Insulating a brick wall from the inside
Applies to: Assumes solid masonry walls without a cavity, internally plastered, in a climate with a heating season.
Last worked on 2015-02-02
Internal insulation is often the only option available, and it is the option that changes the physics of the wall most. It is worth understanding what moves before deciding how much to add.
What changes when you insulate inside
Before insulation, the whole thickness of the wall runs somewhere between room temperature and outside temperature, and most of it is above the point at which the moisture in the air condenses. Heat leaking outward through the masonry keeps it dry.
Add insulation on the room side and you stop supplying that heat. The masonry now sits close to outside temperature all winter. It is colder, it stays wetter for longer after rain, and it dries more slowly because it has less energy to dry with. None of that is a reason not to insulate, but it is the reason the detailing matters more than the thickness.
The two failure modes
The first is interstitial condensation: warm moist room air reaches the cold face behind the insulation and condenses there, out of sight. This is a vapour and air movement problem, not an insulation problem, and it is solved by making the room side continuous rather than by making the insulation thinner.
The second is the thermal bridge at every junction where the insulation has to stop: at the floor, at the ceiling, at the reveal of every window, and at every internal wall built into the external one. These stay cold while the wall around them warms, so they become the coldest surface in the room and collect the moisture that the wall used to.
Detailing the junctions
Reveals are the most common omission and the most visible failure, because a cold reveal next to a warm wall shows as a dark line within a season. Returning even a thin layer of insulation into the reveal and up to the frame changes the surface temperature there far more than the same material would on the open wall.
Internal walls and floor junctions are harder because you cannot follow the insulation through without opening the structure. The usual compromise is to return the insulation along the intersecting wall for some distance, which lengthens the heat path rather than eliminating the bridge.
Choosing a material class
Rigid boards give the most reduction for the least thickness and are the least forgiving of gaps. Semi-rigid batts fitted between battens are more tolerant of an uneven wall but need the batten zone thought through, because the battens themselves bridge the layer. Vapour-open mineral or wood-fibre systems accept some moisture movement by design and rely on the assembly being able to dry inward.
The choice is less about performance figures than about which failure you would rather manage: a sealed system that must not be punctured, or an open system that must be able to dry.
Terms used above
- Thermal bridge
- A part of the construction that conducts heat past the insulation layer, leaving a colder patch on the room surface.
- Interstitial condensation
- Condensation forming inside the thickness of a construction rather than on its visible surface.
- Vapour-open
- An assembly deliberately built to let water vapour pass through it and dry out, rather than to block it.
- Reveal
- The face of the wall thickness at the side of a window or door opening, between the frame and the room.
If only one thing is done well, make it continuity: an insulation layer with a gap in it performs closer to no insulation than to its rated value, and the gap becomes the place the moisture goes.