Underfloor ventilation/insulation question

Pauley2

Hi,

Following advice in a DIY manual, I decided when I had the floorboards up on my suspended timber floor, to place rigid insulation (typical "Kingspan" type stuff - silver on both sides) between the joists - and caulk in any gaps. This has made a big difference in draughts etc, however I'm now a bit concerned that there's now no ventilation between the upper surface of the insulation and the undersides of the floorboards and may therefore encourage rot. I left a gap of about 1", but as it's so well sealed, there won't be much (if any) air exchange in there. (There's still plenty of ventilation in the "crawl" space below...)

(I also did similar in my attic - between the joists, but that's well ventilated on both sides)

Should I be concerned?

Thanks

ircoha

To answer ur question I would have no concerns as if the job is done as you describe the timber will never get cold enough for cold air to condense moisture on it to start any rot.


What follows is for the benefit of other readers who may follow here.
IMO there was no need to leave the 1" gap: I doubt if it was shown in the diy manual so I trust it was you decision.

How did u seal the outer edge of the 1" gap where it meets the walls or is the kingspan up tight against the wall and well sealed.

The reason I ask is that if u have dry-lined walls with a badly fitted wall vent to the outside and shoddy workmanship on the horizontal batten that acts as the fire break it is possible that u will get air passing in the 1" gap

Pauley2

Thanks for the response,

I should clarify: The 1" gap I left was between the surface of the insulation and the underside of the floorboards/top of the joists -i.e. not around the sides. I sealed all sides of the insulation right up to the wall and tight against the joists, so it's this unvented "sandwich" area of air that I was worried about.

However, your note about drylining and the horizontal batten being in place as a firebreak got me thinking about other areas of the house that had drylining retrofitted on top of block and plaster walls (the house is 50 years old).

I didn't check the air ducts, but as I have the skirting off I was able to see that whoever installed it only put floor (well about 1" away from the floor) to ceiling vertical battens in place and packed it with what looks like sheets of white styrofoam (I lighted a piece of this and it burned almost immeadiatly, releasing thick black smoke).

1) is this styrofoam legal for such use?
2) should I place horizontal battens along the bottom to aid as a firebreak (and improve the nailing surface for my skirting!)

Thanks

RKQ

1) Its likely to be Aeroboard. It is legal ( if not ideal)
When was the house dry lined - pre 1992?

All insulation boards burn but the plasterboard should give 30 minutes.
Rockwool won't burn well but Fibre glass, recycled newspaper,sheeps wool
etc will burn easily.

2)It may help as a firebreak, but so will the floorboards! Fire is unlikely to
start in void under floor.
Horizontal battens might help as a fixing for skirtingboards but I assume the
existing vertical battens are fine for fixing skirting too.

ircoha

Pauley2 wrote: »

Thanks for the response,

I should clarify: The 1" gap I left was between the surface of the insulation and the underside of the floorboards/top of the joists -i.e. not around the sides. I sealed all sides of the insulation right up to the wall and tight against the joists, so it's this unvented "sandwich" area of air that I was worried about.

However, your note about drylining and the horizontal batten being in place as a firebreak got me thinking about other areas of the house that had drylining retrofitted on top of block and plaster walls (the house is 50 years old).

I didn't check the air ducts, but as I have the skirting off I was able to see that whoever installed it only put floor (well about 1" away from the floor) to ceiling vertical battens in place and packed it with what looks like sheets of white styrofoam (I lighted a piece of this and it burned almost immeadiatly, releasing thick black smoke).

1) is this styrofoam legal for such use?
2) should I place horizontal battens along the bottom to aid as a firebreak (and improve the nailing surface for my skirting!)

Thanks

So the 1" gap is between each joist, which means that the risk of mass air transfer is much reduced.

Re the Styrofoam, cant really comment and dont wish to be in any way alarmist, except that any wiring in it should be in conduit.

Re RKQ's comment on fiberglass burning, I dont share that opinion.

The other insulation boards have a certain fire resistance which IMO, is much better than styrofoam.

RKQ

ircoha wrote: »

Re RKQ's comment on fiberglass burning, I dont share that opinion.

The other insulation boards have a certain fire resistance which IMO, is much better than styrofoam.

I've seen fibre glass melt to a crystal glass around a flue and it nearly caused a house fire. Believe me there is a vast different between fibre glass (heated glass) and Rockwool.

Foil backed insulation board burns fairly easy with a toxic smoke - its not near as bad as styrofoam, but it still melts and burns very quickly.

IMO I'd replace styrofoam with high desity foil backed, if budget allowed, for insulation u-values. Otherwise there would be no justification to go to the expense of replacing it, re-slabbing & skimming, electric fittings, 2nd fix carpentry etc.

Mellor

I'd agree with RKQ on their being a huge difference between fibreglass and rockwool.


However, not all insulation boards burn readily, the exception being phenolic boards that offer resistance second only to rockwool

Pauley2

Thanks all. My questions have been answered.

Just to ans/clarify some of the above: The drylining was done about 5 years ago. These are loose sheets behind normal plasterboard.

The 1" gap referred to is not between the joists, it's simply the gap left between the top of the insulation and the underside of the floorboards. In other words, if the floorboards were up, you'd see the insulation panels sunken below the top of the joists.