Underfloor Insulation - Suspended Wooden Floor

[northstar]

Some time over the next couple of weeks I'm going to tackle underfloor insulation in the living room of an 1850s house.

Its a suspended wooden floor with varnished boards (possibly Pitch Pine, not sure) that I'll be lifting and intend to reuse. There isn't much of a crawl space, so insulation will be installed from above. Given what I've seen of the joists already the gaps are best described as 'irregular' or perhaps 'erratic'!

With this in mind, and having done a brief bit of reading on this forum and others, I'm thinking that the net hammock & flexible insulation approach would be better suited than using rigid insulation on batons. This approach would then require the use of a vapour barrier over the top of the insulation and the joists, before replacing the boards.


Does that sound like I've got the faintest idea what I'm talking about?

Anything else I should consider?

Any product recommendations for the insulation / vapour barrier?


Thanks!

 

[mointainwalker]

Your idea is fine.

Use any fibre-wool you like, they all insulate equally well. Only thing is that Knauff has introduced a new material that they call Ecover (?) which is brown and much more pleasant to handle than the normal yellow stuff.

If you are sensitive it might be worth looking for that.

For the vb, just use any poly-sheet either on a roll or as sheets ( 4 x 5 for example). Doesn't have to be thick.

Just looked at the packaging and this new material is called "Ecose"

 

[northstar]

Thanks mointainwalker.

Not looking forward to lifting all these boards, but I'm sure it'll be worth the effort, come winter-time. At least they're square-edged, not T&G.

 

[northstar]

Resurrecting this thread due to slight change in plans.

Lifting more of the floor, we found the wall plate DPC layer had been bridged by rubble at one wall where there is a known issue with damp. As a result the joists are rotten at that end and we're replacing them all with pressure treated ones. (This is weekend work only, hence it progressing so slowly )

Chatting to the guy who is going to do our silicon injection DPC in the wall with damp, he advised against placing a vapour barrier on top of the new joists as he felt this would just trap the moisture, causing the same problem again. Instead he recommended placing a VB on the ground (covered with a layer of concrete or at least sand) to stop more moisture coming up from the ground.

Does he have a point here? I've only located one air brick so far (though ground levels to one side need lowered, which may uncover more) so airflow may not be the best under the floor.

Fair to say I'm as confused as ever now.

 

[mointainwalker]

Firstly, what your friendly DPC man is saying makes little sense, which sounds logical to me as I have very little belief in the efficacy of chemical injection and the skills of its practitioners.

Your bridged DPC was physically transferring water by capillary action (whatever ) to the joists and after long exposure/soaking the joists rotted

If this has been fixed and you are replacing the joists with treated- timber, you are obviously not going to suffer the same problem.

Your underfloor ventilation should be taken care of by air-bricks and there must be more than one, so if you can find and clear them that should resolve that issue.

You don't say if your earth sub-floor is damp or if this is just supposition from the above-mentioned purveyor of wisdom.

If it really is damp, the last thing you want is for moisture-laden air to rise through the insulation because there is a very strong risk it will condense within the insulation and this will destroy the insulation value of the imaterial and also surround your joists in a soaking wet blanket !!

You can at little extra cost and effort guard against this by simply using the vb as your support for the fibre-wool( best under the joists if you could manage it , but looped over them would be ok as long as the spacing between joists is well-filled ).

This would keep the moist air on the cold side of the insulation where it won't condense and should be cleared by the air blocks.

I mentioned a little extra expense" this would be because you would need to use a heavier gauge plastic sheet to be able to carry the insulation.

May I ask why, if you have found out that a bridged DPC was the cause of the damp, you are going to have a chemical DPC injected ?

 

[northstar]

Thanks for the reply mointainwalker.

The sub-floor is damp beside this wall, but seems fine everywhere else.

The bridged DPC was the cause of the damp in the wall plate, but the wall itself is damp, so we're lowering the outside ground level (which is higher than internal floor level at this wall) and doing the chemical DPC.

TBH the main reason for the chemical DPC is that it costs less than our mortgage provider chose to retain until completion of these works.

Of course, it occurs to me now that with new, regularly spaced joists, the rigid type of insulation becomes an option again, many of which I believe don't require a separate VB.

 

[mointainwalker]

Rigid insulation boards will cost you 2 - 5 times more than wool and you will have to cut extremely accurately to avoid any gaps at all, although in this case there is no risk of any condensation adversely affecting the boards.

Hopefully your underfloor will dry out with extra ventilation and lowered outside grpound-levels.