Do ground-floor floorboards need to breathe?

[jmo]

I'm planning to paint my living-room pine floorboards with Rustoleum chalk floor paint (no energy and patience left to try finishing them, even if I thought they were good enough to do it, which they're not in my opinion), and concocted a plan to seal the nail heads with Zinsser bulls eye 123 before painting just in case they rust, which became a plan to use it to prime the whole floor to improve the finish of the paint and reduce the sanding.

Bought the primer, and now am wondering if the fact it'll dry to a "tight water resistant film" will be injurious to the boards? But theoretically wouldn't many finishes, such as polyurethane varnish (which many people use even if not recommended on here), also create a water-resistant barrier on the top surface?

I've just got a nightmarish vision of proceeding with priming the whole floor and then painting, only for the boards to start rotting away from beneath. Have probably also done the wrong thing by refreshing the filler between the boards, too, but it's done now. (Is it obvious I'm in a fatalistic while also neurotic mood about the whole thing?)

 

[Craig89]

As long as your air bricks are clear so you have good ventilation below then there shouldn't be a problem

 

[jmo]

As long as your air bricks are clear so you have good ventilation below then there shouldn't be a problem

They are, thanks for your reply

 

[Lacan07]

I'm planning to paint my living-room pine floorboards with Rustoleum chalk floor paint (no energy and patience left to try finishing them, even if I thought they were good enough to do it, which they're not in my opinion), and concocted a plan to seal the nail heads with Zinsser bulls eye 123 before painting just in case they rust, which became a plan to use it to prime the whole floor to improve the finish of the paint and reduce the sanding.

Bought the primer, and now am wondering if the fact it'll dry to a "tight water resistant film" will be injurious to the boards? But theoretically wouldn't many finishes, such as polyurethane varnish (which many people use even if not recommended on here), also create a water-resistant barrier on the top surface?

I've just got a nightmarish vision of proceeding with priming the whole floor and then painting, only for the boards to start rotting away from beneath. Have probably also done the wrong thing by refreshing the filler between the boards, too, but it's done now. (Is it obvious I'm in a fatalistic while also neurotic mood about the whole thing?)

Yes, as Craig has said if you have a good air flow in the void I don't thinks its a problem. If the room / space above floor level is suitably heated to a temperature greater than the void under the floor a weird phenomena occurs whereby moisture, whether in the room or in the void, is forced / sucked downwards into the void instead of rising through the house as you might expect.

 

[jmo]

Yes, as Craig has said if you have a good air flow in the void I don't thinks its a problem. If the room / space above floor level is suitably heated to a temperature greater than the void under the floor a weird phenomena occurs whereby moisture, whether in the room or in the void, is forced / sucked downwards into the void instead of rising through the house as you might expect.

Thanks, I've done steady state heating at 17 degrees with the gas central heating over the winter and it's worked well. I plan to replace the wood burner in the back room this year so it's looking good for next winter.

 

[Cubist]

a weird phenomena occurs whereby moisture, whether in the room or in the void, is forced / sucked downwards into the void instead of rising through the house as you might expect.

I've always been puzzled by that as it seems so counter-intuitive and appears to fly in the face of the science. I've never researched the matter, merely accepted it as a self evident fact, but what passes for my mind tends to latch on to these things and sets my curiosity gland itching. Do you know of any material/information that may explain it?

 

[noj]

Heating air makes it expand/raises its pressure. Heating the floor from above will raise the temperature of the air beneath it. The below floor air is cooler than the room air and so won't rise into it, but is warmer than the outside air and so will push out the air bricks...

That's my logic anyway.

 

[Lacan07]

I've always been puzzled by that as it seems so counter-intuitive and appears to fly in the face of the science. I've never researched the matter, merely accepted it as a self evident fact, but what passes for my mind tends to latch on to these things and sets my curiosity gland itching. Do you know of any material/information that may explain it?

No, I've not looked into it myself. I originally picked this up in a discussion on the Green Build Forum and was equally surprised.

Although not a forum devoted to old houses its a good place to get advice and often times some pretty technical discussions. In this instance it was a well know contributor called Tony, I'll see if I can dig up the original thread.

