K value

[esh21167]

Hi all,

Will be installing a woodburning insert in the future. Winter is almost over here in PA and no deals to be found, so may not be until spring/summer. I have only 13" of hearth flush with the floor and know I need floor protection. Local dealers and some info from the forums here have pointed me to the hearth extensions at American Chimney. (broken link removed)

Some inserts give K or R value requirements and some do not. I contacted American Chimney about the above extensions/stoveboards and they have given me this figure: K=0.84 BTU in/ft2 hr F.

Is the .84 really all I need to worry about? For example if an insert calls for .35 K value, would these extensions more than meet the requirement? Would be placed on hardwood floor in front of insert. Thanks.

Eric

 

[esh21167]

I guess my question is more like this. All the manuals I've seen just give a K or R with a number. Is the BTU in/ft2 hr F just the more complete or accurate listing of the K value?

 

[Jags]

Yes, you have that part figured correctly, but I think you will find that virtually every stove/insert will give the required hearth insulation value required for that model in their manual. Make sure that you meet or exceed the requirements for the hearth and extension for the stove you are going to install.

 

[webbie]

It seems that a lot of stoves call for about a value of 1. Yes, some are more and some are less, but as jags mentions it will be specified exactly in the stove or insert manual. In your case, there are two things to be concerned with - first the actual existing hearth that the insert or stove sits on (especially if it extends out from the fireplace) and then the hearth extension (18" in front of the doors).

Download the manuals for the stuff you are looking at.

 

[esh21167]

Sorry if I'm not getting all this.

This is from a manual of one insert I'm looking at: Hearth protection must be noncombustible insulating board, having an R value of 2.92 (0.343 K-value).
It also list this formula for K to R conversion: If K-value is given with a required thickness (t) in inches: R = 1/K X T.
This would mean that a R increases as K decreases. So if the board mentioned in the original post has a K of .84 (I'm guessing that's per inch from the K=0.84 BTU in/ft2 ), its R is only 1.19, no where nearly high enough.

I was hoping to avoid remodeling and building a new hearth an thought these boards would fit the bill. Now I'm not too sure.

Thanks again.

 

[webbie]

Well. first question is whether they are talking about the heart itself or the hearth extension or both. In most cases, the extension does not have to have the value of the hearth (the area where the unit sits).

It is safe to assume a pretty good R value back in the fireplace, since a fire has been built directly on it.

So what you need to do is find out about the existing hearth extension - can you look in the basement or crawl underneath of it and determine the construction of it?

once you know this - and also whether the stove requires less for the extension part - then we can check out some materials with high R values. Off the top of my head, an inch or so of Micore on top of the existing hearth might do the job.

 

[esh21167]

One insert I have been looking at is the Jotul C450 Kennebec. The manual doesn't seem to differentiate between "hearth" and "hearth extension". In your experience, do some models state different values between the two?

Underneath the existing hearth is some type of masonry material 5" thick that extends about an inch or so beyond the hearth size. I can't see the sides becasue it is boxed in with the floor framing and its form. Beyond that is just the floor material: About 1/4" or so of hardwood on top of 1/2" plank subfloor (not plywood) on the floor joist (2 1/2 x 8 beams).

The house is about 90 years old and we have used the open fireplace quite a bit over the last 14 years, so I agree that whatever is under that hearth is not an issue.

 

[webbie]

Looking at that manual, they are speaking about only the area in front.
The specified R-Value could be obtained in various ways. It will take up a certain height.

For instance, 1" of micore capped with 1/2" of wonderboard or durock(and tile) will- in conjunction with your existing hearth. Even though this may be a little short at the far end of the 18". I think this will be fine - especially with a sheet of metal against the wood underneath. Considering adhesives, this would be a total system height of less than 2".

This hearth could be built only in front and then the insert could rest on solid 2" block set back in the fireplace.
The new extension can be trimmed in wood around the outside to hold the boards and tile.

 

[esh21167]

Craig,

Something you wrote just struck me. Trimming in wood. I've looked at hearth extenders online around that same R value and they are all trimmed with wood. Can they go right up to the front of the insert? Wouldn't the wood trim be a combustible material right next to the insert?

 

[webbie]

No, I mean trimming around the outside of it - to hide the edges and give something to grout to.
see the hearth at:
https://www.hearth.com/econtent/index.php/wiki/Install_a_Pellet_Stove/

the wood still has to be 18" in front.

 

[esh21167]

Got it, Thanks for the input.