> As someone who has built my fair share of houses and worked on many more over the last 25 years, I have to agree that our ability to seal air better than ever is causing problems, especially if you don't have forced air heating and cooling running all the time. People just completely ignore or haven't ever thought about room ventilation, positive or negative room pressures, and where and how to ventilate to the outside. In older houses there was always air leaks, small, but they let the house breath, especially if you had a central fireplace and your house was at negative pressure. I have seen many people purposefully over seal their house in ill conceived ways, to try and improve insulation, but end up causing ventilation problems with mold being especially common. Materials and vents that should let air pass through are commonly replaced with impermeable plastics and sealing tapes, many times in conjunction with a misunderstanding in expansion and contraction joints that have small gaps.

Sorry, but this is just wrong and is the type of "accepted wisdom" that holds back construction, particularly here in the UK.

Repeat: ventilation and permeability are different things.

Yes, permeability can achieve some ventilation but it comes at the cost of:

1) Not being reliable; it is dependent on pressure deltas around the dwelling.

2) Being potentially deleterious to the fabric of the building where escaping warm air cools and water condenses on surfaces in the fabric.

3) Being wasteful of energy.

The solution is to separate the two:

1) Seal the fabric totally, bearing in mind vapour permeability. The seal should be on the warm side of the insulation.

2) Install a ventilation system which provides reliable fresh air and, ideally, exchanges the heat leaving the building with the fresh air coming in.

We can no longer rely on folksy wisdom. Let's engineer our houses.

You may be interested in the "perfect wall" concept by Joseph Lstiburek:

* https://buildingscience.com/documents/insights/bsi-001-the-p...

Also check out Matt Risinger. They both have various good content on building science.

Thanks for the reference. Trouble is there is no one "perfect" - depends on so many other constraints.

Apparently certain residential building codes have air-freshness provisions:

I recently added A/C cooling to a previously heat-only house, and I took the opportunity to ask the company to explain some quirks in the existing system, like how the ventilation fan in the laundry room is on the same breaker as the central heating, and had a hidden timer attached to it.

The house was built (early 1990s) with a particularly powerful vent fan in the laundry room which was on (regardless of the wall switch) a certain number of hours a day, and the forced-air system had an air intake outside the house which mixed with the return air before hitting the filter.

This was all sized such that all the air in the house would be replaced about once every 24 hours, according to WA state's VIAQ ("Ventilation and Indoor Air Quality") code.

That's all been superseded by the International Mechanical Code and International Residential Code.

See also ANSI/ASHRAE Standard 62.1-2016:

* https://www.ashrae.org/technical-resources/bookstore/standar...

I have a 100+yr old house, no vents. Old style radiator. Doors have large gaps underneath them, the holes on the floor that the radiator pipes travel through have gaps that you can feel the draft. It's annoying, but the house really does breath and is noticeable comfortable for me unlike houses with forced heat that are sealed well.

That draft you feel is money going out the (proverbial) window. It's better to stop the air going in/out through walls, doors, and windows, and bring it in in a controlled fashion:

* https://www.youtube.com/watch?v=CIcrXut_EFA

The problem is that many retrofits don't bring in outside air into the mix and just recirculate what's already in the house. This is not current best practice.

Any kind of air exchange will cause heat loss or gain, decreasing insulation. Does not matter if it's through wall or HVAC.

If there is no draft, it means there is no air circulation.

The only difference is if the draft goes through HVAC vents or through unsealed doors and windows, or through walls.

If your local heaters are effective and seal controllable, there should be really no difference in many municipal settings as the heating is waste heat from electricity generation anyway. (Those cooling towers "greens" love to hate.)

Certain baseline air flow is strictly required and it is quite a lot, especially in small volumes.

Now, if you have to burn something to heat, then by all means do install a heatpipe recovery system.

You're not wrong, but one can mitigate energy loss with an HRV (or ERV) and get fresh air in a controlled manner:

* https://en.wikipedia.org/wiki/Heat_recovery_ventilation

With the added bonus that the air can be filtered as well, instead of coming in as-in through leakage.

And I recently remember hearing forced air is bad for air quality inside! Did I hear wrong or is there something more to the story like “forced air brings in outside air, so if your outside air is bad you’ll have trouble”?

Most residential forced-air systems don't bring in air from outside (intentionally at least). They recirculate the air in the building through the heating/cooling unit (typically a combination updraft furnace with AC evaporator sitting on top).

Modern systems (since late 80s, maybe?) have return air ducts so that you get even heating even if interior doors (e.g. bedroom doors) are closed, but older systems may only have one return vent per floor or even just a single one in the house, and depend on gaps under doors and such for return air circulation.

Bigger commercial forced-air systems have had exterior air intakes and heat exchanger systems for a while, but are pretty new for residential (I've never seen one in the wild myself).

Heat recovery ventilators [1] are reasonably common in Canada. Most (all?) EnergyStar homes [2] have one, and they includes most if not all the houses I've looked at (or friends have bought) in the past decade or so.

