Insulation under floorboards

[MatthewC]

I am hoping soon to be in a position to refit the boards on the suspended floor in the ground floor lounge, where there is a 15" gap between the boards and the bare earth. Remembering the value of this forum I did a search for advice and found http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=9972&p=11502 which is very useful.

However, I have a question which doesn't seem to be mentioned on that thread. If I was to decide to install insulation as suggested (probably solid batts held up somehow), it would effectively fill the gap between the floorboards and the wall plate on each supporting wall. In that case, surely it would restrict the air circulation and cause more damage than help? I might as well just block up the airvent(s), which would be a lot easier! (I've only just found that the single airvent, which since last August I have been scrupulously** ensuring was clear outside, was actually blocked with a wodge of concrete inside, below floorboard level.)

I'll probably go for a large rug ...

Matthew
http://houseintheenchantedforest.blogspot.com/


** Are there many other words containing three "u"s?

 

[CatherineB]

scrupulously**

** Are there many other words containing three "u"s?

Oh gawd, don't get us started :lol: :lol: :roll:

 

[Penners]

Are there many other words containing three "u"s?

Well, let's see...cumulus, furculum, succubus, tumulus, untruthful, unusually...

...bored now.

Matthew, the idea with under-floor insulation is to get it above the air vent. It needs to be a tight fit in between the joists, leaving an air gap underneath which is vented by the air-bricks. If your air-brick is too high to allow this, then you might perhaps construct a "tunnel" from the air-brick downwards into the air gap.

 

[Feltwell]

You'd normally have more than 1 airbrick in order to get a cross-flow of air within the void. Have you got a good location to fit an extra airbrick?

Rather than using solid batts of insulation (which are expensive and you have to cut very carefully to get a tight against the joists, if you don't get a tight fit their insulation effect is severely reduced) have you got enough room to put mesh & ordinary loft insulation (or Thermafleece if you want to be extra green)? Cheaper & easier I reckon. You just stuff the loft insulation in, and don't have to worry too much about the fit against the joists.

To support it, chicken wire stapled across the bottom of the joists works well, or I have seen plastic gardening mesh laid over the top of (& stapled to) the joists, allowed to generously sag between the joists, then stuffed full of loft insulation.

Whatever option you take, good ventilation in the void under the insulation is absolutely vital.

 

[AMc]

'Wool' (rock, sheep etc.) insulation on chicken wire between the joists will create a pretty good insulation barrier and as long as you keep it above the bottom of the joists and off the earth then you shouldn't see any issues with moisture transfer and damp bridging (AFAIK).
However having found a dirty great rat skeleton under our floor when we lifted it to fit insulation and underfloor heating I would be a little concerned about creating a nice nesting site.
You might also have to work out how to prevent it being blown/hoovered up between the boards above.
Wool is only about 1/3rd as insulative as solid board (ecotherm, kingspan, celotex etc.) depth for depth - if your floor joist isn't that deep then you may get more benefit from using the higher performing material. In the lifetime of the floor surface above I would expect the energy savings to more than offset the initial outlay.
A clear advantage of the wool approach is that mineral, rock or sheep wool are all able to cope with some moisture - transporting it rather than potentially holding it against the joists.

To ensure a tight fit with the board insulation we screwed tile batten to the joists at the bottom to create a ridge, scribed the board to suit the joists as carefully as possible then pushed in and the edges carefully sealed with expanding foam. Don't go mad with it or it will force its way through and bridge underneath the board or over expand as it cures and blow the insulation apart - the foam continues to expand for several minutes after its been squirted out.
Foam is cheap from screwfix esp. if you buy in bulk.

As regards bridging the floor into the wall plates - as long as the insulation is at the same level as the joists I would expect you to be OK - otherwise the joists would be rotting at the wall ends but you obviously need to make that call on site. With insulation the devil is in the details, there is little point in insulating under the boards if you don't seal to the ends otherwise you get draughts anyway and the effect is severely lessened. Small air leaks can undermine the efficiency of any material very substantially.
In one of the rooms done while I was away there is a noticeable floor level draught which annoys the heck out of me but wasn't apparent until the winter after we'd laid the heating and I'd refinished the floor

If you're worried about the ventilation under the joists then you can consider excavating some of the soil - it's a pain but it might be worth the trouble?