Insulating between floor joists

[CotswoldDoItUpper]

Hi Wizards,

 

We’re about to start ripping up floors ready for insulating between joists but have choices abd can’t decide on best one to use. Cost is a major consideration but if one system is significantly better/easier to fit then we’ll take that into account too.

 

1: PIR expanding foam and tape

2: EPS expanding foam and tape

3: mineral wool laid in net ‘hammocks’ (only what I can think to describe it!) covered with vapour barrier.

 

other things to consider:

 

options 1&2 will be covered by 18mm ply/chipboard followed by UFH and tiles

 

option 3 would use spreader pates, notch the joists and then vapour membrane all covered by 18mm ply/chipboard and tiles.

 

any experience with any of the above greatly received so we don’t fall into common pitfalls!

 

TIA!

[Mr Punter]

It depends a bit on your joist depth.  You could consider fixing battens to the sides of the joists at  the bottom to support the insulation.  You would do better with a polythene sheet on top of the insulation as a vapour barrier because the tape may not want to stick properly.

[Cpd]

If your just insulating between the floor joists will UFH actually work effectively as your going to have some massive and repetitive cold bridging through each and every joust….. 

you say cost is a major factor and it would be a shame to spend all this money on something that is potentially going be to expensive to run. 
i would look into this issue before committing. 

[CotswoldDoItUpper]

3 hours ago, Cpd said:

If your just insulating between the floor joists will UFH actually work effectively as your going to have some massive and repetitive cold bridging through each and every joust….. 

you say cost is a major factor and it would be a shame to spend all this money on something that is potentially going be to expensive to run. 
i would look into this issue before committing. 

Thanks @Cpd definitely a consideration the cold bridging yes. It’s a 1800s cottage so there will be lots of other insulation compromises along the way. It’s about doing what’s the most appropriate option I think. Will eventually (next year prob) be ASHP I hope, but in the meantime a couple of Willis heaters will do the job of warming water.

 

5 hours ago, Mr Punter said:

It depends a bit on your joist depth.  You could consider fixing battens to the sides of the joists at  the bottom to support the insulation.  You would do better with a polythene sheet on top of the insulation as a vapour barrier because the tape may not want to stick properly.

@Mr Punter Joist depth is variable around the property but generally 150-200 at 400 centres. 

Pollythene sheet sounds v cost effective, does the vapour layer not have to be breathable? Have read about battens but sounds fiddly and even more cost to the already more expensive PIR option. Is PIR worth the extra outlay over mineral wool or EPS?

[Cpd]

8 minutes ago, CotswoldDoItUpper said:

Joist depth is variable around the property but generally 150-200 at 400 centres. 

Pollythene sheet sounds v cost effective, does the vapour layer not have to be breathable? Have read about battens but sounds fiddly and even more cost to the already more expensive PIR option. Is PIR worth the extra outlay over mineral wool or EPS?

You want the insulation to be as close to the top of the floor surface as possible and battens will make fitting the insulation much more easy as you can fix them at the exact depth you need to make sure the top of the insulation is where you want it. You can temporarily hold the insulation in place with short bits of wood screwed into the joists  while you foam around the sheets, then remove the wood and fill any holes, cut back and tape. If it was me I would do it in two layers, 100mm foamed in place and then another 100mm foamed and taped. If your going to use less insulation then I would still split it, 50mm and then 100mm etc. It will be near impossible to do it right without battens below. 
I have never used eps so can’t comment but as it’s not as good as PIR I would be looking at using it full depth in two layers of 100mm 

I would not consider using mineral wool as your not going to get the results you need for UFH.

you still need to deal with the cold bridge. 

[saveasteading]

I recently used EPS to insulate a small area of wall in a cottage refurb. Chosen because it was available immediately in Wickes and not expensive.

As the majority of the remaining walls remained uninsulated, it was just sensible and far from an optimum solution.

 

It was not nice to work with. Cutting with a saw leaves thousands of ball droppings on the floor. Inaccuracies of a few mm do not squeeze out of the way, as the sheet will crack.

Cutting with a knife ok, but needs both sides or snaps randomly.

 

Then any little gaps are difficult to fill. I just pushed in offcuts of the eps or of mineral wool.

 

In a ground floor, suspended floor I would probably favour PIR or wall batts, depending on dimensions and cutting waste. 2 layers of PIR sounds handy as any errors in cutting will balance out and the second cut allows correction.

 

I have often tried to find a way of insulating under the whole floor structure, but have not found the answer yet ( and I don't see multi-layer as the answer) 

Easy if there was loads of headroom, but not for the standard gap.

 

Assuming you are talking about a wooden floor, about 400mm above the original ground, what is the surface?

In Scottish regulations there is a requirement for the exposed ground, that seldom seems to have been applied in older English buildings. and the handy word Solum can be thrown into conversations. I do recommend something along these lines before you hide it forever. at least add a polythene sheet and ensure good ventilation of the void.

 

[TonyT]

I put 200mm pir between my suspended floor joists, and then tar paper prior to fitting 28mm t&g floorboards.

