Insulating suspended wood floors

bpk101

We're currently refurbing our small Victorian terrace, electric and plumbing work starts Monday and whilst the floorboards are up I'd like to consider insulating the space beneath them. It's a draughty old house circa 1880.

What are the best options available that don't risk compromising the correct flow of air?

We'd like a relatively easy to fit solution as our budget isn't huge!
At the moment we're considering either wool insulation rolled between the joists, or something like this wedged in between the boards... or both!

Edit: my builder has just suggested / quoted the following:

Use rockwood or Ursa fire rated insulations 200mm raf.: 30-35sqm.
Replace all of the floor boards back.
Cost: £430.00 Labour: £660.00
Total: £1090.00

Any thoughts?

FarsleyGuy

I assume its rockwool, I've not done this but have been researching as I'm also considering it. It seem floor insulation is the least cost effective compared to wall and roof insulation, but I can imagine with draughty suspended floors its still worth it. The accepted way seems to be to use a mineral fibre (like rockwool) suspended by tape or battens between the joists as he has quoted for. as long its done correctly you still get the ventilation required. Someone recommended kingspan to me as superior product, but its very expensive and has to be cut to size etc so would cost more to fir and may not give the ventilation required. Insulating your floor will save 10% (on ave) of your heating bill.so it will take many years to get the money back, but if you're gonna do it, now's the time !!

molerat

Staple garden pea netting to the underside of the joists and lay the fibre insulation on that then seal the board joints if they are going to be exposed.

Mojisola

molerat wrote: »

Staple garden pea netting to the underside of the joists and lay the insulation on that.

That's how we did it.

leveller2911

Fix cheap tile batten to each side of the joists about 50-60mm down from the top of the joists and cut and lay 50mm Celotex insulation, its easy to cut with a handsaw. If you cut it in fairly tight you should be draught free and a good depth of insulation . This will allow an air flow under the floor.

Always a good idea to check that your air brick vents are clear of obstructions and if you have any sleeper walls that support the middle of the floor joists that they also have gaps in to allow the air to flow through the whole floor.

TheCyclingProgrammer

I have done some research into this as I am planing to do this in our new home.

There's a few approaches but as well as the insulation itself you also need to consider air and vapour tightness.

Once you insulate, the space under the floor will get a lot colder so there is an increased risk of condensation.

If you are lifting the floorboards, you can lay a vapour barrier across the top of the joists, up the walls and seal with appropriate tape. Then re lay floorboards and skirting. The vapour/air barrier can just be plastic sheeting or DPM. If you're fitting laminate the underlay you use may be sufficient and can be installed over the floorboards.

Alternatively, if you are doing it from underneath you can fix an air barrier under the joists, and ideal up the sides of the wall and taped above the floorboards to create a seal, however this barrier must be vapour open to prevent interstitial condensation forming on the joists where the warm air from the room meets the cold air. Something like tyvek should do the job:

http://www.dupont.co.uk/products-and-services/construction-materials/tyvek-building-envelope/uses-and-applications/suspended-timber-floor-construction.html

As others have said, ensure air bricks aren't blocked - you may need to duct them - and also lag any pipes.

My plan is to insulate from below as I don't want to remove all the floorboards, using Rockwool or similar, holding in place with battens and fixing the vapour open membrane, up the sides and taped to the wall.

In addition, as I will be laying laminate, I will use a suitable vapour barrier overlay on top of the floorboards, followed by fibre board underlay for a bit of extra insulation and to handle unevenness.

That should result in a fairly air tight, well insulated floor.

There's some good info here:

https://www.english-heritage.org.uk/publications/eehb-insulation-suspended-timber-floors/

TheCyclingProgrammer

Incidentally, does anyone know if it's necessary to get building regs approval for this? I know that you should be meeting the minimum U value but is it necessary to get building control sign off?

FarsleyGuy

TheCyclingProgrammer wrote: »

Incidentally, does anyone know if it's necessary to get building regs approval for this? I know that you should be meeting the minimum U value but is it necessary to get building control sign off?

I think it only applied to new builds, if you are just insulating an existing floor then they don't apply , I think!!

TheCyclingProgrammer

FarsleyGuy wrote: »

I think it only applied to new builds, if you are just insulating an existing floor then they don't apply , I think!!

I think you might be right although it can't hurt to try and achieve current building regs levels. I might drop an email to building control.

Seronera

We used ready encapsulated Rockwool and just stapled it between the joists. It works and it's a lot easier to do. We have sliding cast iron vents in the sub floor space to outside.They are opened in spring and closed in winter.

Unless there is a source of damp I doubt condensation will be a problem.

TheCyclingProgrammer

Seronera wrote: »

We used ready encapsulated Rockwool and just stapled it between the joists. It works and it's a lot easier to do. We have sliding cast iron vents in the sub floor space to outside.They are opened in spring and closed in winter.

Unless there is a source of damp I doubt condensation will be a problem.

The issue, I believe, is that leaving the insulation exposed to the elements, particularly if it's quite drafty under your floor, can reduce the effectiveness of the insulation.

Putting up a membrane on the underside of the joists or any kind of vapour open barrier prevents this. However, putti any barrier up then increases the risk of condensation where the colder air below meets the warm air from the room, hence the need for it to vapour open. The problem is interstitial condensation, not any exterior source of damp.

I think you can also mitigate this risk by having a sealed vapour barrier above the insulation, either between the joists and floorboards or on top of the floorboards if you're covering them up anyway.