What Does Insulation Solve and How Do You Decide?

What does insulation solve and what does it not?

Insulation does two things at once. In winter it slows heat escaping, and in summer it delays outside heat coming in. The benefit is not limited to one season.

The second gain is in the surface temperature of the wall. The inside face of an uninsulated wall stays cold in winter, the moist air in the room condenses when it meets that cold surface, and mould follows in time. Insulation warms the surface and reduces that condensation.

What insulation does not solve is ventilation. If the moisture is being generated inside, through drying laundry indoors or insufficient ventilation, insulation alone will not be enough. The two have to be addressed together.

It also does not solve losses through the windows. In a building with an insulated facade and old windows, a meaningful part of the loss continues through the joinery, which is why the two jobs are usually considered in the same plan.

Should insulation go on the inside or the outside?

The right place is the outside face. When insulation is applied externally the wall itself stays on the warm side, and the condensation point moves out of the wall and into the insulation layer. That is what is wanted both for heat performance and for the health of the structure.

Internal insulation leaves the wall on the cold side. Condensation can then form behind the insulation, and because it is out of sight it takes a long time to notice. A badly configured internal system can enlarge the very damp problem it was meant to solve.

Even so, internal work is sometimes the only option. If a building wide decision cannot be reached, or the structure is not suited to external work, the inside comes onto the agenda. In that case the vapour balance has to be calculated separately and the decision made against the specific situation.

How is board thickness chosen?

In external wall insulation the board thickness is usually chosen between 4 and 8 centimetres. There is no single correct figure, the decision combines several factors.

The first is the local climate. Where winters are harder the thicker end is preferred. The second is the wall itself, since a thick solid wall already offers some resistance while a thin wall increases the requirement.

The third is which way the elevation faces. Elevations facing north and getting no sun lose more heat, so different thicknesses on different elevations of the same building are possible.

The gain from added thickness is not linear. Beyond a certain point each additional centimetre contributes less. Rather than choosing a thicker board than needed, pairing the right thickness with correct application gives the better result.

What is a thermal bridge?

A thermal bridge is a point where the insulation is interrupted and heat takes a short route out. Even where the whole elevation is insulated, losses continue if these points are left open.

The most common thermal bridges are where a balcony slab passes through the facade, along column and beam lines, at window sills and around joinery. They look unimportant because they are narrow, yet a noticeable share of the loss happens there.

The symptom is recognisable. If mould still appears in one particular corner of an insulated building, that corner is very likely a thermal bridge that was missed. Correct application treats these points separately so the continuity of the insulation is not broken.

Can one flat be insulated on its own?

It is technically possible, but the gain stays limited compared with doing the whole building. The reason is the walls shared with neighbouring flats. As your flat warms, part of that heat moves towards uninsulated neighbouring walls.

The second limit is appearance. Work applied to the elevation of a single flat creates a visible difference on the face of the building, and that can become a matter for the owners as a body. Speak to the building management before starting.

The third is thermal bridging. In a single flat application the insulation stops at the edges, and those lines become bridges that reduce the benefit. Where possible, a building wide decision is both more effective and more economical.

What other work should go with it?

External insulation needs scaffolding, and the period while the scaffold is up is the best time for any other work on the facade.

The items worth including are facade painting and repair, renewing rainwater downpipes, correcting window sills and any eaves work. Carried out separately, the cost of putting up scaffolding is paid again each time.

Replacing windows belongs in the same plan. If the windows are being renewed, doing that before or alongside the insulation allows the detail around the frame to be resolved properly. Joinery changed afterwards means opening up a newly finished facade.

What season should it be done in?

External insulation depends on dry weather. Every layer needs time to cure, and rain interrupts that and lowers the quality of the surface.

Spring and autumn are therefore the best periods. Extreme summer heat is also avoided, since a surface drying too quickly can weaken adhesion between the layers.

On timing, insulating two elevations of a five storey building runs 15 to 25 days, and a single elevation is shorter. What governs the schedule is the scaffolding and the weather rather than the application itself.