Hearth Pad Building Questions

[Poult]

I'm getting ready to start building the hearth for my Isle Royale that's going to be installed later on this summer. The R requirement for this stove is 2.22. It would be nice to get that 2.22 with an air space in the pad, so it wouldn't have to be such a thick hearth (low ceilings in the house, don't want to have the stove up too high).

I've been reading https://www.hearth.com/econtent/index.php/articles/k_values_what_does_it_all_mean
http://www.chimneysweeponline.com/horvalue.htm
http://www.woodstove.com/pages/hearthplan.html
plus a few threads on here to figure things out.

The only mention on air space in these articles says "ventilated". If you build a hearth pad with an air space in it, how do you ventilate it? Leave the back of the pad open? Is it that simple?

I could meet the requirements with three sheets of 1/2" Duroc board, one inch of air space, and the ceramic tile topping, all setting on a sheet of 3/4" plywood, and will add some flashing material to the sandwich to help dissipate heat as well. Is there anything else to be thinking about?

I really like the look of this stove, firebox size, top loading feature, but I sure wish it were a little easier on the floor pad!

Poult

 

[begreen]

I haven't seen a ventilated hearth pad, but it would be relatively easy to do it I guess. Somewhere I thought I read that rule of thumb is R1=1" air space. If that's correct, metal studs should give you R 3.5 or on it's flat face R 1.5, + durock on top. But if you want a shallower pad, then using 2 layers of micore (1" total) + 1/2" durock will get you there with only 1.5". No need for an airspace. You can add a layer of sheetmetal between the micore and durock if you want to be super safe.

 

[webbie]

You don't need to ventilate a hearth pad to get the standard R-Value of air.

As BG says, there are various ways to do this. "Hat" channels for sheet rock are about an inch (I think), so you could put one sheet of wonderboard on the ground, a hat channel and then 3 sheets on top and some tile - or something like that. Total thickness would be about 3.25" and R value would be enough. Having three sheets on top with some glue between and screwed to the Hats should allow some spanning - maybe the hats a foot apart.

Edit: looked up hat channels and they are 7/8" or 1 1/2". Either would do - the R value for an air space is given as approx 1 - for a range of thickness. The insulating value does not increase much without a barrier in the air - in your case, the wonderboard over and under. Add one more thickness above to be at a 2.2+ for certain. So that would be:
1/2 under
7/8 channel
2" over
1/4" or 1/2" tile
----------------
still under 4"

 

[fossil]

In this case, "ventilated" certainly doesn't mean forced convection. You don't have to install any sort of fan or anything real fancy. It simply means that the airspace that you're counting as insulation isn't completely sealed up airtight. Unobtrusive gaps provide all the ventilation you need. Rick

 

[webbie]

In the case of a hearth, even a "dead" space should do the trick. The ventilated air spaces are only required in walls where conditions are different - for instance, most walls only have one layer of protection, then an air space, then the combustible wall.

Most air insulated chimney uses dead air space (addressed in another thread somewhere), which achieves pretty amazing insulating values...again, in combination with the barriers.

A perfect example is insulated glass - another dead air space which does the job.

 

[Poult]

Thanks, everyone. I'm probably obsessing over this a little too much and trying to make it more complicated than it is, but I thought I'd ask before doing something stupid that would make more work later.

I've got some old garage door rails to use instead of metal studs or the drywall channel, but it's the same idea, not to mention "free" from my "jeez, that looks like good metal and I might need it some day so I'd better not throw it out" scrap pile. Cheap is good these days.

Poult

 

[fossil]

...I thought I'd ask before doing something stupid that would make more work later.

Aw, c'mon...what fun is that? Rick