HardieBacker instead of FoamGlas?

[JoeT62]

I came across a type of HardieBacker board that is touted as "100% waterproof"
The Home Despot (not a typo!) nearby is selling it for less than $25 for a 3' x 5 ' sheet

Any thoughts on using this rather than Foamglas for the under CalSil layer?

[mongota]

Hardi is "waterproof" in that it won't be physically damaged by moisture wetting and drying cycles. But it is moisture permeable, in that it will allow moisture to pass through it. Think of it as being a rigid yet still porous sponge.

[JoeT62]

This is a different version of the usual HardieBacker that is ANSI A118.10 waterproof. It is called HardieBacker with HydroDefense. It definitely claims true "waterproofness"

[david s]

Assuming it is totally waterproof, you still need to provide some channels for the moisture to escape. Cutting the stuff into pieces and replacing it so that there are gaps between them and positioned so the pieces don't cover the weep holes will still be required.

[mongota]

My apologies. The Hydrodefense board is indeed ANSI 118.10 'waterproof'.

[JoeT62]

Assuming it is totally waterproof, you still need to provide some channels for the moisture to escape. Cutting the stuff into pieces and replacing it so that there are gaps between them and positioned so the pieces don't cover the weep holes will still be required.

Thanks for the replies! Things have gotten a bit more sophisticated since my Pompeii build a decade ago!

My understanding is that weep holes and channels are not needed with FoamGlas - why would this stuff require them? I'm sure I'm missing something, but Foamglas (as a closed cell foam) is also not permeable - wouldn't FoamGlas also trap moisture?

[david s]

Yes I think channels between pieces of foam glass are also required. But as it at least raises the calsil off the surface of the concrete slab it reduces the wicking effect, as well as not giving the cal sil wet feet from possible water entry from any crack that may develop at the base of the outer dome render. This is less likely to occur if the design is an enclosure rather than an igloo. I use an additive in the concrete for my supporting slab which reduces permeability to prevent or at least to reduce water wicking. The fire will eventually push out the moisture, but methods to allow its exit and prevent its entry save both time and fuel.

[CoastalPizza]

What is unknown is whether the waterproofness of HardieBacker with HydroDefense is maintained in a high heat environment. Since it is intended for floors and walls of places like bathrooms, the additive used to make it waterproof may break down at high temperatures.

[JoeT62]

What is unknown is whether the waterproofness of HardieBacker with HydroDefense is maintained in a high heat environment. Since it is intended for floors and walls of places like bathrooms, the additive used to make it waterproof may break down at high temperatures.

Agreed. But if the far side of 2" or 3" of ceramic board is still at high heat there are bigger problems!

[NCMan]

A bit confused why Foamglas is being compared to this type of Hardiebacker, as they each do totally different things (?) One insulates, the other doesn't.

[Stlavsa]

Anything ever come of this? I do not want to use 2" foam glass as I have not budgeted for the additional height it will add, but I was thinking that a 1/2 slab of waterproof cement board may work to keep the Calsi board elevated off the hearth?