Insulating a suspended timber floor

[stuart noble]

I thought it was the underfloor insulation that wasn't worth installing. Maybe I misread

[fred]

Let's see how the month end goes, it may end up on beer and pizza before then.

[Jeremy Nicoll - news posts]

Who he? Do you mean me?

[Bert Coules]

You see? Incompetence. Apologies again.

[GMM]

I had exactly the same issue. My living room floor was replaced last year (not diy sadly but a competent builder), incorporating tightly cut

75mm celotex on battens, laid from above before the boards went down. Results were instantly apparent so I resolved to do the same with the dining room, which is above the cellar. The amount of 'stuff' in the way was, as you say, considerable and there was no way I was going to be able to get celotex in there easily. The only practical way would have been to empty the room, lift the carpet and floor boards and lay it from above.

So I took the easy way out and used 170mm loft insulation, which can be pushed past the obstructions, especially if you're quick and manipulate it before it expands to its final size. Held it up with polypropylene twine strung in a zig zag between staggered clout nails into the inside faces of the joists, although it mostly holds itself up. The result is that after months of thinking about it, I insulated it in half a day without having to arrange for 8 x 4 sheets to be delivered (always a pain if you work all day).

On the side topic, I doubt very much that it has sealed every last draught but now the room feels very much warmer than it did before, even without having any cold weather yet.

Of course, I would have preferred the celotex option but, with winter coming, a 90% job is better than no job at all. If I decide in time that it doesn't cut it and do the job the other way, I'm only out by £100 of materials (which I'll probably get back on this winter's gas bill) and half a day of my time. Alternatively, the celotex would have cost me twice that in materials, even using 50mm, which would likely do the job as well as 75mm does, and a few days of work (plus associated swearing).

[Bert Coules]

That's exactly the situation I face.

And that's an approach I might use, albeit working from above rather than below.

That's interesting, thanks. I have some areas where items of furniture have been built in on top of the previously laid laminate and I was planning to leave the old stuff in situ and lay the new floor covering only to the edges of the furniture. I doubt if I can get insulation (or whatever sort) effectively underneath the built-in items (though maybe wardrobes and bookcases contribute a certain amount of insulation of their own?).

[<me9]

I insulated my kitchen floor two years a go after a flood meant lifting the floorboards. I used a mixture of sheepwool and other cheaper blanket but not glass fibre because of the itch.

The kitchen is much warmer as a result. I am thinking of doing the rest of downstairs but there is limited crawlspace (max 300mm) so I was thinking of lifting a couple of boards at strategic intervals and pulling through mineral wool encapsulated in a plastic sleeve (I can't remember its name but the loft is topped up with it) and supporting it on tile laths.

[Bert Coules]

I can't get to the bottom of the joists (at least, not without more contortion than I care for or am capable of) to nail battens across, but I did wonder about glueing them: putting the adhesive on the batten, feed it into position and then firmly pulling it upwards, perhaps with temporary clamps.

In all this discussion, I've rather neglected the question of raising the floorboards in the first place. From my limited previous experience (in the same property and with the same t&g boards) that isn't going to be as easy a job as perhaps it sounds.

[<me9]

My plan is to use the tile laths in line with the joists supported on the same supports as the joists, with perhaps an intermediate support, and pull the encapsulated insulation along on top.

Very tedious. In the kitchen I used a PMF 180 multitool to cut through the boards above a joist just to one side of the nails and the same a few joists away, then punch through the nails in 3 boards and lever up the boards. I had a handy hatch to aid this where I started. Then I levered up the rest, being careful(unsuccessfully in some cases) not to break off the tongues or one side of the groove. I had to lever up the next board or two slightly to release the tongue of the board being lifted. A trolley jack, crowbar and long 3x2s as levers were all deployed.

When I first removed them after the flood (dishwasher drain hose had a pinhole leak which led to me discovering the damp but the main cause was the fridge condensate tray having been cracked for someconsiderable time and draining under the lino and skirting board where it was unnoticed) I re-laid them loosely to walk across. I couldn't fit the last board, it had to fit a gap of about 1/4 inch. However, a month later when screwing down the then much drier boards I could almost get in an extra board so had to carefully space them to fit.

Whilst the floor was up I took the opportunity to add some stiffening (herringbone strutting) partly as the floor was weakened by the board ends not being staggered, and that it had been too springy. What a difference that made. (as did the insulation, SWMBO complains /much/ less about cold feet in the kitchen!)

[Bert Coules]

That's interesting. I'd thought in terms of fixing batten perpendicular to the joists rather than parallel to them.

Thanks for your comments on lifting and relaying the floorboards. I wonder if it's worthwhile using screws rather than nails when the boards go back?

[stuart noble]

"Lost head" nails and a decent concave punch would be my choice. Much neater IMO

[Bert Coules]

Thanks. It's just a peculiar personal prejudice but I've always preferred the quiet, controlled, driving in of screws to the noisy, clumsy, hammering home of nails. Besides which, as far as my bungalow's floor structure is concerned, I rather think the less vibration the better.

[GMM]

I'd have thought the built in furniture would provide a good degree of insulation although, if you're lifting the floorboards (presumably cutting by the built in stuff), you'll probably be able to slide some insulation under that bit too.

[GMM]

My prejudice would be for screws too. Much easier to get the boards up again in the future if you need to. And you know Sod's law says you will have to if you nail them but not if you screw them....

[GMM]

I did something like that in my last place after a roof leak wrecked the ceiling in part of a single storey area. To get the encapsulated insulation into the space where the original ceiling was still intact (but completely uninsulated from new), I attached the leading edge to the edge of a peice of thin ply and slid that along the top of the plasterboard. Proved very easy and made a difference there too.

[Bert Coules]

Yes, I'm hoping so. It's difficult to be sure until the boards are actually up, though. One part of the furniture in question (a long bookcase) has its bottom shelf some ten inches from the floor with both the back and front below that fitted with MDF sheeting - a bit akin to the foot plinths on kitchen cabinets. I'm wondering if, in the absence of insulation under the floorboards, it would be of any benefit to pack that space with the stuff.

[GMM]

Well, if there turns out to be no easy way to insulate under it, instead of wondering, you might ask what harm it could do. Especially as you're bound to have a bit of insulation left over.

[Bert Coules]

That's very true in both instances. Thanks.

[<me9]

I used screws. If I ever need to remove the boards they'll come up much easier next time.

[chris French]

Hmmm, I've got a similar situation with our cellar which is under the hallway/dining room. We particularly notice it in the hallway, as bare boards, and gaps between some of them. And the cellar is ventilated as it is a little damp.

But like you say, I've not been getting the roundtuits for the celotex route, so might try this.. Though I think I'd have to use something other than bare fibreglass insulation as I seem sensitive to it and we do use the cellar for storing some stuff. Maybe encapsulated insulation, or wool maybe?