Topic: Insulated Concrete Slab Conundrum

Insulated Concrete Slab Conundrum 

Hallo everone,

From the many articles I've read on the subject, approx 10% of heat can be lost through an un-insulated concrete slab.

According to most builders I have spoken to and articles on the web, insulating around the permiter of the slab *as well as* underneath it, is necessary to combat heat loss to any meaningful extent.

However, I can't seem to find anywhere that offers a product or service that provides for this permiter insulation and the builder we're in negotiations with at the moment says that it isn't actually possible here in New Zealand at the moment.]

I have ti say, whenever I hear statements like this I am automatically sceptical, hence my post here.

Can anyone shed any light on this subject or point out any providers of such a service that I could contact in the Wellington area?

Thanks a lot,

Re: Insulated Concrete Slab Conundrum 

Hi Mr R, nice to see you here. I'll leave this to the kiwis or I'll only rant on that their building practices are a bit like winning rugby world cups .... they did it that way 20 years ago when everyone was an amateur and they're still trying the same old methods today

Pete

Re: Insulated Concrete Slab Conundrum 

Insulation under the slab is essential to prevent heat loss with in slab heating, insulation round the edge is quite difficult to achieve. One suggestion is not to put pipes near the edge (allow 800mm), which stops much heat loss in an case.

JK

Re: Insulated Concrete Slab Conundrum 

Insulating the sides of the slab are NOT difficult if it has been considered before hand. 800mm from the edge of the slab is VERY old school and not usually practiced now days. Once the house is finished you WILL notice the differance if standing or sitting in those areas without floor heating because someone decided 800mm from the slab edge was a good idea.

Re: Insulated Concrete Slab Conundrum 

Above by VB

Re: Insulated Concrete Slab Conundrum 

Hi phptek
Insulating the perimeter of a concrete floor slab is even more effective and therefore more important than insulating underneath the slab. I seriously recommend you do insulate both the edge and under the slab.
Edge insulation is always recommended practice in all New Zealand climates.
Underslab insulation is sometimes only recommended for heated slabs (this is based on the timeframe for payback being quicker), but as underslab insulation is not hugely expensive or difficult, and not a retrofit option if you change your mind, I would always recommend it. After all, I make the assumption that the floor should be at room temperature!
Insulating the slab is especially effective on damp building sites where the ground conducts heat away very well.
There are not proprietary systems for perimeter insulation, which will be why you are having difficulty finding information, but there are several methods possible. They use standard products and are easy to build. It may be that the builders you have spoken with are just not familiar with them, as they are unfortunately not common building practice in NZ.
The exact method to use will depend on the detail design of your foundation and the junction between your cladding and your floor. Your building designer should be able to draw them up with no difficulty, unless they have also not worked on a well insulated building as is unfortunately common!
Let me know what your building details are at the floor/wall junction if you need more info on how to achieve the level of insulation you want.

Re: Insulated Concrete Slab Conundrum 

oops... wasn't logged in when I posted the above, sorry.

Re: Insulated Concrete Slab Conundrum 

HI Phptek,

This page offers a pretty good outline on ways to achieve perimeter insulation of the slab (although some of the sections they use are from overseas, still applicable to NZ) http://www.design-navigator.co.nz/phpBB3/viewtopic.php?f=1&t=8

It seems like you have a few options;

150x50 h3.2 timber around perimeter of slab butted up to foundation wall (this works better if you have 140mm walls or carpet/tiles as you can hide the top edge of the timber

or 25mm polystyrene in the same place as timber above

or use polystyrene around the whole foundation as it looked like they did here : http://goldenbayhideaway.co.nz/design_build/construction This site has lots of other pretty cool ideas too!

Or as mentioned in the first link about using ICF blocks (Insulating Concrete Formwork), which I'm pretty sure you'd be able to do just the foundation walls with the ICF then pour the slab then carry on with normal timber framing, above that well thats the theory anyway.

Cheers,

Re: Insulated Concrete Slab Conundrum 

I think any polystyrene manufacturer would be able to help out, if it was polystyrene you were after, or in the case of the timber above....

Re: Insulated Concrete Slab Conundrum 

Hi phptec
You could very easily use a polystyrene block foundation the same as you would lay a concrete block foundation to get the edge foundation...most builders would be able to manage that.

Otherwise it is just a matter of lining the inside of the outside boxing of your foundation with at least 50 mm of polystyrene which can be purchased from normal suppliers of building materials in sheets.

