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