Insulation above timber floor

MichaelR

So I have this room with a suspended timber floor; the rest of the house has concrete floors. The room with the suspended floor is the coldest in the house and there are drafts from the floor too apparently; when a rat died under it a year and a half ago, there was a stench for weeks, despite me trying to tape any holes I found.

I would like to insulate this floor. However, I could not yet find a contractor in Limerick to do a proper job of lifting the floorboards and installing insulation, or else just putting in concrete plus insulation. I also am not sure if either of these options might lead to long term issues (as moisture from the ground is now free to evaporate to a ventilated space which would be changed etc), and I don't want to hire an engineer too just to be sure.. So I am thinking of putting insulation above the floorboards instead.

The idea is one or two layers of floor insulation and then laminate; I actually prefer a vinyl floor, but I would need to put plywood between the insulation and the vinyl, and thin plywood tends to warp, so I'd need to screw through insulation a lot - creating many cold bridges. So I guess laminate will do, maybe with some vinyl rugs on it.

Ideally I'd want to keep the level of the floor, to avoid working on the door. The existing floor is thin vinyl on top of old, circa 12 mm engineered wood floor, then bubble plastic then floorboards. As far as I can work out this is about 15 mm, so 8 mm insulation and 6 mm laminate sounds about right. And I am considering a DIY job, or else getting some handy person to do it, or else maybe asking at the flooring shop for an installer?

I do have questions here though. It's either DIY or I need to make all decisions and supervise, so I need to understand what I am doing either way.

My biggest question is how to ensure airtightness in this scenario, as drafts are a major source of heat loss. I understand that the laminate will have to be spaced from the wall to allow for heat expansion. So should I put the insulation flush to the wall and tape its joints to the wall (and joints between pieces of course)? Or does XPS also expand like laminate? What other things, too, should I do to improve airtightness?

Another question is how to pick the insulation, so that I could have the maximum TOG from the 8 mm. Price is not a major issue as the room is small, circa 10 sq.m.

Also, can I get thin waterproof laminate? Spills sometimes happen, that's one of the big reasons I like vinyl, so waterproof laminate would be ideal.

...or is this a fool's errand and do I have to keep looking for floor insulation contractors?..

shesty

We have a house with suspended floors in several rooms.One of the rooms is quite big and cold.

We've pondered all the options for insulating it, much as you.Both of us have construction backgrounds.We've concluded the "cleanest" way would be to lift the floorboards, cut out the joists, and put down insulation and concrete.

We aren't going to do that tbh.There are options where you can "hang" insulation from the joists under the floor, giving you insulation but still allowing airflow, to avoid the timbers rotting.An architect we spoke to said he was on a job that was using pumped insulation to fill the cavity under the floorboards.We (and he) are unsure of that, again for airflow/rotting reasons.

In one of the smaller rooms we have used a thick underlay and 10mm laminate boards, and it has worked a treat.Taped from the bottom of the wall down to the floorboards, installed the floor and replaced the skirting.It is a vast improvement - most noticeable to me because I work in that room so I'm sitting still in it all day. We are going to take a similar approach in the big room, a thick underlay, and a 10 or 12 mm wooden floor on top of the floorboards.Taping all around the edges.

It's probably not the most "by the book" way but weighing up time, cost materials, we are going for this approach to make the room far more comfortable.

The Continental Op

This guy has done it right https://www.boards.ie/discussion/comment/119942025/#Comment_119942025 .

loyatemu

I've contemplated the same. An airtight membrane over the existing boards taped to the wall, as shown in the thread linked above; then an insulated underlay (this one maybe?), then the laminate. It won't have the same level of insulation as going under the boards, as you're limited in how deep the underlay can be, but you're eliminating draughts, the underfloor is still ventilated. Seem like a less disruptive option than ripping up the floor.

Also, can I get thin waterproof laminate? Spills sometimes happen, that's one of the big reasons I like vinyl, so waterproof laminate would be ideal.

you can certainly get laminates that are rated for kitchen and bathroom use - we've had laminate in our kitchen for 10 years, it's still fine despite many spills.

10-10-20

Key thing is the system composition.

Warm-side to cold side:

Flooring - vapour control membrane - open faced insulation (rockwool/glasswool) - windtight and vapour permeable (diffusion open) - joists.

MichaelR

https://www.boards.ie/discussion/comment/119990550#Comment_119990550

Thanks! So the idea is to separate the membrane (which I tape to the wall) and the insulating underlay. The thread lined above puts the membrane under the floor boards but I guess I can put the same kind membrane over the floor boards as well?

There is one more thing I found online that is available in Ireland. something called FloorQuilt https://www.screwfix.ie/p/ybs-floor-quilt-insulation-1-5-x-10m/925pt . This is circa 6mm floor insulation in rolls, which one tucks behing skirting boards and tapes to the wall. It is intended, however, to be put under a screed, so I wonder if putting laminate right over it is a good idea.

Another product is described for my exact use case, but does not appear to be available in Ireland https://www.multifoilsdirect.co.uk/product/ecotec-floor-foam/ . (They deliver to Ireland but the delivery is super expensive and one might be hit with customs charges on top of that)

shesty

https://www.boards.ie/discussion/comment/119990345#Comment_119990345

That's the "hanging" option.He's done a good job.Our room is substantially bigger than that space🙈And I suspect the cavity underneath is deeper also.However fair play to him, he did it well.

10-10-20

Those reflective foils are snake-oil. They can't reflect radiated energy when sandwiched up against another material. Additionally the reflective surface degrades from compression and friction when in contact with floor coverings, so can't retain their full resistance to vapour.

Anyway I know that we're talking about floors here, but have a listen to what Steve Roofer says about vapour control layers on top of other vapour control layers. That's why he recommends open-faced insulation and one VCL.

https://youtu.be/ygdC9eB-QEw?t=149

Of course your VCL will be the top layer instead of being wrapped around the joists. But maybe as this is a floor and the vapour is less loaded with humidity and the pressures are less, the rules will be somehow different.

loyatemu

I think the big problem with suspended timber floors (certainly in the case of ours) is they're bloody draughty! Fix that issue and you should be much more comfortable and less inclined to stick the heating on to compensate.

The proper way to insulate the floor is as shown in that other thread (Kingspan also make rigid insulation designed to go between the joists on battens). I have seen people on other threads saying the joists are still a cold bridge and really you should insulate below the joists, whilst maintaining some form of ventilation; though I'm not sure how you achieve that without making the floor cold again...

MichaelR

I understand the proper way involves removing floorboards - I just want to avoid this huge work!

Degradation from friction is something that probably does make FloorQuilt less desirable in this case; it is intended for putting under a screed, where friction does not happen. So maybe, indeed, a thin draught barrier membrane (which is probably sold as a vapour control membrane?) all the way to the walls and tucked under the skirting boards, plus foam floor insulation put on top of it with expansion space at the sides, is the way to go.

The sides are then a cold bridge but at least not a draught bridge; and maybe I should stick in cork expansion strips to minimise cold bridging?

Another question is how to fix that all. I don't want to screw or staple through the membrane or the insulation. Dab glue on the membrane to stick it to the floorboards? And should I also glue the foam insulation to the membrane, or shoudl the foam insulation underlay be floating too, like the laminate above it?