Three control jobs, one continuous enclosure.
Attics and roof assemblies
Does adding attic insulation also stop air leaks and moisture?
Insulation resists heat flow. An air barrier limits air movement through the enclosure. A vapor-control layer slows diffusion through materials. One product may perform more than one job, but the jobs remain different. Identify where each belongs in your attic or roof, connect the layers at edges and penetrations, and select the vapor strategy for the climate and assembly.
Ask what each layer controls.Section link
| Function | What it controls | What to specify |
|---|---|---|
| Thermal insulation | Heat flow through the assembly. | Material, installed R-value, thickness, and continuity. |
| Air barrier | Air leakage through openings and joints. | The continuous layer, its transitions, and the sealing details. |
| Vapor control | Water-vapor diffusion through materials. | Required permeance and location within this particular assembly. |
Read this table one item at a time
Thermal insulation
- What it controls
- Heat flow through the assembly.
- What to specify
- Material, installed R-value, thickness, and continuity.
Air barrier
- What it controls
- Air leakage through openings and joints.
- What to specify
- The continuous layer, its transitions, and the sealing details.
Vapor control
- What it controls
- Water-vapor diffusion through materials.
- What to specify
- Required permeance and location within this particular assembly.
R-value measures thermal resistance; it does not tell you whether the attic hatch seals. A sheet that resists diffusion can still leak air around its unsealed edge. Conversely, an airtight layer need not be highly resistant to vapor diffusion. These distinctions explain why “add a moisture barrier” is too vague to be a useful scope item.
For example, rigid insulation may contribute to thermal control while sealed joints make it part of an air barrier. The product’s vapor behavior is a separate property. Document all three functions instead of assuming that the product name settles them.
Decide where the conditioned building ends.Section link
In a conventional vented attic, insulation and the principal ceiling air-control layer separate the living space from the attic. Outside air uses designated ventilation paths above that boundary. In an unvented design, the relevant control layers move toward the roof plane, with the attic brought within that enclosure strategy.
PNNL describes condensation control as central to an unvented roof: the design must manage the deck temperature or prevent moisture-laden indoor air from reaching a vulnerable cold surface. Closing existing vents and adding some insulation does not, by itself, establish that design.
Draw the chosen boundary through the actual building section. Include the hatch, kneewalls, dormers, dropped ceilings, and connections to exterior walls. If the line cannot be followed across an intersection, the proposal has a detail to resolve. This is a useful owner question even when an installer or designer develops the construction detail.
Worked example: doubling a label does not halve the energy bill.Section link
For a simplified, uniform layer under steady conditions, conductive heat flow is proportional to area × temperature difference ÷ R-value. Imagine 1,000 square feet with a 40°F temperature difference and an effective R-value of 20. The simplified heat flow is 1,000 × 40 ÷ 20 = 2,000 Btu per hour. At R-40 it is 1,000 Btu per hour.
That halving applies to the example’s conduction through that idealized layer. It does not say the whole house uses half as much energy. Windows, walls, equipment efficiency, air leakage, weather, and occupancy still contribute. The calculation also assumes the effective assembly resistance really doubles; a product label cannot establish that by itself.
This is why the inspection question should include coverage and continuity. PNNL’s insulation-quality guidance identifies gaps, voids, compression, and loss of contact with the intended surface as defects. Ask how the contractor will fit insulation around obstacles and preserve access without flattening the material.
Preserve drying and make the scope specific.Section link
Vapor-control choices depend on climate, indoor conditions, and the layers already present. Building Science Corporation distinguishes slowing diffusion from controlling moist-air movement and explains how an added low-permeance layer can restrict drying. Avoid applying a universal “plastic goes on this side” rule to every roof.
Before work, ask the contractor to list existing facings and membranes, the proposed air-barrier line, insulation placement, and the intended vapor-control strategy. Include how roof leaks or wet materials will be addressed before concealment. If a conversion to an unvented attic affects ducts or equipment, identify who coordinates those changes.
Air sealing should also preserve the building’s intended ventilation and required clearances around heat-producing equipment. Ask for the appropriate details rather than filling every opening indiscriminately. Keep photographs and product records so a later repair does not accidentally interrupt a control layer nobody can see.
Sources and further readingSection link
Understanding Roofing / Updated
Scope and limitations
- A suitable R-value and vapor-control arrangement depend on climate, the existing assembly, and the applicable project requirements.
Continuous Air Barrier in Exterior Walls
Pacific Northwest National Laboratory
The air barrier must connect around the enclosure, with sealed joints and alignment with insulation, including at ceilings.
Insulation Installation Achieves RESNET Grade 1
Pacific Northwest National Laboratory
Gaps, compression, voids, and misalignment reduce insulation installation quality.
Unvented Attic Insulation
Pacific Northwest National Laboratory
Unvented designs move control layers to the roof plane and need condensation control.
BSD-106: Understanding Vapor Barriers
Building Science Corporation
Air transport and vapor diffusion are distinct; vapor-control location depends on climate, materials, interior conditions, and drying.