy model:
| Climate Zone | Code Minimum R-Value | Passive House Target R-Value | Required Insulation Thickness |
|---|---|---|---|
| Zone 4 (Mixed) | R-10 | R-40 to R-50 | 8 to 10 inches EPS/XPS |
| Zone 5 (Cold) | R-15 | R-45 to R-55 | 9 to 11 inches EPS/XPS |
| Zone 6 (Very Cold) | R-20 | R-50 to R-60 | 10 to 12 inches EPS/XPS |
| Zone 7 (Extreme Cold) | R-25 | R-55 to R-65 | 11 to 13 inches EPS/XPS |
Continuous Insulation Layer
The defining feature of a superinsulated slab is a continuous layer of rigid foam insulation beneath the entire slab area. This layer must be uninterrupted by structural elements that would create thermal bridges. In Passive House construction, the slab insulation extends under the entire foundation footprint, including under interior load-bearing walls. The foundation walls themselves are either insulated on the exterior or built as insulated concrete forms (ICFs) to maintain the continuous thermal envelope. For more on ICF systems, see our guide to insulated concrete forms.
Material Selection for Superinsulated Slabs
Choosing the right insulation material is essential for achieving the required R-value while maintaining structural stability and moisture resistance.
Expanded Polystyrene (EPS)
EPS is the most commonly used insulation in Passive House slab construction due to its favorable balance of cost, compressive strength, and long-term thermal performance. Type II EPS with a compressive strength of 15 psi or higher is typically specified for under-slab applications. EPS does not lose R-value over time when protected from moisture, and its cellular structure resists wicking. The standard R-value is R-4.2 to R-4.5 per inch.
Extruded Polystyrene (XPS)
XPS offers higher compressive strength (25 to 40 psi) and a slightly higher initial R-value of R-5 per inch. However, XPS loses R-value over time as its blowing agents diffuse out of the foam, stabilizing at approximately R-4.5 per inch after several years. XPS is more expensive than EPS but provides better moisture resistance, making it a good choice for sites with high water tables.
Polyisocyanurate (Polyiso)
Polyiso offers the highest R-value per inch at R-6 to R-7, but its performance degrades significantly in cold temperatures, making it unsuitable for below-slab applications in cold climates. It also has lower compressive strength than EPS or XPS and can delaminate if exposed to prolonged moisture. Polyiso is generally not recommended for Passive House slab insulation.
Construction Sequence for a Superinsulated Slab
Installing a superinsulated slab requires careful sequencing and attention to detail at every step. For guidance on rigid foam types and properties, see our rigid foam insulation technical guide.
Site Preparation and Sub-Base
- Excavate to the required depth, accounting for the thickness of the insulation layer plus the slab thickness. For a 10-inch insulation layer and 5-inch slab, the excavation must be at least 15 inches deeper than the finished floor elevation.
- Compact the subgrade to at least 95 percent standard proctor density to prevent differential settlement that could crack the slab or damage the insulation.
- Install a 4-inch layer of clean, well-draining gravel or crushed stone as a capillary break. This layer prevents soil moisture from wicking upward into the insulation and slab.
- Install a 4-inch perforated drain pipe around the perimeter of the excavation, sloping at 1/8 inch per foot toward a sump or daylight outlet.
Vapor Barrier and Insulation Installation
- Place a 15-mil polyethylene vapor barrier directly on the compacted gravel base. Use reinforced polyethylene for puncture resistance. Lap all seams by 12 inches and seal with double-sided butyl tape.
- Install the first layer of rigid foam insulation boards in a staggered pattern, similar to brickwork, to prevent continuous thermal seams. Butt joints tightly and seal with acoustical sealant or foil tape.
- For multiple layers, stagger the seams between layers by at least 12 inches in both directions. Seal between layers is generally not required if the boards fit tightly, but a thin bead of spray foam can be used at seams for added air tightness.
- Install foam edge strips around the perimeter of the slab. These strips extend vertically to isolate the slab from the foundation walls, preventing thermal bridging at the edge.
Reinforcement and Slab Pouring
- Place welded wire mesh or reinforcing bars on plastic chairs that hold the reinforcement in the middle third of the slab thickness. The chairs must not penetrate the vapor barrier below the insulation.
- Install a 6-mil polyethylene slip sheet over the top of the insulation if required by the insulation manufacturer, particularly for EPS. This prevents concrete slurry from wicking into the foam.
- Pour the concrete slab in a continuous operation to avoid cold joints. Use a concrete mix with a low water-to-cement ratio (0.45 or lower) to minimize cracking and reduce curing shrinkage.
- For radiant floor heating, install the tubing on top of wire mesh or专用的 clip rails before pouring. The tubing must be pressure-tested before the concrete pour.
Thermal Bridge-Free Detailing
Eliminating thermal bridges is essential in Passive House construction. Even small thermal bridges at the slab edge can reduce the effective R-value of the entire assembly by 20 percent or more.
Slab Edge Insulation
The slab edge is the most vulnerable point for thermal bridging in a superinsulated slab. Several strategies address this:
- Extended insulation wrap: Extend the rigid foam insulation up the exterior of the foundation wall to meet the wall insulation, creating a continuous thermal envelope around the slab edge.
- Insulated concrete forms: Use ICF blocks for the foundation walls, which provide continuous insulation on both the interior and exterior of the concrete core.
- Perimeter insulation strips: Install vertical strips of rigid foam around the slab perimeter before pouring, isolating the slab from the foundation wall.
- Double-layer edge insulation: For extreme climates, install two layers of edge insulation with staggered seams to eliminate any direct thermal path.
Penetration Sealing
Every penetration through the slab from plumbing drains, water supply lines, and electrical conduits creates a potential thermal bridge and air leak:
- Group all penetrations together in a single service core where possible to minimize the number of thermal breaks.
- Use insulated sleeves or foam wrap around pipes that pass through the slab to maintain thermal separation.
- Seal each penetration with both a physical gasket and a bead of acoustical sealant or expanding foam.
- For large penetrations like floor drains, insulate the sides of the penetration with rigid foam before pouring the slab.
Quality Control and Performance Verification
Achieving Passive House performance requires verification that the slab assembly performs as designed.
Pre-Pour Inspection Checklist
- Verify that all insulation layers are installed with staggered seams and tight joints (no gaps wider than 1/8 inch).
- Confirm that the vapor barrier is continuous and sealed at all seams and penetrations.
- Check that edge insulation strips are properly positioned and extend the full required height.
- Ensure that radiant tubing (if used) is pressure-tested and that all connections are accessible.
- Document the installation with photographs for future reference and Passive House certification.
Post-Construction Blower Door Testing
After the slab is poured and cured, a blower door test measures the air tightness of the completed building envelope. The slab contributes significantly to the overall air barrier, and any leaks at the slab perimeter or through penetrations will appear in the test results. For more on how insulation and air sealing work together, see our building envelope predictability article.
Constructing a superinsulated slab for a Passive House requires careful planning, quality materials, and meticulous installation. The effort is rewarded with a foundation that virtually eliminates heat loss through the floor, provides exceptional comfort, and contributes to a building that uses 80 to 90 percent less heating energy than conventional construction. For more on Passive House certification and other green building standards, see our guide to green building certification programs.