I lived in a house built around 1985 that had one, which was R-2000 [3], so they're at least that old.

[1] https://en.m.wikipedia.org/wiki/Heat_recovery_ventilation

[2] https://www.nrcan.gc.ca/energy/efficiency/housing/new-homes/...

[3] https://www.nrcan.gc.ca/energy/efficiency/homes/20575

Oops, I meant houses that were built in the last ~decade are mostly EnergyStar.

You can add filters on the blower to clean up the air. Especially important in cities.

Also important in the middle of nowhere when the clean, forest air turns to smoke each August.

This is a bit of a side quest comment.

South Africans are (at least historically) quite different in this respect. Everyone is cold in the winter due to complete disregard for insulation (at least, when compared to Europe). People also don't get layering. The comments about construction do hold mostly. In the warmer areas of the country usually you are cold in the winters for only a couple hours per day.

When we go overseas we tend to open all the windows. Maybe this is because we are not used to long winters.

My grandparents were experts at opening select windows in their houses to create drafts in the pre-aircon age.

There is a meme: "If a guy invites you to a braai at his home at 3am, it really is a braai."

A house should not breathe. A catchy mantra I've run across is "seal tight, ventilate right":

* https://www.youtube.com/watch?v=CIcrXut_EFA

Mold is not a ventilation problem, but rather a moisture control problem. Keep the humidity down (especially with whole house dehumidifiers), and eliminate condensation points (by eliminating thermal bridging), and it goes away.

While the above video, which is from the really good channel by Matt Risinger, is a good introduction, I recommend you also check out the building science stuff put out by Joseph Lstiburek for a more science-y explanation:

* https://en.wikipedia.org/wiki/Joseph_Lstiburek

On the "perfect wall":

* https://buildingscience.com/documents/insights/bsi-001-the-p...

Presentation on control layer (especially starting around 15m):

* https://www.youtube.com/watch?v=rkfAcWpOYAA

Good webinar:

* https://www.youtube.com/watch?v=sBcNFeiWjrg

> Mold is not a ventilation problem, but rather a moisture control problem. Keep the humidity down (especially with whole house dehumidifiers), and eliminate condensation points (by eliminating thermal bridging), and it goes away.

This sounds like one of those kinds of answers that only works with spherical cows in a vacuum.

If my shower were completely open and in the middle of the living room, perhaps. Alas, it's in a small private room that usually has the door closed while in use, and now we've circled back around to ventilation...

Yup, ventilate right:

* https://www.youtube.com/watch?v=Ulaz3aUSZlQ

* https://www.youtube.com/watch?v=-3CRGhu8C5I

There are houses that are almost hermetically sealed, and do not have issues because they control temperature and humidity properly:

* https://en.wikipedia.org/wiki/Passive_house#Airtightness

> A house should not breathe Well - first you have to define what "breathing" is, because I've talked to tradesmen who thought it meant allowing air in and out...

Closer to the correct definition shared amongst building scientists is to allow the passage of water vapour. Many engineers would disagree with the approach of having non-breathable walls. If you're renovating old properties you are putting a LOT of faith in your VCL if you go this route.

I like the concept of breathability personally, allied with high air tightness. But you have to use the right materials and, crucially, install it right.

Yes, moisture transport is a lot higher with air leakage:

* https://www.ecohome.net/guides/2316/the-difference-between-a...

However, if you're doing an air barrier, you can often get a moisture barrier "for free" nowadays. Since a lot of housing uses OSB/plywood, if you go with something like Huber's Zip System--which has a coating on it--you can save a lot on labour by only doing one pass around the house. Though you do have to seal the seems as well.

Otherwise something like the popular Tyvek will get the job generally done, but there are peal-and-sticks as well that tend to be tighter (and vent as well, like Delta Vent).

If one is okay with the cost, there's also the idea of simply closed cell foaming the entire house. :)

What's the solution?

My house is absolutely frigid during the winter, and I got quote for resealing it. He told me that afterwards I am either going to have to leave my windows open (no thanks!) or live with mold.

I've heard that you can install vents, but can't find useful info. My grandparents had a chute vent, and it somehow became a breeding ground for flies and all sorts of ichies. Not sure why.

And short vents sound to me like an always open wondow, which just brings in the cold.

I want the insulation, the circulation, the breathability and the warmth. Is there any way to have it all?

Heat recovery ventilation can bring in fresh air with about half the heat loss, but it needs regular maintenance.

You can also look at whole house dehumidifiers to take the water out of the air to help reduce the mold problem.

Yes. Matt Risinger has a really good channel on building science, and this video is a pretty good intro on air tight houses and ventilation:

* https://www.youtube.com/watch?v=CIcrXut_EFA

Go through the archives as well. Lot's of good stuff.

He's based in Austin, TX, so needs to worry about heat and humidity being kept out, but the principles also apply for colder climate where the cold needs to stay on the outside.

Another good person is Lstiburek, and his concept of the "perfect wall":

* https://buildingscience.com/documents/insights/bsi-001-the-p...