Is small gap in micore a concern for me not meeting hearth pad R value, and what should I do?

[KennyK]

I am getting everything prepped for my upcoming Woodstock Ideal Steel installation, and I ordered a modular hearth pad that has three interlocking pieces (https://www.amazon.com/dp/B01MSAG5ZP/?tag=hearthamazon-20). The IS requires minimum of 0.41 R value and this hearth pad has 1.03 and ember protection as it's micore 300 covered in powder coated 16 gauge steel. The sheet metal has interlocking pieces so an ember can't reach the floor below, however, there is a small gap between where the micore ends on one piece and begins on the next (about 1/4 inch or so). I will be putting this hearth pad partially on top of my existing marble fireplace hearth and partially on top of 3/4 inch plywood over a wood floor. One of the two seams where the pieces of the hearth pad meet (and where the gap is), will be directly under the middle of the stove and on top of the plywood, and the other will be a few inches in front of the stove (also on top of the plywood). Should I be concerned with this little gap in the micore or is it a non-issue. If it is a concern, I suppose I could try to get some micore 300 (or something else with at least R 0.42) to put below the hearth pad instead of plywood, but I can't go over 3/4 inch thick below the hearth pad or I won't be able to fit the rear vent flue. Any input is greatly appreciated. If it helps here's a link to the hearth pad manual (http://files.drolet.ca/upload/image...65.1860424340.1504699920-508711186.1502221010). And, by the way, the hearth pad, while very simple, looks really nice with a great finish and the seems don't look bad.

Thanks!

 

[KennyK]

Any thoughts on this?

I found a place in my area that sells Micore 300 for $42 for a 4'x8' sheet, so I'm thinking that I might get a sheet of that and use it instead of plywood under the hearth pad - would that work? The Micore is 1/2 inch, so if I need another 1/4 inch, I could put 1/4 plywood under the Micore. Thoughts?

 

[yooper08]

So just thinking about the cross section of this. You have the sheet metal, then the Micore 300 that came with the pad, then an additional Micore 300 sheet at 1/2" as a continuous piece to close the gaps, then 1/4" plywood?

Micore 300's R value is 2.06 per inch, so a 1/2" sheet will have an R value of 1.03. You're effectively doubling the R value of the pad except for where you have the small gaps, so you should be more than safe to put the 1/4" plywood underneath.

Put differently, with the additional Micore sheet, your pad will be 5x the IS requirement.

 

[RobbieB]

An eighth in of air has a R of .98

 

[KennyK]

So just thinking about the cross section of this. You have the sheet metal, then the Micore 300 that came with the pad, then an additional Micore 300 sheet at 1/2" as a continuous piece to close the gaps, then 1/4" plywood?

Micore 300's R value is 2.06 per inch, so a 1/2" sheet will have an R value of 1.03. You're effectively doubling the R value of the pad except for where you have the small gaps, so you should be more than safe to put the 1/4" plywood underneath.

Put differently, with the additional Micore sheet, your pad will be 5x the IS requirement.

Thanks @yooper08! What you described isn't exactly what my concern was - perhaps I didn't outline it clearly. My concern is that there is a small gap in two places in the hearth pad where there is NO micore, rather a 1/4 or so where the micore ends, but the metal on top of it continues. I think this is so there is room for the three pieces of the modular hearth pad to clip together, though it seems to be more space than needed in my opinion. My concern is that this would leave two 1/4 inch spaces, one right under the stove and one just in front of it, with only sheet metal and 1/4 inch of air between the stove and the wood floor/plywood, thus not meeting the R value required for the stove. Now, if I put micore 300 down under this hearth pad, instead of the plywood I had planned on, that will take care of that issue as it will run underneath these gaps.

An eighth in of air has a R of .98

This is interesting, @RobbieB. However, being a "better safe than sorry" kind of person (and a bit of a worry-wart), I'd feel a little better with a little more than air between the hearth pad and wood, plus the metal bends over each edge of the pad, so that will be touching the wood, and I'd likely worry about heat conduction! My latest thought, as I wrote about above, is to use micore 300 under the hearth pad instead of plywood (and if I need a little more height, adding 1/8 inch plywood between the floor and that layer of micore). This would cover me for the 1.03 R value of the lower sheet of micore under the gaps and for the rest of the pad it will double up on the micore R value, which will do no harm. At $40 for a 4'x8' piece of micore it's not a huge expense.

My two questions now are:

1) How will micore hold up well under the weight of this very heavy stove (650 pounds or so with no wood in it!)

2) How is micore to cut and do the cut edges hold up like the plywood? Can I cut it with an electric circular saw?

Any input is welcome!

Thanks!

 

[Simonkenton]

I have never used micore, but, if you are concerned that it won't hold the massive weight, just use concrete board, Durock. It is made of concrete. Easy to cut.

 

[yooper08]

Thanks @yooper08! What you described isn't exactly what my concern was - perhaps I didn't outline it clearly. My concern is that there is a small gap in two places in the hearth pad where there is NO micore, rather a 1/4 or so where the micore ends, but the metal on top of it continues.

We're on the same page.

 

[KennyK]

I have never used micore, but, if you are concerned that it won't hold the massive weight, just use concrete board, Durock. It is made of concrete. Easy to cut.

I can't seem to find the R value for Durock - do you know? I know it's not a huge concern, but as I'm trying to meet the R values for the gaps in the hearth pad I have, I would want to know.

My guess is that the micore should withstand the weight, as many hearth pads use it, but it would be great to hear if anyone here has experience with it, especially as the Ideal Steel is heavier than most stoves. Also would be helpful to know how it cuts.

 

[begreen]

1/2" Durock NexGen is R=.39. 1/2" original cement board Durock is R=.26

 

[KennyK]

1/2" Durock NexGen is R=.39. 1/2" original cement board Durock is R=.26

Thanks @begreen! Do you have any experience with Micore 300? In particular, any thoughts on me putting it under the hearth pad I bought and on top of a wood floor instead of plywood? Thoughts on how it will hold up in this situation under a 650 pound stove? Do you know how it is to cut and if you can use an electric circular saw on it? How the edges where cut hold up? Etc...

Thanks!

 

[georgepds]

Consider the average heat flux, which is ~half that of a smaller r pad

Now consider the heat transfer in the metal cover on the bottom.

It will tend to transfer heat from the no micore hot spots

You should be ok unless the gaps are~ 50%of the hearth area