Hearth thickness for insert- need advice

[Chicago fire]

I want to purchase new Montpelier insert cause my old VC model (0044) fan is caput. Looking at required hearth thickness I am pretty sure I don't have enough R value, although the salesman is saying otherwise. I have 4 in brick under 1/4 slate. Old house (1936) so it is framed in wood. My thoughts are to possibly slide in some steel underneath the brick to increase the total R value. I need approx 1.5 R and think I have about .9 R. Not sure yet if I can slide the metal in from underneath but I would do this from my basement where I can see the framing. Does this sound possible? Otherwise I believe my only alternative is to rip the slate and existing brick out and replace the brick with alternate material. I had really hoped to use the existing black slate which will go well with the new insert. Not looking forward to the work.

I don't think the R value was mentioned in the manual of the old VC insert. I have used this insert for over 20 years. Am I just lucky the house never went up in flames? Thanks for the input.

 

[midwestcoast]

I don't see how getting a bit of steel (sheet metal Iassume) under the brick would help as it would have next to 0 R-Value.
Go through the manual very carefully. Determine whether the stated R-Value is required for the entire hearth area or only directly under the insert. It may not need the state R-value out front of the insert. IF that's the case you could slip a sheet of Micore (and protecting layer like cement-board) under the insert to bump-up the R-value there.
Are you sure you have the required hearth distance out from the front of the door?
I'd also study the current hearth very carefully to get as accurate an estimate as possible for it's R-value (Including mortar, checking for double or triple coarses of brick...)
You could always raise the hearth to get your R-value without ripping out any brick.

 

[Chicago fire]

Thanks for the reply. Unfortunately they are talking about hearth in front of the insert. I have the distance out front but I fear that all I have is 4 inches of brick under the 1/4 or 1/2 slate for depth. Not much chance of raising the hearth because my fireplace opening is already pretty small - not too much room to fit the insert but I'll take a look and do the math. I had not thought of the mortar. With luck there will be plenty of it.

 

[oconnor]

I'd look for an insert that doesn't have an R value requirement. It can be a real pain to build up the hearth to gain R value when it is an existing hearth. My C450 called for R 2.92, which I just could not get, but I already owned the stove and had it installed when I discovered the R value requirement (damned "certified" installer). That said, I have never had anything more than a warm hardwood floor in front of the stove - definitely not any temps I would worry about.

 

[Chicago fire]

Thanks Brent, I have recently checked under my hearth in the basement and I think that I could remove the layer of wood that originally supported the bricks. My plan is to replace it with some mineral fiber board. The pieces of wood are 1 X 6's, each about 18 in long. Actual thickness in approx 3/4 in. If I can get mineral fiber baord in that thickness or close to it I can achieve the R value needed when you include the existing brick & slate. But this will be a pain to do.
I did check my old installation manual from my last VC insert and the olny requirement mentioned was that the hearth should extend 16 inches out - no R vales mentioned. I too have never had more than a warm floor ahead of the hearth. But who knows what heat was below the hearth. Hard for me to believe that a lot of heat could be transfered through 4 inches of brick and 1/2 inch of slate? I think VC is protecting themselves by stating this R value in the Montpelier manual. Lawyers are involved.
I have done a little shopping but it is hard to find an insert to fit in my small opening (29 X 23 inches) and also fit a small surround (Caprice) like the one offered with the insert. What I also like about the VC Montpelier is the 40,000 btu and variable speed ,heat activated fan. If any one knows of a comparable insert please let me know.

 

[midwestcoast]

If you want suggestions of other inserts I'd start a new thread with that title.

Don't know what you mean by "wood that originally supported the bricks", so I'll just suggest to be very carefull that you're not ripping out any structural support for the fireplace. That's a lot of weight sitting there & needs to be properly supported. Cement board below the brick would help on the r-value of course, but wouldn't provide support that the 1X6's may be giving. If in doubt, don't rip it out

 

[oconnor]

Thanks Brent, I have recently checked under my hearth in the basement and I think that I could remove the layer of wood that originally supported the bricks. My plan is to replace it with some mineral fiber board. The pieces of wood are 1 X 6's, each about 18 in long. Actual thickness in approx 3/4 in. If I can get mineral fiber baord in that thickness or close to it I can achieve the R value needed when you include the existing brick & slate. But this will be a pain to do.
I did check my old installation manual from my last VC insert and the olny requirement mentioned was that the hearth should extend 16 inches out - no R vales mentioned. I too have never had more than a warm floor ahead of the hearth. But who knows what heat was below the hearth. Hard for me to believe that a lot of heat could be transfered through 4 inches of brick and 1/2 inch of slate? I think VC is protecting themselves by stating this R value in the Montpelier manual. Lawyers are involved.
I have done a little shopping but it is hard to find an insert to fit in my small opening (29 X 23 inches) and also fit a small surround (Caprice) like the one offered with the insert. What I also like about the VC Montpelier is the 40,000 btu and variable speed ,heat activated fan. If any one knows of a comparable insert please let me know.

So, your hearth is 4" brick over 1/4 slate, but what is under the brick? Normally the hearth base is beneath that, and is likely poured concrete of up to 4 inches thick. Your insert needs a total R value of 1.49, so 7.44 inches of brick and concrete give you what you need. If there is 3.44 inches of concrete beneath your brick and slate, no problem.

The 18" 1x6 you mention are likely just forms - there is still likely joists that butt against the hearth. Once you remove a combustible, there is nothing to insulate, so no need to replace it. But you will need to get something between the bricks/cement and the adjacent joists if you don't have the 7.44 inches mentioned above.

If you raise your insert 3", the manual reduces the distance you need thermal protection from 18" to 16". If the joists are more than 16" from the front of the insert, then you are free and clear.

As for how much heat can transfer thru 4" of brick and slate, the answer is lots of heat. In the chimney, enough heat can transfer to turn adjacent timbers to charcoal. That said, you won't likely reach temps of that value out in front of the insert.

I looked at the manual for your stove, and the calculations in the manual for R and K value are wacky! Someone in quality Assurance at VC needs to go back to grade 9 math class.

 

[Chicago fire]

Yes, I noticed the math error also in the VC manual.
I agree my 1 x 6's are supports for the hearth. If I removed them I actually have solved my problem - no more combustable material to worry about and as long as I repace them with a non combustable to get back my support I'm ok. One small issue remains. And that is the joist that holds up the supports.
I can't remove them and I still need the R value there. My guess is that I'll probably end up removing my 1x6's and geting some 1/2 in. mineral fiber board in place of it and supporting that either by putting the 1 x 6's back underneath or using Durock. Thanks again for the inputs guys.

 

[oconnor]

Once you expose the cement, I wouldn't insulate it - there is no need. If it its insulated you will cause it to retain heat, whereas if it is exposed, it will radiate. The 1x6 boards don't likely hold anything up, but where there just to form the concrete hearth. It is supported by it's own weight under the chimney and fireplace.

 

[Chicago fire]

Brent, I still have a joist that looks like it supported the 1 x 6. That still needs the R value. Either I chisel away some wood from the top of this joist or I put something between it and the hearth. Mineral fiber board seems to be the best insulator for the thickness. Not sure where to get this stuff from. Anyone have other suggestions for insulators with good R value that is easily obtained? Thanks.