 

[Cubist]

Heating air makes it expand/raises its pressure. Heating the floor from above will raise the temperature of the air beneath it. The below floor air is cooler than the room air and so won't rise into it, but is warmer than the outside air and so will push out the air bricks...

That's my logic anyway.

No, I've not looked into it myself. I originally picked this up in a discussion on the Green Build Forum and was equally surprised.

Although not a forum devoted to old houses its a good place to get advice and often times some pretty technical discussions. In this instance it was a well know contributor called Tony, I'll see if I can dig up the original thread.

Thank you both, irritating itch scratched.

The source of my consternation was the knowledge that any heating source(s) in a given room would draw cold air toward it and initiate convection currents that would accelerate that draw thus pulling cold air from the void below the floor-boards and, in turn, from outdoors via the air-bricks. The obvious, and natural, conclusion of this thinking is that the air-flow is inward rather than outward. Hence my puzzlement regarding the claims/suggestions that the air-flow is outbound from the house.

The penny has dropped and I now see that I have consistently failed to 'think outside the box' - which is particularly galling for me!

Yes; when there is a heating source in the room cold air will be drawn toward it from the underfloor void and ultimately 'in' to the house from outdoors via the air-bricks. BUT; that thinking completely neglects the fact that the outdoors temperature is constantly cycling between warmer and colder and therefore during the day, and assuming no indoor heating is used, the natural airflow will be outbound. Hardly a eureka moment but I now at least understand that the predominant direction of airflow through an air-brick can indeed be outbound.

My thanks again to both - even though you've deprived me of what may have been a chance to bust an old myth.

 

[Toby Newell]

The direction of airflow through airbricks is dominated by differential air pressure.

If there is an air parcel moving at 20m/s perpendicular to the front of the house and due to walls and other constraining factors the air parcel is only moving at 10m/s at the back, there is then a pressure differential, as faster moving air has a lower pressure.

Airbricks literally work like vacuums.

You experience this often but familiarity takes away the significance.

Your window is open, fine. Someone opens the front door and slam! an interior door closes, air pressure.

Say you are upstairs in your study with the window slightly ajar, you can often tell immediately when someone comes in the house because of the pressure change.

As regards moisture wicking there are a number of conflicting factors depending on the materials. In general nature likes to equalise enthalpy. Hot things cool to ambient, cool things warm to ambient, damp things dry to ambient, dry things absorb moisture to ambient.

In general moisture wicks from damp to drier areas. If the dry side is heated this will draw more moisture through until the dry side ambient air is 100% saturated, this vapour pressure means no more water can escape from the surface and so the timber will wick moisture towards the warm side until the timber itself is 100% saturated, then a steady state will occur.

Applying a DPM or non breathable coating will have the same effect.

Bear in mind there is not complete air mixing at micro scales, you may have an air pocket that is 100% saturated, say under a floor covering, whilst the ambient air in the rest of the room is not.

Moisture will wick downwards towards the cold side if the airbricks create a lower vapour pressure than that above on the hot side, especially in summer when the warmer hot side air is likely to be more moist (higher dew point)

Having said that, capilliary action is a separate force that may need to be taken into account.

In general the driving force is vapour pressure (you can look at that like literal pressure forcing the water molecules to stay in the wood) more accurately, vapour pressure differential, and the mechsnism of movement through the timber as capillary action.

Fun thought, vapour pressure and capillary action work together in trees so that via expiration at the leaves the lower vapour pressure sucks water and minerals sometimes several hundred feet into the air.

Anyone who has worked with pumps that have a 100m head will know how mind boggling powerful that is for a 'machine' with no moving parts.

 

[Toby Newell]

I'm planning to paint my living-room pine floorboards with Rustoleum chalk floor paint (no energy and patience left to try finishing them, even if I thought they were good enough to do it, which they're not in my opinion), and concocted a plan to seal the nail heads with Zinsser bulls eye 123 before painting just in case they rust, which became a plan to use it to prime the whole floor to improve the finish of the paint and reduce the sanding.

