Ventilation systems: Sort out causes ...
Home  »  Hub blog  »  Ventilation systems:...
Vicki Cowan
Ventilation systems: Sort out causes of moisture first
Blog, Internal Enviromental Quality, Ventilation
, ,

Recently I was contacted by a homeowner who had been part of Beacon’s HomeSmart Renovation project.  She had received a renovation plan to make her home warmer, healthier and more energy/water efficient, and she had been working through this, replacing windows and adding insulation while recladding.

Her question to me was: What kind of ventilation system do we need?

This is a question that always gets me a little hot under the collar!  That’s because there is increasing evidence that these systems often don’t solve dampness problems, and in most instances don’t even improve heating.

If you have a tile roof, for example (a lot of 1950s houses do) then most types of ventilation systems won’t even work – because the tiles let in so much airflow.  Similarly  if you have a skillion roof (with no attic space) then most systems won’t work either – for systems taking air from the roofspace, there isn’t a space there!  Yet I’ve still seen them installed in those circumstances.

Most older homes, in fact are classed as draughty.  That means that lack of airflow is not a problem – if there’s dampness in the house you need to look for the causes.

Positive pressure/forced air roof cavity ventilation systems (the type that has mostly been installed in New Zealand) act by pushing air from the roofspace through the house and out through the gaps.  Because the air in roof spaces is often quite dusty they rely on having very good filters to clean the air – changing the filters frequently is essential otherwise the air being blown into the house can be quite dirty.

In some houses (but not all) they might reduce the amount of condensation you see on places like windows – because the air is moving so fast the moisture doesn’t have time to settle, but in doing so they will often make the house colder – especially in winter.

Both University of Otago and Beacon research show that positive pressure or roof cavity systems bring cold air into the house in winter, and bring in warm damp air during the day and in summer.  Neither EECA nor Consumer NZ recommend these systems for heating.

Heat recovery systems are a new product to New Zealand but are widely used in the colder parts of Europe and North America.  They use a heat exchanger, bring fresh air in from the outside and are usually more effective at heating.  However, they don’t work in older houses because they require very airtight homes (more airtight than most new construction in New Zealand) combined with a severe climate (think Central Otago) before you save more energy than you spend in the running costs.

Whole house ventilation systems are an expensive investment.  Before you invest, you should be sure that you actually need one. If you have moisture and dampness problems or if your house is cold then you should sort out the cause not the symptoms.

For this homeowner I made the following suggestions:

To address dampness, first get rid of any moisture inside:

  1. Make sure you have a range hood in the kitchen, an extract fans in the bathroom, and for the dryer.  Most importantly, these should all be extracted to the outside NOT the roof space.
  2. Put a polythene vapour barrier down over the ground under your house if there’s room to get under there.  A tremendous amount of moisture (about 25kg) comes off even quite a dry underfloor each day – and that just travels up into the house.
  3. Make sure you can leave windows open safely and air the house regularly.
  4. Get rid of any unflued gas heaters – they pump out a litre of water every hour!
  5. Fix any drainage problems or leaks in or under the house.
  6.  Don’t dry clothes inside – all the moisture from the clothes goes into the house.

Next keep warm inside

Warm air holds more moisture and keeping the air inside your home warm will also keeps surfaces warm so moisture won’t form on them when it gets cold outside. An insulated room will have fewer problems with condensation on walls and windows. Insulating, double glazing and using passive heating or an efficient heating system to keep your home warm will also help to reduce moisture.

  1. Insulate, insulate, insulate!  Insulation is also a key way of reducing dampness – and actually for older homes it’s critical that you don’t just insulate ceilings and underfloor, but the walls as well.  If you are planning to reclad or reline then installing insulation at the same type (with building wrap around the outside) is a must.  In fact I would prioritise it ahead of any other intervention, just because you so rarely get the opportunity to install wall insulation easily.   I’d stick as thick a batt as will fit – and make sure it’s a rigid product that won’t slump in the walls.  Stiff fibreglass (eg R2.6 or R2.8), thick polystyrene or stiff polyester are the best options.   And try to get the highest R value possible. Of course if you haven’t already you should take advantage of the Warm Up New Zealand subsidies (while they are still there) and get your ceiling and underfloor insulated. With all insulation products good installation is critical.  Ideally get someone who has been trained by the Insulation Association of New Zealand (IAONZ) because they will do a good job.
  2. Depending on how extensive your renovations are, consider replacing single glazing with double glazing – there are a lot of options here so I might save these for a separate blog.
  3. Heat your home efficiently (don’t forget the bedrooms) and use heat transfer systems to move the heat around your home.

So my advice is: Before you think investing in a ventilation system, address moisture sources and insulation levels in the home first – it’s simple, relatively low cost, and you get the added benefit of a warmer home.

If you really want to install a heat recovery ventilation system get your house tested for draughts first (often called a blower door test) and I recommend you only consider one of these systems if you live in a cold climate and your house is very airtight.