Re: Insulated Concrete Slab Conundrum 

I have seen a number of web sites showing polystyrene insulation used on the outside of the slab edges showing how it deteriorates very badly over time. In the USA they tend to use rigid polyurethane for this purpose, covered by some sort of sheathing.

Re: Insulated Concrete Slab Conundrum 

It is not great science and done so for many many years.
PU sheeting is ideal but polystyrene (high density) is ok too.
You need to put the sheets against the boxing ,the plasic nails which normally come with it hold it on the concrete once it is poored need to be poked through beforehand.
The finish is usually just cement based plaster.

Re: Insulated Concrete Slab Conundrum 

Thanks for the info, and everyone is preaching to the converted! I know that permiter insulation is good as-is under-slab insulaton and purchasing it is a doddle and el-cheapo givn we have an EPS recycler who will custom extrude sheets of any sie for us.

I was after some ideas for existing and BRANZ appraised OR viable "Alternative Solutions" for standard 90mm timber framing on concrete slab.

Incidentally, and I neglected to mention, our builder is going to flow me some BRANZ testing documents from NZ AND overseas, which claim the heat-loss out of a slab is nothing major. Obviously, "Nothing Major" is subjective, those are my words, I'll let you know what the docs say when I get 'em.

The house is a 146sqm replica colonial villa, centrally heated, well insulated walls, ceiling (and hopefully floor - concrete) and as far as I can tell - and we're only at sketch plan stage, this is likely to be a standard 90mm bottom-plate sitting on concrete.

Can a rubberised or plastic material also be placed between the slab and the bottom plate to reduce thermal breaking?

Many thanks and I look forward to any and all ideas :-)

Re: Insulated Concrete Slab Conundrum 

Perimeter insulation on concrete slabs has been done for decades and is not a big deal.
No need for BRANZ Ltd appraisals.
They talk like thermal efficiency has been "just" invented here in New Zealand "first" in the world and they earn their golden share bei "appraising" dried paint.

Re: Insulated Concrete Slab Conundrum 

Plus your builder appears to have "no major" understanding of heat transfer and heat loss.
Most likely he has already scaremongered you away
from the thermally efficient PVCu joinery with the same old UV/uPVC fairytale for which he has no proof (apart from some poorly made spouting some times ago) and he just "heard" something as usual with these guys. Am I wrong ?

Re: Insulated Concrete Slab Conundrum 

Yes, unfortunately, you are wrong. In the 7 or so builders, architects and design/build companies we've come across, although not as clued-up as some, the present one is exceedingly wiling to not only accept any alternative soluton that I throw at him, but actively go ahead and investigate it himself. He's very hands-on, fantastic about offering ideas we'd not thought of before.

He ad his wife even invited us into his own Federation style villa (we're building a Colonial) with up to 25 of his neighbours having built their houses with him (He pointed them out to us on the way over) this is the most enthusiastic builder we've met!

FWIW we'd use thermally broken ally (or timber if we could afford it - we'll still get quotes from Eurovision mind) even if we were "scaremogered" away.

Cheers.
Russ

Re: Insulated Concrete Slab Conundrum 

http://www.maxsystem.co.nz/products.html

These guys have a great insulated slab and wall system. Well worth a look if you are considering building.

Re: Insulated Concrete Slab Conundrum 

Thanks for the Maxsystem link. I forwarded that to my builder as I did a Firth product called "HotBLoc" (http://www.firth.co.nz/product-information/home-building/masonry-blocks/hotbloc.aspx) it's an insulated, structural block that I suggested could be used in a single course to box the concrete+EPS instead of timber, thus serving as boxing and insulating perimeter.

I also "came up with" this (see: http://www.theruss.com/images/ICS.png - I put it in inverted commas as I'm not a builder or a tradesman!)

I forwarded this to the builder too, he replied quickly saying that he hadn't used the HotBloc for some years but is keen to see whether it might be fit for purpose and will consult his engineer about this as well as possible solutions regarding my "solution".

If anyone has any other tips, I'm all ears.

Thanks again people,

Re: Insulated Concrete Slab Conundrum 

Not sure if you've had a look at this site http://goldenbayhideaway.co.nz/design_build/construction . Maybe worth emailing them and seeing if they could send you any details of how they did this for their house?

I think the Max super floor system looks good too, although their website seems remarkably free of any details of the products. Sure there's the section of how the product works, I guess if you contact them they may have more literature you can look at?

Re: Insulated Concrete Slab Conundrum 

I heard that the Max Panel system is a straight out "steal" from the "Cornerstone Building System" along with the flooring. The guy was a "Cornerstone Licencee" who did not live up to the quality control systems for Cornerstone and left builders to cope on their own with the new system, instead of training them and obviously abused his Licence with his action in making his own panel as well.

