# Looking for expert advice for adding insulation under a gas fireplace



## MileHigh80228 (Dec 9, 2012)

Hello, 

Newbie to the forum. I'm looking for some advice regarding insulation below a gas fireplace. 

We recently remove a large brick hearth. As you can see there is no insulation under the fireplace. The cold air comes in unabated and we would like fix that. 

The three areas under the fireplace that need attention are all made entirely of wood (2x on 3 sides and 3/4" plywood on the top). My understanding is of course wood has a much lower combustible temperature than either fiberglass or rock wool insulation. 

Is there any reason that I can't simply insulate the three cavities with product (non-paper faced) from the local home improvement store? 

The town home was built in 1985. I'm unable to find any brand info/model number/specs pertaining to the fireplace. 
Thank you for your thoughts and advice


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## joecaption (Nov 30, 2011)

Got quite a mess there.
I'd like to know how air is coming in from what looks like a sealed area.
Is that funky framed area cantileavered out the back side of the house?
If so that should have been insulated outside under the cantileaver.
Trying to imagine what would make someone think using 1 X's for joist was a good idea.


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## MileHigh80228 (Dec 9, 2012)

joecaption said:


> Got quite a mess there.
> I'd like to know how air is coming in from what looks like a sealed area.
> Is that funky framed area cantileavered out the back side of the house?
> If so that should have been insulated outside under the cantileaver.
> Trying to imagine what would make someone think using 1 X's for joist was a good idea.


It is 20 degrees out this morning, no insulation, equals cold air.

The joist are 2x10s. Please see photo for cantilever build. 

Any thoughts on insulating these areas?

Thank you.


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## joecaption (Nov 30, 2011)

Why is there what looks like a dryer vent in that outside wall? That alone will let in some of the air in.
I'm concered now you have cut away all the support that was supporting that whole bump out. 
It really should have been designed into the floor joist system not just sitting on top of the floor like that.
If you can reach in far enough I'd use some expanding foam to try and seal all the areas where the plywood meets the framing then use 2" foam as insulation.
http://www.totaljoist.com/Assets/details/B1/iSPANDetail_B111.png


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## Windows on Wash (Aug 30, 2011)

Seal all the seams in the framing and fill the cavity with insulation.

Pull that fiberglass out and seal up wherever you can. Roxul is a good insulation because it does not compress overtime.


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## MileHigh80228 (Dec 9, 2012)

joecaption said:


> Why is there what looks like a dryer vent in that outside wall? That alone will let in some of the air in.
> I'm concered now you have cut away all the support that was supporting that whole bump out.
> It really should have been designed into the floor joist system not just sitting on top of the floor like that.
> If you can reach in far enough I'd use some expanding foam to try and seal all the areas where the plywood meets the framing then use 2" foam as insulation.
> http://www.totaljoist.com/Assets/details/B1/iSPANDetail_B111.png


No sure about the vent. My guess is it has something to do with providing air to the gas fireplace. We don't get appreciable air leaks through the fireplace. Only the area below is cold. Thank you for thoughts on insulating the area.


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## MileHigh80228 (Dec 9, 2012)

Windows on Wash said:


> Seal all the seams in the framing and fill the cavity with insulation.
> 
> Pull that fiberglass out and seal up wherever you can. Roxul is a good insulation because it does not compress overtime.


Thank you.


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## Windows on Wash (Aug 30, 2011)

MileHigh80228 said:


> No sure about the vent. My guess is it has something to do with providing air to the gas fireplace. We don't get appreciable air leaks through the fireplace. Only the area below is cold. Thank you for thoughts on insulating the area.



Yes.

Combustion air for the fireplace.


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## Gary in WA (Mar 11, 2009)

Where are the rest of the joists that supported the cantilevered section? Now the sidewall sheathing/roof rafters are carrying the f.p. load alone. You need to somehow side nail new joists or exterior support the box permanently. The outside combustion air supply vent is normal for the unit. The box/cavity is radiation-coupled to the decking temperature outside; http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/

Gary


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## joecaption (Nov 30, 2011)

That had to be a DIY add on.


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## MileHigh80228 (Dec 9, 2012)

GBR in WA said:


> Where are the rest of the joists that supported the cantilevered section? Now the sidewall sheathing/roof rafters are carrying the f.p. load alone. You need to somehow side nail new joists or exterior support the box permanently. The outside combustion air supply vent is normal for the unit. The box/cavity is radiation-coupled to the decking temperature outside; http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/
> 
> Gary


 
Thank you for you valuable advice. I removed three feet of the cantilever that was under the old hearth in the living room. Long story short, we will be removing the fireplace in the spring.


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## MileHigh80228 (Dec 9, 2012)

joecaption said:


> That had to be a DIY add on.


DIY demo.


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## Windows on Wash (Aug 30, 2011)

GBR in WA said:


> Where are the rest of the joists that supported the cantilevered section? Now the sidewall sheathing/roof rafters are carrying the f.p. load alone.


We see them like that all the time. They are not full height units usually and are direct vented to the exterior right there out of the picture frame.

Still very questionable construction but certainly not the first unit I have seen like that.

Getting that stuff sealed up will really warm that area up quickly.


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## Gary in WA (Mar 11, 2009)

It was answered in post #11... , I have built close to 200 fireplace chases (for both wood and gas) as pictured--- never one *without floor* *joists extending* into the house for the exterior cantilever. lol Now, the sidewalls sheathing of the chase is holding up the box, good thing most of the f.p. weight is on the forward edge over the concrete wall. 

Gary


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## Windows on Wash (Aug 30, 2011)

GBR in WA said:


> It was answered in post #11... , I have built close to 200 fireplace chases (for both wood and gas) as pictured--- never one *without floor* *joists extending* into the house for the exterior cantilever. lol Now, the sidewalls sheathing of the chase is holding up the box, good thing most of the f.p. weight is on the forward edge over the concrete wall.
> 
> Gary


Missed that part.

Thanks for the catch. I have seen a couple this week with suspended bump outs and zero cantilever.

They were smaller but they were still bumped out.


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