  1. I’m pleased Lois referenced CONSUMER whose reports are invariably pretty up to date when it comes to new products and technologies on the market. If you are not a member it costs a mere $25 to access a report; a good investment to make the right purchase decision where hundreds or thousands is involved and one lives with the consequences for years. Not that NZ households have a strong track record of spending money to get independent advice on energy. Its so unmanly, and uncessary, when as a kiwi bloke you know everything. All is explained by reading Funnily enough its written by a women. Cheers, Norman

  2. Great post Lois, I think your comments are very topical & bang on!

  3. Couldn’t agree more Richard, thanks Lois, for a very relevant and ‘laymans’ terms post, really appreciated because I am receiving enquiries every week about ventilation systems from the CEN website, fortunately I have already been responding with similar information and sending people to various links other hub users have shared (for public use), but post is a fantastic quick reference check list.

  4. Ok, I have a dirty little secret to confess…I have an HRV system in my house, but in my defense, it was installed approximately 6 years ago (before I had a clue) and, to cut a long story short, we got an extremely good deal ($3000 for a $5000 system).

    So not believing HRV systems to be the silver bullet and over the years learning more about energy efficieny, I’ve slowly started to improve the thermal envelope of my house and this point we have:
    – Ceiling and floor insulaton (including polythene as ground cover)
    – an extractor fan in the bathroom
    – most doors and windows have seals on them, we are working our way through the back of the house re-weatherising the windows which were in a dire state (very thin glass)
    – thermal Curtains in all living areas
    – I leave doors and windows open all day every day, and some stay permanently open all year round
    – We have an old but reconditioned kent fire place and I burn dry wood
    But we don’t have:
    – an extractor fan in the kitchen (which is open plan, lounge, dining)
    – we do dry clothes inside, there is often no other choice
    – no wall insulation

    We can get the house warm with the fire, and this is the first winter with all of the things below ticked off, so we are hoping for a much warmer one than last year.

    To get the point of my post, last night I decided to turn my HRV system off to see what would happen re condensation. Within 3 hours, condensation was on ALL windows…keeping in mind there were no clothes drying in the house at the time, but I had cooked dinner over that time, BUT windows throughout the house were open.

    Does this mean, for my situation, I need the ventilation system to avoid condensation? I live in a 1960s brick home, she’s an old girl but I love her. Wooden floors, lots of natural light and warmth from the north facing lounge/windows.

    And no, I didn’t disclose that I had an HRV system when I was interviewed for my role with CEN….its out now though.

    Very keen to hear thoughts on this.

  5. Should have qualified my post a bit more, in the sense I am not actually seeking advice on my home – what’s done is done, I already have the system in place, I am seeking advice on how to communicate the question I get asked the most in my role with CEN via our website, similar to what prompted Lois to write the first post: Do I need a ventilation system? I am now stumped with the answer, because it seems in my house, failing getting an extractor fan for the kitchen, wall insulation and double glazing…$20k+?, I do need an ventilation system…

  6. I visited a house this morning that was in a similar state, water literally pooling on the window sills, aluminium joinery single glazed with average (ie poor) curtaining, late 80s house, probably R1.8 batts in the roof (now down to 60mm and needing top up) and foil underfloor. Bathroom had a 3 in one fan ventilating into the roof. Kitchen had a lovely rangehood that recirculated moisture back into the room. So they could have put in an anti-condensation system and not fixed all the other problems, and would probably have got a good result. But at what cost? $3500 for the system, plus annual maintenance ($200?), plus energy costs (not great admittedly). Or they could get better insulation, get their fans ventilated outside (and preferably throw out the 3 in one in the bathroom for one located over the shower, and get added benefit of no recessed downlight fitting). And most importantly put a groundsheet under the floor. Cost with subsidy probably less than the ventilation system (maybe $3000). So what do they do? For my money, get everything else except the ventilation system and then see if you need it. Maybe they still will, maybe by then they’ll decide the condensation is not such a problem and spend that money instead on good curtains and a decent heater.

  7. Thanks for a great discussion.

    Jo, this wouldn’t happen in your house because you ventilate it properly, but I suspect that, in a fairly airtight house that isn’t well ventilated (e.g. post 70s with aluminium joinery)a PPV system would be forcing moisture into the wall cavities, since the pressurised air has few other places to go. Once inside it would condense on the coldest surface, either the outside of the insulation or (where no insulation) the inside of the wall underlay or cladding. And 70s-80s houses with foil-backed plasterboard are another nightmare again, don’t want to think about that one …

    Many people don’t manually ventilate their houses. And we know there are lots of small gaps where lining meets doors, windows, skirtings, scotias, light fittings, ducts etc. The damage from hidden condensation there might take years to show up.

    It would be interesting to know if this has been found to be a problem in houses that have had PPV systems for a few years and where walls have been opened up.

Leave a reply

You must be logged in to post a comment.