# Forum Home Renovation Bathrooms  insulating a bathtub

## cathynoosa

hello
it is worth to put insulation under a bathtub , and if yes , wicht one?
maybe then it stay warmer a bit longer ?
thanks :2thumbsup:

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## ed @ ecoclassic

> hello
> it is worth to put insulation under a bathtub , and if yes , wicht one?
> maybe then it stay warmer a bit longer ?
> thanks

  I guess you've got a steel bath... I have used acrylic & steel - and the backside stays warmer in acrylic for much, much, longer... in fact steel is just cold to sit on even in a warm bath. I think the idea is great, just stick batts directly to the bath.  Old immersion heater tanks used to be lagged this way.
Ed

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## Bloss

> hello
> it is worth to put insulation under a bathtub , and if yes , wicht one?
> maybe then it stay warmer a bit longer ?
> thanks

  Yes - I live in the ACT have insulated all metal baths using batts or Anticon (batt+Foil). For two reasons - 1) depending upon the way it is installed it is a thermal bridge to the underfloor area when not being used as bath so heat from the house heating system is being effectively wasted. Even if it is above a timber floor or slab heat will be lost through it in winter. 2) In use as a bath the bath stays warmer longer and the area above the water stays warm to the touch not cold and you can get the temp you want with less hot water. 
Even a layer of double side foil drooped underneath in the frame before installation (sealed all round though) improves the thermal performance. And cheap as chips to do. In North America and Europe (although fewer homes have baths there) the metal baths come with styrofoam insulation already sprayed on. Have seen that here, but not often. I have done a couple using a couple of tins of expanding foam gap filler before installation and that has worked well too.

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## SilentButDeadly

We filled our bath hob cavity with the left over offcuts of wall insulation.....we can lie in a warm bath for a couple of hours as a result. The bath itself is fibreglass. Highly recommended.

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## Insulvac

The best product to use would be an expanding spray foam insulation such as icynene, (there are other brands that work just as well) it is excellent for this purpose. Not too sure if you can get it in a D.I.Y kit, I know installers charge roughly $30 per m2. 
Cheers 
Mick

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## Guv

Just note that if the 'bath' is also used as a shower base, then IIRC, the entire base needs to be fully supported - I believe it is a requirement of Building Authorities and Australian Standards that acrylic baths be fully supported with a mortar mix... not 100% sure on other bath material type requirements though.

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## Bloss

> Just note that if the 'bath' is also used as a shower base, then IIRC, the entire base needs to be fully supported - I believe it is a requirement of Building Authorities and Australian Standards that acrylic baths be fully supported with a mortar mix... not 100% sure on other bath material type requirements though.

  Not just if used for a shower - the assumption here is that the bath of whatever type is installed to its own specs (which will be compliant) then the question becomes do I then insulate & how - which is what the suggestions have been. The consensus is that insulation is useful if it can be done initially or as a retrofit: a) to break the thermal bridge (that even an acrylic bath creates) and stop heat loss from the air when not in use and water when it is)  and b) to allow the bath water to stay at a warmer temperature and for longer.

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## Guv

> Not just if used for a shower - the assumption here is that the bath of whatever type is installed to its own specs (which will be compliant) then the question becomes do I then insulate & how - which is what the suggestions have been. The consensus is that insulation is useful if it can be done initially or as a retrofit: a) to break the thermal bridge (that even an acrylic bath creates) and stop heat loss from the air when not in use and water when it is)  and b) to allow the bath water to stay at a warmer temperature and for longer.

  Interesting - my understanding when requesting something along the lines of insulation between a steel supporting base frame and a fairly large acrylic bath (100mm gap) was that if it was also planned for use as a shower base (it was), then they had to fill the cavity with a mortar mix and specifically told me it was against the regulations to use foams in this scenario - although we could insulate around the edges without issue.  
Obviously concrete slab to mortar mix to bath is going to be a good conductor of heat - and not what you want if you can help it. I was just pointing out what I had been advised when installing ours... if this was specific to the bath we chose, you could possibly ignore this - but that was not the impression I was given. 
Cheers, 
Guv