"Cornerstone Licencees" have been told directly by the people in the Max Panel factory that this is the "Cornerstone System" which leaves me thinking if they are blatant enough to behave in such a manner, why would anyone trust them to possibly their largest purchase?

Re: Insulated Concrete Slab Conundrum 

Hi, plenty of comments above about edge insulation systems so I won't add to that other than it is needed, it is relatively easy and that there is a big difference in longevity between 'expanded' polystyrene and 'extruded' polystyrene when it comes to using it in or near the ground.
(Unfortunately I'll need to go and dig out boxes to find my books on which is which though so can't give best advice tonight, but one absorbs water over time making it less useful, while the other doesn't)

re underslab insulation - it's effectiveness depends on the type of ground that you are building on. If you are on rock or other conductive ground types, or have a high water table at times (eg at bottom of a hill or slope), then it is very important.
However if you are building on some types of clay that are themselves insulating, then the clay under the middle of your slab has the potential to become part of your thermal mass. It is not quick to heat though. It can take one or two years for your 'system' of floor and ground to warm up and stabilise before starting to return benefits. In this system, perimeter insulation is still needed to reduce horizontal losses under/around your footings.
What is very right for some sites can be very wrong for others.
regards, Russell.

Re: Insulated Concrete Slab Conundrum 

We're having in slab hw heating and the small amount of research I've done on this means we are having 100mm of poly under the slab and the timber thermal break around the perimeter of the slab as suggested in several BRANZ articles.
My issue though is that the builder insists on putting reinforcing bars from the footing into the slab every 600mm.
Surely this is making an easy path for any heatloss from the slab into the footing, making the timber thermal break useless.
He claims slabs should always be tied into the footing, yet there is no mention of the reinforced bars entering the slab in any article i can find.
I don't really want to put something round the perimeter of the footing as well.
Is he talking rubbish?
Have BRANZ just ignored the fact these bars tie the slab to the footings?

Re: Insulated Concrete Slab Conundrum 

Using timber as thermal break sounds not right to me.
Why don't you just use 40mm PU foam sheet with solid plaster on top to insulate the footing and slab.
It is really not rocket science and is has been done for decades!!

Re: Insulated Concrete Slab Conundrum 

Timber break is one of the solutions BRANZ suggests to isolate the slab from the footings. (maybe you misread where it was going, its not on the exterior).

Re: Insulated Concrete Slab Conundrum 

Simple reinforced raft foundations with a thickened edge are good for lightly loaded buildings, good in earthquakes and relatively cheap, it combines the wall foundations and the slab in one hit. This makes it the usual choice for kiwi houses. It is by no means the only way to construct your foundations and slab, but it is familiar to designers and builders who dare I say have probably "always done it that way" and unless poked up the jaxi with a sharp stick, will see no reason to change.

It is however as you have noted a thermally poor solution. This has not been a concern to the average kiwi until fairly recently, why bother with the slab when the windows hold heat like a colander holds water? For anyone wishing to install any form of "proper" underfloor heating, getting a highly insulated slab is paramount. Any heat you store in the slab will tend to leak into the ground if no insulation beneath or to the outside air if the edge is not insulated.

In Europe and to some extend North America, trench fill and block foundations are more common - even in earthquake prone areas. The slab is cast as a separate entity which allows full insulation to be added before pouring the slab. Even where raft foundations are used a separate sub raft is cast, this is insulated and a separate screed enclosing the UFH pipes is poured afterwards. So a bit more expensive to construct and hence not so attractive to your "do it cheap, never mind the quality" average kiwi.

Having said all this though, the steels are important to prevent differential movement between the slab and the foundation wall. The foundations will have a greater load and may settle into the ground more. Without tying it to the slab, there could be a tendency for the floor to migrate towards the ceiling slightly, or dramatically if an earthquake happened - not very desirable. In terms of insulation loss having steels poking through the insulation at 600mm centers will have an effect. If you have 100mm x 25 mm timber break then a length of 10m has approx area of 1m2. With steels at 600mm you would have 16 poking through. If they were D10 the the total steel cross sectional area is 16*25*pi = 1256mm or about 0.1% of the area of the insulation. However the thermal conductivity of steel is 50 compared with about 0.11 for timber. Without the steel the edge insulation value is about R0.22, with the steel it drops to about R0.18. The poster at 1.51pm suggested using 40mm PU foam. This is not a bad idea as this has a low thermal conductivity of about 0.21. Using this would give an edge insulation with steels of about R1.8.

Pete