Insulating under a floor from beneath

Adam Funk · 2015-01-08T10:48:05+00:00

Having upgraded our heating system last year, we now notice that the kitchen & dining room still don't get very warm, although most of the rest of the house is well-heated. I want to insulate under the floors of those 2 rooms. There are crawl spaces below them that I can sit in, and holes (professionally made, with lintels) that I can crawl through to get to them. I envisage nailing laths up to the joists near one end of each room, sticking a strip/slab of fairly inflexible insulation into each gap, & nailing more laths up & repeating as necessary to fill all the spaces between the joists. Does that sound right? Is it possible (or too expensive) to get strips already in the right width (14", I guess), or should I get big boards & cut them into strips? What tools do I need to cut them? For the small gaps where joists are close to the side walls, should I just stuff rockwool or something like that up in there? Thanks.

Jan 08, 2015 24 Replies

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Adam Funk 11 years ago

The hole into the crawl space is 65 X 60 cm, so I can't fit whole boards into there anyway.

The space under the kitchen has two air bricks to the outside & a square hole (a bit smaller than the main one) to the space under the dining room. That space has one air brick & the 65 X 60 cm hole to the front cellar, which has some air bricks. The cellar is musty (especially near the cellar floor) but all the joists are OK.

I'm inclined to think that putting the insulation between rather than under the joists is better for ventilating the joists themselves.

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Adam Funk 11 years ago

(Sorry for the delay in replying to this thread --- my leafnode server went down & has just been fixed.)

The builders who are doing some work elsewhere in our house now are insulation enthusiasts so I've seen a lot of Celotex in the past few days. They've been using an ordinary crosscut saw to cut it --- but possibly (probably?) not the same one they use on timber.

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Adam Funk 11 years ago

All the Celotex I've seen (in the past few days) is like that too.

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Tim Watts 11 years ago

Not so relevant - the key factor is whether the air bricks are *lower* than the bottom of the joists. You need to ventilate below the insulation, not in the space you insulated.

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Adam Funk 11 years ago

Thanks (& to others) for the advice. (I've got some other irons in the fire around the house right now, so I'm not sure when I'll get round to this.)

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