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Ventilation in a SIP panel house
In an airtight house ventilation is part of the services design, not an accessory. Why opening a window is not a system, the three options and what decides between them, and the point in the build when it has to be settled.
Published: September 13, 20265 min read
A correctly assembled SIP house comes out very airtight. That is exactly the quality you are buying, because the low heating bills come from it. But the same airtightness means air no longer finds its own way in through gaps, old windows and badly closing doors, the way it did in your grandparents' house. The air exchange has to be organised rather than hoped for.
So ventilation goes into our services design from the start, not as an option discussed once condensation appears on the glass. Below: why opening a window is not enough, what the options are, what decides between them, when it has to be settled, and where it most often goes wrong.
Why an airtight house needs designed ventilation
People produce moisture constantly: cooking, showers, laundry dried indoors, breathing. In a leaky house that moisture leaves on its own, together with a good share of the heat. In an airtight house it has nowhere to go, so it accumulates, condenses on the colder surfaces and, in time, feeds mould.
Worth saying plainly: mould is not a disease of SIP panels. It appears wherever moisture, the right temperature and a surface to grow on meet, including in a well-insulated brick house. The difference is that an airtight house does not forgive missing ventilation, and the consequences show up sooner. We wrote about it without softening in the list of SIP downsides.
"I will open a window" is not a ventilation system
Airing by hand works, but only for as long as somebody does it, every day, in every room, including the rooms nobody walks into. Winter adds two problems: you throw out the heat you paid for, and people predictably air less precisely when the moisture load indoors is highest.
The second common confusion is the kitchen hood. A hood pulls air from one point while you cook; it does not provide the air exchange of the house. In an airtight house a powerful hood with no make-up air can even create negative pressure, with unpleasant effects at the flue if there is one.
Three options and what decides between them
| Option | How it works | When it makes sense |
|---|---|---|
| Mechanical extract | Fans pull air out of bathrooms and kitchen, fresh air enters through vents in the rooms | Smaller budget, holiday house, seasonal use |
| Supply and extract | One system brings fresh air in and takes stale air out, balanced room by room | Permanent living, family house |
| With heat recovery | The same, but the warm outgoing air pre-warms the incoming fresh air | Permanent living in a cold area, where the heat loss matters |
There is no single best system, only the one that suits your house. What separates them is who moves the air and what happens to the heat in the air going out.
What tips the choice: whether you live there permanently or arrive at weekends, how many people are in the house, how many bathrooms it has, how cold the area is, and how much you are willing to put into the installation now to pay less for heating later. On the small holiday designs in the catalogue, mechanical extract is usually enough; for a family house lived in all year we seriously discuss the heat-recovery version.
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Where the air comes in and where it leaves
The rule is simple and the same for every system: fresh air enters the clean rooms, meaning bedrooms and living room, and leaves from the wet ones, meaning bathrooms, kitchen and the utility room. In between it passes under the doors or through transfer grilles, which is why interior doors are not fitted tight to the floor.
That is also why the ventilation layout is drawn with the house plan rather than after it: the runs have to pass somewhere, and in a panel house the openings through load-bearing walls are made to a detail, not with a drill when the installer arrives.
When it has to be settled
Before the finishes, together with the electrics and the plumbing. Ducts, grilles and the position of the ventilation unit are placed in the floors and walls while everything is still open. Added after the plastering and the floors, the same installation costs more, looks worse and sometimes no longer fits where it should.
In the turnkey version ventilation is in the package and is discussed at the consultation along with the rest of the installations. With a kit it stays on your side, together with the electrics, the plumbing and the finishes, but it deserves to be thought through at the same moment: what exactly each formula contains is in what the insulated kit includes.
Four mistakes that get paid for
The first and most common: the system switched off to save money. It brings back exactly the problem it prevents, and the moisture does not wait for you to change your mind.
The second: an installation sized by eye. Too weak does not do the job; too strong dries the air and consumes for nothing. Sizing belongs to the designer, not to anybody's intuition.
The third: filters never taken out. A clogged filter turns a good system into a noisy useless one. It is a few minutes of maintenance, at the intervals the equipment manufacturer states.
The fourth: openings left unsealed in the walls after the work. It is the same mistake that shortens the service life of the house, because it lets moisture reach where it should not, as shown in the article on service life.
What it costs
We do not publish a per-square-metre price for ventilation, because the sum depends on the option chosen, the number of rooms, the duct runs and the equipment available at the time of the offer. In a turnkey house the installation is part of the price fixed after the consultation, alongside the electrics, the plumbing and the heating, and the general anchors for the two formulas are on the price page.
What can be said is the right order of the conversation: first how you will use the house, then which system suits it, then the figure. The other way round you end up with equipment chosen on price and a house where it is switched off by the second winter.
Does a SIP house really need ventilation?
Yes, like any modern airtight house. Airtightness cuts heat loss, but it also stops the air exchange that used to happen by itself through gaps. Without organised ventilation the moisture from cooking, showers and breathing stays in the house and condenses on cold surfaces.
Is opening the windows not enough?
It works only for as long as somebody does it daily, in every room. In winter you throw out the heat you paid for and, predictably, air less exactly when the moisture load is highest. A kitchen hood does not replace ventilation: it pulls air from one point while you cook.
Which system should I choose?
For a holiday house, mechanical extract from the bathrooms and kitchen is usually enough. For permanent living a system that also supplies air makes sense, and in cold areas the heat-recovery version. The choice follows how the house is used, how many people live in it and the climate.
When is ventilation decided?
Before the finishes, together with the electrics and the plumbing. The runs and openings are placed while the walls and floors are still open; in a panel house, openings through load-bearing elements are made to a detail rather than improvised on site.
Does mould appear in SIP houses?
Mould appears wherever moisture, the right temperature and a surface to grow on meet, in any technology. What is specific to an airtight house is that it does not forgive missing ventilation: the consequences show up sooner than in a house full of gaps.
What we do at the consultation
We go through how you will use the house, how many people live in it, how many bathrooms it has and which area it is built in, and out of that comes the ventilation option that suits it, with the installation placed in the design along with the rest. If you want to see first how the wall it all mounts to is built, the panel and the assembly are described on the technology page.
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