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## Bloss

Then the impression was incorrectly given - sure the physical placement needs to be in accordance with the manufacturers instructions - but they are generally written to meet ease of install not any thermal performance needs. That is has been changed for years in EU and in US too. The issue is generally with a pressed metal bath that flexing of the metal in use does not eventually cause breakdown of the brittle enamelled surface - the bath itself sitting in a frame around the rim with no other support will hold several people safely! Vibration of the bath in use can also disturb tiles or other inflexible materials that are tied to it in some way. 
Depends much on your climate - in Canberra or other cool climates then it's worth looking at and taking care to get as good thermal breaks in place as you can. In cold climates floor insulation too is more important - especially a tiled slab with an underside exposed to ambient temperatures. These should be fully insulated using stryrofoam board or anticon or even batts as they will be a source of significant heat loss. The added advantage is that the floor will feel much more comfortable all year round especially in winter. In cool climates all slabs should be poured with under slab insulation and there is much benefit in retrofitting existing slabs at the edges and out from the walls to constrain heat loss to the soil. An easy DIY activity too mostly although in established houses it needs all round access so paths and gardens abutting the slab would be disturbed. 
In temperate or warm climates this is not really as great a concern so would be lower down the list.

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## Guv

Bloss, 
Just found the link to the install instructions for the bath we installed. Unless they are telling great big ferphies, it may be worth your time to read it - Particularly Parts A & D: http://www.decina.com.au/pdf/instal_instr.pdf 
I'm interested in your thoughts. 
Guv

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## Black Cat

Just to put  a spanner in the works here ... I was planning to put insulation batts all round my pressed steel bath (external wall, on new slab) here in Tassie but my plumber (who is generally a pretty reliable and intelligent guy, up on matters environmental) suggested that in this environment it might cause major problems with condensation build up dripping from the exterior walls of the bath, soaking into the insulation and causing rot. Since the bath is not quite installed yet (give me a couple of weeks) it would be interesting to hear your views.

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## Moondog55

I do not see how that could be, surely if insulated the bath will be at the same temperature as the house which should be well above the dew point.
Also if the bath is laid on a slab there should be a vapour barrier under the slab to stop vapour rising though the concrete, if there is a danger of condensation then the solution would be to use a styrene foam based insulation board.
I have insulated a bath with fiberglass batts myself, in a WB house on stumps in the suburb of Northcote, and there was no discernible condensation in that instance

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## Black Cat

Oh good. Well I will ignore him then, lol. It is, however, optimistic to assume the bathroom might stay above dew point, though since baths are an evening occupation, rather than a morning one, then it should be fairly cosy in there due to the thermal mass of the slab (NE aspect). Thanks Moondog!

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## multiblade

I'm about to have a bath in my old tin bath. It will be cold, unless i screw the environment and keep topping it up. I had never thought of insulating a bath, what an idiot. When I had to renovate the bathroom in my old place I just installed an acrylic bath, which fixed the temperature issue. Mind you, I installed it cause I was tight and couldnt afford anything dearer.
cheers  :Smilie:

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## Moondog55

I just thought i'd update this thread.
We just installed our new bath, our builder insisted on putting thee bath itself on a bed of mortar, insisted because his insurance was invalid if the bath wasn't installed as per manufacturers instructions. All remaining haps plus the wall cavity are filled with polyester fiber tho and under the huge thermal mass of the concrete sheet floor there is a layer of :"Expol" and when everything is finished I have some spare Foilboard that will go under the lot.
Insulating the under floor space will work almost as well I think, the combination of 25mm Foilboard 40mm airgap and the Expol should give about R4.6

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## Black Cat

Did your plumber have any thoughts about condensation. I am in a running argument with mine (which will finish when the side panel is permanently fixed) about this. He reckons that since the bathroom is not heated, there is a strong chance that filling the bath will result in condensation on the outside of the bath which will, in turn, result in mildew in the insulation. Though, on my windows, the condensation seems to form on the inside, where the temp is warmer, not the outside where it is colder. So I am still open to discussion on the subject. Either way, I suspect I will be installing some vents in the side wall just in case ...

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## Belair_Boy

From my understanding, condensation occurs when warm moisture laden air (eg in a bathroom) contacts a cool surface (eg bathroom window in winter).
So for condensation to occur on the under side of the bath, the bath will have to be at a lower temperature than the air in contact with it and the air must contain moisture.  Filling the bath (I assume with warm water) will raise the temp of the bath and it will be hotter than the air under it and condensation will not occur.
If you fill the bath with ice water and beer, then there is a chance of condensation but the area under the bath must have air with a high enough humidity.
It is likely that the air temperature in the space under the bath will be lower than the bath itself and if everything else is as it should be, dry as well, so condensation will not be an issue.

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## Moondog55

Nope BC no discussion at all; taken as a given by all parties that the space needed to filled with insulation,  think Belair_Boy is right on the money, if the plumber is worried tell him to throw in some sachets of silica gel sitting in a soup bowl, or just finish the job of sealing it all up with the silicon when it is hot and dry
I wouldn't worry about the vents myself

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