Which insulation type goes where?
Start with the assembly: location, required R-value, available depth, air-control layer, water exposure and fire requirements. Then choose the material and installation method. ENERGY STAR warns that not every insulation is designed for use in every part of the house. ENERGY STAR seal-and-insulate guidance.
| Location or need | Useful starting point | Main condition |
|---|---|---|
| Open, regular wall or floor cavity | Fiberglass or mineral-wool batt | Fit without gaps, voids or compression; establish air control separately |
| Open attic floor or irregular horizontal area | Fiberglass or mineral-wool loose fill | Air-seal first, preserve ventilation paths and install to documented coverage |
| Tight, irregular or air-leaky junction | A compatible spray-foam system | Qualified installation, substrate and cure conditions, plus required ignition or thermal protection |
| Continuous exterior wall, foundation or slab | Assembly-approved rigid insulation | Coordinate water, vapor, insects, fire protection and attachment |
| Interior sound or fire-control cavity | Mineral wool or fiberglass assembly | Rate the complete wall or floor system, not the batt alone |
No material fixes a broken assembly. Insulation controls heat flow; the enclosure still needs deliberate air, water and vapor management. Use the insulation R-value guide for climate and code targets, then use this guide to choose a practical form.
Fiberglass batts: regular cavities with visible quality control
Batts make sense when framing is open, cavity dimensions are regular and the installer can see every cut. They are easy to inspect before drywall: insulation should contact the cavity boundaries and fit around wiring, pipes and boxes without large gaps or crushing.
The North American Insulation Manufacturers Association says proper installation is critical for insulation to be effective. NAIMA installation guidance. That is the real batt tradeoff. The product is straightforward; the field result depends on fit and on a separate air-control strategy.
Do not use the facing as a generic answer to vapor control. Climate, assembly layers and local code determine whether a vapor retarder belongs there and which class is appropriate. Confirm the tested product, facing orientation and enclosure design before closing the cavity.
Pick batt when access is good and workmanship can be inspected. Move to another form when obstructions make a continuous fit unrealistic or when the insulation layer must also deliver a specific air-control function.
Blown-in: broad attic coverage and irregular spaces
Fiberglass or mineral-wool loose fill is a supported starting point for an open attic floor because it can cover a broad horizontal area and fit around framing and services. It is also useful when topping up existing attic insulation, provided the existing assembly is dry, safe and suitable.
NAIMA says fiberglass or mineral-wool loose fill can be used in attics, and a combination of batts and loose fill can be effective around roof trusses and hard-to-reach areas. NAIMA where-to-insulate guidance. The same guidance says the proper amount and minimum thickness must be installed to achieve the intended R-value. NAIMA where-to-insulate guidance.
Treat depth markers and bag count as quality-control records, not decoration. Follow the specific product coverage chart, keep material out of heat-producing equipment clearances, and maintain the ventilation and access details required by the attic design. Before adding insulation, correct roof leaks and complete the air-sealing work that will be difficult to reach afterward. Coordinate that work with the attic ventilation reference so insulation does not block the intended air path.
Spray foam: use it for an assembly problem, not a slogan
Spray foam belongs on the shortlist when geometry is difficult, an air-impermeable layer is part of the design, or limited space makes another assembly impractical. Open-cell and closed-cell products do not have identical vapor, density or water behavior, so “spray foam” is not a complete specification.
The basement example shows why assembly matters. Building Science Corporation says basement walls should use non-water-sensitive insulation that prevents interior air from reaching cold concrete and rim-joist surfaces; it identifies foam-based insulation as the best fit for that condition. Building Science Corporation basement guidance. The same article requires attention to inward drying and warns against generic interior vapor barriers.
Specify the product, thickness, substrate condition, temperature and humidity limits, fire or ignition protection, ventilation and re-entry instructions. Ask who owns substrate inspection and what happens if adhesion or cure is unacceptable. The Spray Polyurethane Foam Alliance describes its Professional Certification Program as an installer, contractor and project-manager credential focused on safety, quality and professionalism. SPFA. Certification is one screening input, not a substitute for the product instructions, code inspection and a written scope.
For a direct material decision, use the spray foam vs fiberglass comparison after the assembly requirements are fixed.
Rigid foam and mineral wool solve different problems
Rigid insulation is useful when the design needs a continuous layer across framing or a moisture-tolerant layer at concrete. It can reduce the thermal bridge through studs, but joints, edges, fasteners and transitions still determine whether the layer also serves as air, water or vapor control. Product type and facing matter.
Building Science Corporation says basement floor slabs are best insulated underneath with rigid insulation and notes successful use of expanded and extruded polystyrene. Building Science Corporation basement guidance. It also describes a continuous rigid layer behind an interior frame wall as part of one basement-wall approach. That is assembly guidance, not permission to place any foam board anywhere below grade.
Mineral wool belongs on the shortlist for cavities or continuous layers where fire resilience, acoustic absorption or water repellence matters. ROCKWOOL says its stone-wool insulation is resilient to fire. ROCKWOOL says its stone-wool insulation repels water and resists moisture. ROCKWOOL says stone wool can reduce noise and absorb sound. ROCKWOOL stone-wool overview. Those are manufacturer claims about its products; the required wall, floor or roof rating still belongs to the complete tested assembly.
Do not choose between rigid foam and mineral wool from R-value alone. Choose the control function first, then confirm attachment, exposure limits, compatible membranes, cladding details and code-required protection.
A location-first insulation checklist
Before naming a product, answer these questions:
- Where is the thermal boundary? Draw one continuous line around the conditioned space.
- What R-value is required? Use the adopted code and project documents, not a generic national target.
- Where is the air-control layer? ENERGY STAR lists sealing leaks and adding attic insulation as related but distinct work. ENERGY STAR seal-and-insulate guidance.
- How will the assembly handle bulk water and drying? Fix leaks before insulation and identify the intended drying direction.
- What space and access are available? Measure the cavity and plan around services, clearances and future access.
- What fire, ignition, acoustic or exposure rules apply? Check the complete listed assembly and local code.
- How will quality be verified? Define photos, depth markers, bag counts, coverage, thickness or inspection hold points before work starts.
Openings must stay connected to that boundary. When the scope includes a new entry unit, compare exterior door types and materials, then detail insulation and air sealing continuously around the rough opening. Door material does not excuse a leaky perimeter.
Finally, calculate the quantity from the approved assembly and product coverage. The insulation calculator can help with bags for attics and walls, but its result is only as sound as the area, target and product inputs.
Insulation types FAQ
Which insulation type is best for an attic floor?
Fiberglass or mineral-wool loose fill is a supported starting point for broad, irregular coverage. Air-seal first, preserve required ventilation and clearances, and follow the product coverage chart and code.
Is fiberglass batt bad insulation?
No. It is useful in open, regular cavities where it can be fitted and inspected. Gaps, compression and a missing air-control layer are installation or assembly problems, not proof that every batt is wrong.
Should I use spray foam in every rim joist?
Not by default. Define the air, vapor, moisture, fire-protection and drying requirements first, then select an approved assembly and qualified installer.
Where does rigid foam make sense?
It is a common starting point for continuous insulation and for some foundation or slab assemblies. The product, facing, joints, attachment and protection must match the complete design.
When should I choose mineral wool?
Put it on the shortlist when fire resilience, acoustic absorption, water repellence or a dense friction-fit product matters. Confirm the tested assembly rather than assigning a rating to the batt alone.