Bought the primer, and now am wondering if the fact it'll dry to a "tight water resistant film" will be injurious to the boards? But theoretically wouldn't many finishes, such as polyurethane varnish (which many people use even if not recommended on here), also create a water-resistant barrier on the top surface?

I've just got a nightmarish vision of proceeding with priming the whole floor and then painting, only for the boards to start rotting away from beneath. Have probably also done the wrong thing by refreshing the filler between the boards, too, but it's done now. (Is it obvious I'm in a fatalistic while also neurotic mood about the whole thing?)

No.

Unless you have a serious damp problem, then yes.

You would be able to tell if you had a serious damp problem because your boards would likely be full of holes.

 

[Toby Newell]

Thank you both, irritating itch scratched.

The source of my consternation was the knowledge that any heating source(s) in a given room would draw cold air toward it and initiate convection currents that would accelerate that draw thus pulling cold air from the void below the floor-boards and, in turn, from outdoors via the air-bricks. The obvious, and natural, conclusion of this thinking is that the air-flow is inward rather than outward. Hence my puzzlement regarding the claims/suggestions that the air-flow is outbound from the house.

The penny has dropped and I now see that I have consistently failed to 'think outside the box' - which is particularly galling for me!

Yes; when there is a heating source in the room cold air will be drawn toward it from the underfloor void and ultimately 'in' to the house from outdoors via the air-bricks. BUT; that thinking completely neglects the fact that the outdoors temperature is constantly cycling between warmer and colder and therefore during the day, and assuming no indoor heating is used, the natural airflow will be outbound. Hardly a eureka moment but I now at least understand that the predominant direction of airflow through an air-brick can indeed be outbound.

My thanks again to both - even though you've deprived me of what may have been a chance to bust an old myth.

see my other post, depending upon the size of structure, exposure and wind level the mass of air moving through the air bricks is likely to be tens if not hundreds of kgs per day, mostly from the outside, some of that will leak into the area above if the pressure above is less. Likewise some of that air may be drawn from the house above if the pressure is greater in the house.

I guess in unreal conditions, ie assume 0m/s windspeed outside, then greater air pressure inside due to higher temperature would force air out, but this is farcical as the entire M.O of air bricks are to work as a differential pressure vacuum due to wind.

Temperature of slightly warmer air above is neglible and is only relevant as it relates to air pressure.

Anyone who states air mostly travels out of airbricks from the house above has little grasp of the facts or basic physics.

EXPERIMENT:

Wait for a cold day. Put your heating on maximum, so, highest pressure.

Open your 30kg back door a foot, then go open your 50kg oak front door a few inches, unless it's a very calm day one of them will slam, or an internal door.

Now, did any doors slam when you turned the heating up?

Not really.

And its not the force of the wind either as the doors slam almost instantly.

It's air pressure, or the differential between one side of the house and the other.

Tornadoes can lift trucks and train cars, not by the wind, even at 200mph, but by low pressure. Differential wind pressure is very powerful, it is actually what causes tornadoes.

It's the everyday nature of air pressure that makes our brain switch off about how powerful it is.

14.7 pounds on every square inch, in a tornado pressure is reduced by 10%, ie 1.47lbs.

Take a Ford F150 at 5000lbs weight with a floor area of 16,800 sqins. 1.47x16,800 = 24,696lbs force. Thats almost 5 to 1 ratio.

The Saturn V rocket had a thrust to weight ratio of 1.2 to 1.

This is the reason why a 200mph hurricane will move your truck, might even flip your truck, but a 200mph EF4 tornado will lift your truck and land it upside down at the top of a tree quarter of a mile away. The difference? air pressure.

Search youtube for mythbusters train car implosion.

Air pressure is so mind boggling because its a thing and yet we cannot feel it, the roughly 100kg column of air on our heads and 1,800kg on our entire body... because we are immersed in it so it balances out, almost... but it's there, actually you appear around 90g lighter on your scales due to the buoyancy of air. Air actually weighs 1.2kg p cubic metre, our brain just ignores this.

After saying all that, I only realised all this around 5 years ago, before that I assumed that air bricks simply allowed wind to blow through their tiny holes and out the other side, which seems ridiculous now, and yet perfectly reasonable.