 

the tar paper is generally used by flooring contractors in my area laying wooden floors

 

[CotswoldDoItUpper]

Ok so it transpires that my joists are 100x50 at 400 centres but the gap underneath is 550 before I can see what looks like a plastic sheet on top of concrete. I guess I could therefore fit 100mm under and perpendicularly to the joists  and 70mm in between solving the issue of the cold bridge? Then still have space for a spreader plate and pipe work for the UFH. Does this make sense? 
 

Is there a scenario in which I could use mineral wool to make this cheeper? Can’t work out how I’d ‘hang’ nearly 300mm of mineral wool from the joists.

 

also, there are 15mm copper water pipes atratched to some of the joists. How do you work around them when laying insulation between the joists?

 

thanks all, nearly got a plan! 

Edited July 7, 2021 by CotswoldDoItUpper

[Cpd]

In the perfect world you would put 100mm or less of PIR below the joists and fill the rest with rockwool, however it’s always recommended that the most breathable insulation goes on the cold side and then the less breathable materials on the inside. BUT I have recently seen a new build where they put PIR on the outside and rockwool on the inside……if this can be confirmed as acceptable  then this would be an easy and cost effective way to do it. 

[CotswoldDoItUpper]

So I’d have 100mm PIR below and then 100mm rock wool or similar? Surely the 70mm PIR would give better results? Happy if not, saves a bit of cash!! How on earth would I hang rock wool under the PIR?!? Seems like madness!

[Cpd]

As an example….. I have 50mm PIR sheeting on the inside of my roof joists  (over the ends)  170mm rockwool between them on the cold side, I have found it to be very effective and when I stick my hand between the PIR and the rockwool it is toasty warm….. now I am no expert but I am assuming rock wool actually stores a lot more warm air than PIR and if the rockwool can be made to work on the warm side without causing condensation  build up on the PIR then I think you could be onto a winner….. 

 

yes 70mm of PIR between the joists would give you better performance on paper but (and this is just my observation……) you would miss out on how rock wool performs by storing heat within its layers, ( I am well out of my depth in technical terms and knowledge now) I just think that 100mm of PIR bellow the joists will be your main insulator and the fully filled joists with rock wool will solve your problem about pipes and it can easily be compressed from the top so that it does not interfere with your UFH and subsequent floor layers. 
 

personally I don’t think that rock wool suspended below the floor joists will be as effective. 
 

anyway this is just me chucking some ideas around……. They need verification ! 

 

[saveasteading]

21 hours ago, CotswoldDoItUpper said:

 

also, there are 15mm copper water pipes attached to some of the joists. How do you work around them when laying insulation between the joists?

 

Cutting around just has to be done as best you can, and probably involves stuffing mineral wool into any odd gaps.

If cold water then there is a possible danger of freezing, if now in the void below the insulation. 

If hot, you will lose heat into the void.

 

So I would suggest you insulate the pipes first, before doing the floor.

If the pipes are going to be 'in' the insulation then there should be no issue.

 

you have said that the solum has a plastic sheet on it, so good.

You have not mentioned ventilation of the void, which is important. Are there air bricks, and are they working? They are often covered by earth or paving.

[Ferdinand]

You need to consider preservation of your joists, too. They need to be ventilated or guaranteed dry, which can be done by making sure that they are sealed from outside and sufficiently warm.

 

The last one I did I used 100mm rockwool between joists (staplegun), with bottoms exposed to he void, with 50mm PIR above the floor (doors !), and a floating floor on top. I did that because it was simple, and I had just taken 100mm of rockwool out of the loft so it was empty enough to get me free loft insulation to 250mm.

[CotswoldDoItUpper]

5 hours ago, Ferdinand said:

You need to consider preservation of your joists, too. They need to be ventilated or guaranteed dry, which can be done by making sure that they are sealed from outside and sufficiently warm.

 

The last one I did I used 100mm rockwool between joists (staplegun), with bottoms exposed to he void, with 50mm PIR above the floor (doors !), and a floating floor on top. I did that because it was simple, and I had just taken 100mm of rockwool out of the loft so it was empty enough to get me free loft insulation to 250mm.

There are air bricks around the perimeter below the level of the joists. Hopefully this will be ok!

 

So you need to make sure they are exposed to the draught.

 

If you have any standing water or dampness under there, it will be at least as bad.

 

Unless you seal it entirely and makes sure the joists are far enough inside the insulation layer to be kept warm and dry - which is the more unusual technique of filling the whole void with Leca or Polybeads.

[Lee66]

Hi all, jumping in on this topic as I intend something similar. I am only insulating under floor(bungalow) to reduce draughts but was intending 100mm mineral wool between my joists, which are only 100mm themselves, and insulating the pipework and hanging below the joists. the floor itself below has been concreted and there is a gap of 300+mm between bottom of joists and concrete with good ventilation from air bricks. would I need a vapour barrier or is it ok to just replace the tongue and groove floorboards? rooms will be carpeted.