The house that leaks

On this page

1. Two ways out
2. The U-value, from the bottom up
3. The second layer does much less
4. What Swedish houses actually have
5. The hole you are not allowed to plug
6. From a leak to a yearly bill
7. The same kilowatt-hours, different money
8. Check yourself

Two ways out

The heat leaving your house takes two entirely different routes. They look identical on the bill, but they are fixed in entirely different ways, which is why they have to be kept apart.

Through the material. Heat walks straight through a wall, a roof, a floor, a pane of glass. No air moves. It happens silently and without pause, day and night, all winter.

Riding on the air. Warm indoor air leaves and cold outdoor air takes its place — through the ventilation, and through every crack. Here the air itself carries the heat out.

Let us put numbers on it with a worked house: 150 m² on one floor, built around 1970. That is the most common house type in Sweden. It measures 15 × 10 metres, the walls are 2.5 metres tall, with 20 m² of window and 4 m² of door. The dimensions are chosen so the sums can be followed, not taken from a register.

We need a measure of leak rate, and it is watts per degree, W/K: how many watts run out for every degree of difference between inside and outside. A 100 m² wall with a U-value of 0.40 leaks 40 watts per degree. That is the whole operation: area times U-value.

Two rows deserve a comment. The windows are 20 m² and leak nearly as much as 101 m² of wall — the most striking line in the table, and the reason old windows get so much attention. The floor is the shakiest row. The ground under a slab is not as cold as January air, so in reality a floor leaks less than this multiplication says. A careful calculation gives the ground its own, milder temperature.

Where the 62.4 W/K for the air comes from

The rules require at least 0.35 litres of outdoor air per second per square metre of floor. For 150 m² that is 52.5 litres a second, which is 189 cubic metres an hour.

0,35 × 150 = 52,5 l/s → 189 m³/h

Warming a cubic metre of air by one degree costs about 0.33 watt-hours. The standard method writes the leak as 0.33 times the flow, times whatever the recovery does not take back.

0,33 × 189 × (1 − 0) = 62,4 W/K

The U-value, from the bottom up

The U-value appeared in that table without explanation. Time to build it up from the bottom, because then the rest of the module becomes obvious. There are three numbers, and they are one fact seen from three sides.

R = thickness / λ    U = 1 / R_total

High R and low U are the same claim, said twice. U is R upside down.

An example with numbers. Twenty centimetres of mineral wool is 0.20 metres divided by 0.04, so a resistance of 5.0. A wall made of nothing but that wool would have a U-value of 1 divided by 5.0 — that is 0.20.

0.20 / 0.04 = 5.0 m²K/W  →  1 / 5.0 = 0.20 W/m²K

Now the anchor, which makes the numbers physical. An old single-glazed window has a U-value around 4.5. A modern high-performance triple sits around 0.8. Turn both upside down and compare the resistances:

Why no U-value can rise without limit

The air closest to a surface stands almost still and insulates a little by itself. The standard method counts 0.13 m²K/W on the inside of a wall, 0.10 on the inside of a roof, and 0.04 outside.

Those two films are always present. Even an infinitely thin sheet therefore has at least 0.17 of resistance, and cannot reach a U-value above roughly 5.9.

1 / (0.13 + 0.04) = 5.9 W/m²K

That explains why single glazing's 4.5 sits close to the ceiling on how bad a thing can be. Much worse than that is barely possible to build.

The second layer does much less

Here comes the most important line in the module, and the reason many people over-buy insulation. Resistances add up in a straight line, but resistance is not what you pay for — the U-value is. And U is one divided by the total.

Take the worked house's wall. Its U-value is 0.40, so its resistance is 1 divided by 0.40, which is 2.5. We add twenty centimetres at a time, and each helping is worth 5.0 of resistance.

0.053 / 0.267 = 0.20 → one fifth of the benefit, for the same money

Resistance grows in a straight line. The U-value does not.

The lower box is the wall you already have, the upper one is the wool you buy. Every helping bought is as tall as the last — but the bottom row shows what each one took off the U-value, and that figure nearly collapses.

What Swedish houses actually have

The construction year says more about the envelope than anything else you can know without opening a wall. The watershed is Svensk Byggnorm 1975, which followed the oil crisis and raised insulation requirements sharply. A house from 1974 and one from 1977 differ more than 1977 and 1990 do.

