Vapour Barrier, Insulation & Floorboards

[artemecionkupo]

Hello! We're doing a mix of carpet and engineered wood floor for our renovation project - it's a 1930's bungalow, so at least we only have one floor layer to worry about.

We're unwilling to pull up the floorboards to insulate from the top-down as there is plenty of space beneath the floor to work. Airflow is good and the void is generally tidy and clear.

The plan is to insulate the floor using (from the bottom up) a breathable layer, some form of insulation material (either rockwool-style or kingspan-style) and then a vapour barrier layer to protect the floor from moisture.

As we won't be pulling the floorboards up, I'm wondering if it's sensible to add our vapour barrier layer on top of the floorboards as installing it from beneath means we can't insert this layer between the joists and floorboards.

I've read lots of different opinions and thoughts about this, and I'm swaying away from installing the vapour barrier layer from beneath to prevent moisture or spills from above ending up in the space between the barrier and the joists (see diagram). Another curiosity is whether, if we were to use our vapour barrier on top of the floorboards, could we use this as an airtight layer to get better use from our MHVR system?

Would it be sensible to do this? We could also add an entire layer of thin plywood on top of the vapour barrier to double-down on the airtightness... Lots of ideas, no clear understanding of what the correct / most sensible approach is, so would appreciate some input and advice!

Thank you

 

[JohnD]

No.

Heat loss from floor is almost entirely due to draughts, mostly round the edges of the room where there is a gap under the skirting, but also round holes for pipes and cables, and between ill-fitting boards.

As you can imagine, there is no convection loss, and very little loss through conduction.

Mineral wool can easily be stuffed tightly into irregular gaps to block draughts. It will also insulate the tiny amount of conduction.

 

[robinbanks]

Another curiosity is whether, if we were to use our vapour barrier on top of the floorboards, could we use this as an airtight layer to get better use from our MHVR system?

Yes, a vapour control layer should separate house air from any part of the structure you wish to protect from interstitial condensation of the moisture in that house air

When JohnD said "no" they weren't answering the question you asked, they're just grinding their "floors don't need insulating because no heat is ever lost through a floor" axe. I personally wouldn't heed John's advice in this instance

 

[JohnD]

If you have a house where the cool, well-ventilated void under the floor contains more water vapour than the warm air in the room above, or where warm air from a room travels downward through the floor into the cooler void, then you have a very strange house.

It sounds like Robin does not understand the concept of RH.

 

[JohnD]

"floors don't need insulating because no heat is ever lost through a floor"

Not something I have ever said, though I have pointed out which of the mechanisms of heat loss is most applicable to a floor.

Insulating a ceiling to prevent heat loss to a cold loft, or a solid external wall, are quite different, because the reasons for heat loss are different.

 

[artemecionkupo]

Would there be any harm in covering the existing floorboards with a vapour / airtightness control layer, then covering over that with plywood to help create a seal? I'm thinking it might be best to tape the VCL against the wall with airtight tape / sealant and cover with skirting. What do we think?

 

[JohnD]

Unless you are boiling kettles under the floor, the air in the void will be cooler and contain less water than the air in the house. What are you expecting your vapour barriers to keep in (or out)?

 

[robinbanks]

What do we think?

I think you should do it. John thinks you should not.

Your house, your choice

 

[vahids1639]

Hey! Sorry to revive an old topic. I am in the same place. What did you do in the end? I am dubious on using PIR boards or mineral wool + breathable membrane underneath without a vapor control layer tbh as I thought similar to @JohnD. But, when I read more about if this VCL is needed or not, I learnt about the air pressure difference which could lead taking air from the room to void area. I asked ChatGPT how warm, moist air can move from a room into the below void (if this is possible at all), here is the answer:

Why Warm, Moist Air Can Move Downward​

While it’s true that warm air rises due to buoyancy (convection), moisture (in the form of water vapor) doesn’t always follow the same pattern. Moisture movement is primarily driven by vapor diffusion and air pressure differences, not just convection. Here's why warm, moist air can travel downward in a subfloor scenario:
Vapor Diffusion:

• Vapor diffusion occurs when water vapor moves from areas of high vapor pressure to areas of low vapor pressure.
• In a heated room, the air is warm and holds more moisture (higher vapor pressure). The crawl space or subfloor is typically much cooler and drier (lower vapor pressure), especially in winter.
• This vapor pressure difference causes moisture to move downward through the floorboards, regardless of the direction of air movement.

Air Pressure Differences:

• Air pressure differences between the living space and the crawl space can also drive air downward. For example:
  • If the crawl space below is ventilated or exposed to wind, it can create a negative pressure relative to the living space above. This pressure difference can pull warm, moist air downward through gaps, cracks, or permeable materials like floorboards.

 

[vahids1639]

I read that if I use PIR boards and seal the edges (skirting boards), no vapor control layer is need anymore.

 

[vahids1639]

Also, I don't like the idea of sealing the joist from breathing from air circulation in the void, as shown in the figure on the first post.