Insulating between floors

I have the kitchen ceiling down at the moment, the bathroom is directly above the kitchen.

I have a load of 150mm insulation board left over from insulating the bathroom ceiling – does it make sense to chuck it all between the joists in the kitchen ceiling? I can see any downsides with this as yet? Potential for trapped moisture and damp issues? They’ll be an extractor in the kitchen once dine

We did this in the lounge, with the foil backed version, as it deadened what sounds passed up to the bedrooms above.

The plaster board is the barrier for rising moisture, before you do this though check all pipe works for any potential leaks, they are easier to sort now

Do you need to prevent heat going from kitchen to bathroom? Both are within the insulated ‘envelope’ of your building, and you will save more energy looking at walls, floor and upstairs ceiling/roof instead.

Or are you looking to reduce sound transfer? If so, cheap fluffy rockwool or similar is better, even a proper sound stop membrane or double plasterboards.

Yeah, all the external walls and roof spaces have been insulated, I get that it’s transfer out of the building to be worried about in the first instance, which is mostly done.

I have about two full boards of 150mm PIR and the joists exposed in the kitchen so just wanting to put it to some use before I take it to the skip, if it deadens sound and keeps the heat in the kitchen a little (prob coldest space in the house) it’d be an advantage

Just thinking I’ll have about 50mm air gap, best to put the air gap at the top or bottom of the joist space?

Yes do think about air movement. It will keep more heat in the kitchen.

However, some PIR is known to carry sound rather too well, and has minimal sound deadening. It may be worth just putting it on Gumtree and using fluffy stuff instead, which will insulate and sound deaden much more.

To prevent bass travel you need the dense foam, the fluffy stuff does the rest of the sound, like Matt says though there is advantages to allowing the heat to rise, warming the bedrooms

If it keeps the heat in the kitchen a little (prob coldest space in the house) it’d be an advantage

If the kitchen is the coldest space then insulating between that and the bathroom can only make the kitchen even colder by stopping heat transfer from bathroom to kitchen.

Yes I would do, but I would try and leave a bit of an air gap to ventilate the bathroom floor a little.

Main thing it to get your extraction sorted in the kitchen and the bathroom – decent extractors (inline or centrifugal) + clear straight (as possible) runs of insulated ducting with backdraught shutters and appropriately large vents on the exit. 150mm ducting on the kitchen extractor is good. Might need condensation traps on the ducts too, depending on the configuration. The best way to avoid damp is simply to remove the source, and unfortunately extraction is often an area that becomes an afterthought or an opportunity to save a bit of money, which doesn’t work well in the long run.

With the bathroom above I thought it’d work the other way, the bathroom is well insulated already so trapping as much heat as possible into the kitchen spade would keep it warmer for longer, it’s trapping the moisture along with it is my main concern when doing this

Ive started putting rockwool under the floor upstairs and in the stud walls. It definitely cuts down on the sounds carried through the house.

On eof the back bedrooms is above the utility where the dishwasher and boiler are. You used to be able to hear the water sloshing around in the dishwasher if the house was quiet where now, you can just about make out the hum from the pump.

It does work. I’m only doing it because all the chipboard needs replacing. I wouldn’t have bothered doing it just for the insulation.

Thermally, it’s made a difference to the front bedrooms in summer. I’ve packed the cavity of the internal walls between the rooms. The front rooms are a few degrees cooler in the evenings. The back rooms stay hotter for longer so it’s a bit of a trade off.

I also have the upper rooms heat like crazy in thes summer, even during the night, the heat stays inside, it cannot escape which is a headache in the rooms with low AC reach

I wouldn’t be messing with an insulation with zero moisture permeability unless I had guidance form a building envelope engineer, I’d be even more hesitant about allowing for an air gap in a space with no air exchange and in both cases little to no drying potential. You’re potentially creating a vapor pocket with faced foam below tile. The wooden sub floor will take on any moisture and any small leak potentially ends up as a structural issue.
Rockwool solves that as it’s moisture permeable so allows for drying potential.
I’d sell the foam, or use it where designed

Read the manufacturer’s fitting instructions carefully to avoid condensation if putting under the floor. Most say to place the foam in contact with the floorboards when insulating under a suspended floor.

BUT bathrooms are prone to leaks from above and are inherently damp with less good airflow than the space under a house. PIR board is also potentially flammable so may not be suitable for a kitchen ceiling. Check whether building regs. recommended the plasterboard with enhanced fire resistance for this job as they do with staircases.

If it were me I’d either not bother or use rockwool for slightly less insulation but better soundproofing. YMMV.

Ive started putting rockwool under the floor upstairs and in the stud walls. It definitely cuts down on the sounds carried through the house.

This +1, as I used to live in a flat, my bathroom above their kitchen below.. they had a leak in the pipe under my floor (their ceiling), so removed the plasterboard and 4″ of dust/dirt that had been put into it, when it was refurbed from lather and plaster to plaster boards. The sound proofing effect was almost completely removed, and I do not think they appreciate my morning ablutions, while they ate breakfast… Just a thought to bear in mind

I used to live in a flat, my bathroom above their kitchen below.

That sounds like terrible flat design…

Heat flows from hot to cold so if your kitchen is the coldest room then adding insulation to the ceiling will keep the bathroom warmer and make the kitchen colder as you’re creating a barrier to that heat flow. However, the heat transfer will also be very low already as the difference in temperature across the two rooms is likely to only be a couple of degrees C.

You might also create a condensation issue on the kitchen (cold) side of the insulation but you won’t know unless you calculate it.

Personally, I wouldn’t do it. No great benefit and some potentially bad drawbacks.

Thanks all, appreciated, though thought of creating some high humidity pocket with no ventilation circulating is what made me post – I’ll leave it, the insulations being picked up by a gumtree’er tomorrow

This building envelope engineer is it not just an up to date architect ? I served my time as a fired clay technologist many moons ago

Who gets the blame for all the badly designed 1960’s70’s buildings that are all being demolished and replaced , and then there are the RAAC designed building due to be flattened, then there is the Celotex used on so many buildings you may not know you have it

A bit like woofers and tweeters they’re in your speakers

I’ll leave it

Less labour for you if I understood that corectly.. you will just leave it as it is?

I have about two full boards of 150mm PIR and the joists exposed in the kitchen so just wanting to put it to some use before I take it to the skip,

Disappointing if thats the only outcome you can think of