The last column is what BBR 29 demanded of a new build, plus a combined requirement of 0.30 for the house as a whole. Note that a house built after 2010 already sits at or below the ceiling — the requirements followed practice rather than the other way round. Note too that Boverket's rulebook is being replaced, so a date always belongs with a requirement figure.

Open the guide: U-values by construction period

The hole you are not allowed to plug

A fifth of the worked house's leak went out with the air. Anyone who has just understood this chapter gets a very natural idea: block it up. That idea is the most dangerous one in the whole course, and it has made more houses sick than it has saved money.

Damp comes first. A household pours water into its own air all day long — cooking, showering, laundry, wet outdoor clothes, houseplants, and the people themselves breathing. It is litres a day, not decilitres. We have found no Swedish source that puts a figure on it, so treat that as an order of magnitude and nothing more.

That air has to leave, or the water condenses wherever the surface is coldest: in a window reveal, in an outside corner, behind a wardrobe against an outer wall. That is where mould starts. Mist on the inside of the pane on a cold morning is exactly that water becoming visible.

Then carbon dioxide. A bedroom with the door shut and a taped-over vent turns stale before morning, and the air in it shapes how you sleep. Radon last. Radon rises out of the ground and out of blue concrete, and the only everyday defence is air being replaced. A house sealed up without its ventilation being reviewed concentrates whatever was already there.

So the rules set a floor, and it is not negotiable: at least 0.35 litres of outdoor air per second per square metre of floor area, and at least 4.0 litres a second per person in a room. That is where the worked house's 189 cubic metres an hour came from.

The Swedish system types

The exchanger does one thing only, and it is easy to picture. Exhaust air at 21 degrees passes close by supply air at −1 degree, and hands over most of its heat without the streams mixing. If the exchanger takes 80 per cent of the temperature difference, the air enters the room at 16.6 degrees instead of −1.

−1 + 0.80 × (21 − (−1)) = 16.6 °C

The same airflow, with and without an exchanger

Computed as 0.33 × 189 × (1 − 0.80) = 12.5 W/K against 62.4 without an exchanger. The annual figure is the difference times 3 600 degree days times 24 hours.

Does a house need a ventilation inspection?

A building's owner must have the ventilation checked before first use and regularly after that. But one- and two-dwelling houses are exempt from the recurring check.

On top of that the requirement does not apply at all to a house with natural draught or plain exhaust without recovery. The net: an ordinary house with natural draught or exhaust needs no check whatsoever. With a recovery unit it needs a first inspection, but no recurring one.

If the owner suspects a fault it must be put right as quickly as possible. Supervision sits with the municipal building committee.

From a leak to a yearly bill

We have a leak rate: 331 watts per degree. It says nothing about money until we know how many degrees, and for how long. That is exactly what degree days are for.

The idea is simple. Take each day of the year. If the day's mean temperature sat below 17 degrees, count how many degrees below — at 5 degrees that day scores 12 degree days. If it sat above 17 the day scores zero. Add up the whole year, and you have one number describing the winter of a place.

Why 17 rather than the 21 degrees you actually keep indoors? Because people, lamps, the fridge, the cooker and the sun heat the house for free. The last few degrees up to 21 come as a bonus. Those four degrees are the method's crude way of subtracting the free heat, and crude is the word — anyone wanting precision has to model solar gain and how many people live there.

kWh = W/K × degree days × 24 / 1000

Three caveats belong with those figures, and they belong in the same sentence as the figures. The bands are wide because a degree-day number depends on which normal period and which base temperature were used — our own guide computes a table per locality from typical-year files, and it agrees with station data within 4.3 per cent. Hot water sits outside: degree days are only about warming rooms. And the worked house is untouched since 1970: 28 600 kWh over 150 m² is 191 kWh per square metre, which is deep in the worst energy class. A house with a new attic and new windows lands far lower.

Open the guide: degree days and normal-year correction

The same kilowatt-hours, different money

Now comes the thing that makes two identical houses get two different answers to the same quote. It is the single most common slip in a measure calculation, and it is easy to correct once you see it.

There is a consolation in that. A house with a poor heat source has the most to gain from its envelope, and a house with a heat pump has already banked much of the prize. So the order matters: work out the envelope before the pump is bought, because a tighter house needs a smaller pump — and a smaller pump is cheaper.

Check yourself