Insulation and R-value

What the number measures

R-value is thermal resistance: how well a material resists the conduction of heat through it. It is measured under laboratory conditions on a sample of the material, at a stated thickness, with no air movement and no interruptions.

Every one of those conditions is absent in a wall. The material is interrupted by framing. It is installed by a person under time pressure. There is air on both sides of it and often within it. So the rated R-value of the insulation is an upper bound on a property of one component, and the building gets something lower.

Assembly R-value is the number that matters

A framed wall is a composite. Between the studs there is insulation performing at close to its rated value. At each stud there is wood, conducting heat past the insulation. The wall as a whole performs somewhere between the two, weighted by how much of its area is wood.

That fraction is larger than most people assume, once corners, wall intersections, headers, sills and plates are counted. It is why the framing decisions on the previous series belong in an insulation discussion: reducing unnecessary framing raises assembly performance without touching a single insulation specification.

The other way to address it is to interrupt the bridge entirely with continuous insulation on the outside of the framing, so the studs are no longer connected directly to the exterior. That is why continuous exterior insulation appears in modern energy codes: it addresses the part of the wall the cavity insulation cannot reach.

Installation quality is a specification, not a hope

Batt insulation compressed behind a pipe, cut short at a corner, or left with a gap at the top of a cavity performs worse than the same product installed properly, and the gap does not need to be large to matter. A void is not a slightly worse insulator, it is a bypass.

Blown and sprayed products avoid many of those failures by filling the cavity, at the cost of depending on installer technique and equipment settings. Rigid boards depend on the joints between them being taped or sealed, because an unsealed joint in a continuous layer is a hole in the continuous layer.

The consequence for an owner is simple and it is about timing. Insulation is one of the few things in a house that can be inspected properly for about two days, between installation and drywall. Look at it then, in every room, with a light. It is the highest-value walk-through in the whole build.

The materials, briefly, on what actually separates them

Fibrous products, glass and mineral wool, are inexpensive, non-combustible in the case of mineral wool, vapour open, and entirely dependent on being installed without gaps. They do nothing for air sealing.

Cellulose, usually blown, fills cavities well and around obstructions, and dense-pack installation reduces air movement within the cavity without being an air barrier.

Closed-cell spray foam is the outlier. It has a higher R-value per inch than the others, and it is an air barrier and, at sufficient thickness, a vapour retarder as well, which is why it is used where space is tight or where several jobs must be done by one product. It is the most expensive per unit of R-value, it is difficult to inspect once installed, and its performance depends heavily on mixing and application conditions.

Rigid boards are the usual answer for continuous exterior insulation. Their differences matter more than their R-values: some absorb water, some are vapour open, some are not, and the choice interacts with the vapour strategy of the whole wall rather than standing alone.

How to find the value that governs your building

Required insulation levels come from the energy code your jurisdiction has adopted, and adoption is local and amendable. [1] There is no national requirement and this page does not publish one.

Ask the building department three things: which energy code and edition, which climate zone the jurisdiction is in, and whether there are local amendments to the envelope requirements. That produces the actual required values in a form you can hand to a designer.

Then ask a separate question of the designer: what is the assembly performance, not the cavity insulation rating. If those two numbers have never been distinguished on your project, the specification has not been done. the contents sheet sets out how the four control layers relate to each other.

A second question exposes whether anybody has looked at the building as a whole: which part of this envelope is the weakest, and was that deliberate. Every assembly has a worst element, usually a junction, a header or a band of framing at a floor line. A designer who can name it has thought about the building. A designer who says the whole thing meets code has answered a different question, since meeting the code minimum at every point still leaves one point that governs how the building behaves.