Hi everyone, We recently purchased an old house (built in 1952) that is brick exterior, cinderblock exterior walls, and plaster finishing on the inside. The basement was finished in the 90's some time and has sheetrock walls. I doubt there is much insulation between the foundation wall and the sheetrock in the basement.

The furnace (along with the water heater) is currently in the only unfinished room in the basement and that room is vented to the outside for the combustion air source. Because it is vented to the outside, it has a solid door separating it from the rest of the basement. The HVAC system is exhausted correctly up a dedicated flue of the chimney which also backs on that room. However, this room is not insulated in any way. Obviously, without insulation, this lets a lot of cold air into the rest of the house even with a solid door on the room. The heat is lost from the rest of the house through the ceiling of this room and the walls of this room. Note that I don't have any plans to ever finish that room - I'd rather just keep it as storage and furnace space. It's the size of our Master bedroom (which isn't huge), but provides adequate space for storage shelves, a water heater, and the HVAC unit. From what I understand, I basically have two options to make that situation better, from an energy efficiency standpoint:

Option #1: Leave the external combustion air source and insulate the interior walls and ceiling of the room from the rest of the house. I was thinking denim or fiberglass insulation on all interior walls and ceiling of that room. Obviously, because it is vented to the outside, there is not much reason to insulate the exterior walls of this room in this scenario. I was also considering a better insulated door to that room since the room has an external combustion air source and therefore doesn't need a louvered door. If I went this route I would also insulate the hot water pipes in that room and put a blanket on the water heater - even though the water heater is a decent one and is new.

Option #2: Close off the external combustion air source and insulate all the exterior walls of that room. If I go this route I'm not sure what insulation would be best yet, so I'm open for ideas there if you think this option #2 is better than option #1 above. If I do this, I will also have to install a louvered door instead of the solid one I have on the room now so that there is adequate combustion air available.

So, which approach is best? :)

How old are your furnace and water heater? if they are more than 10-15 years old I would say that a third option would be to replace both the furnace and water heater with new direct vented appliances. In this way you could close off the outside combustion air, insulate the exterior walls of your mechanical/storage room and enjoy lower utility bills and significantly greater comfort.

Posted By KI7OM on 03 Oct 2013 06:09 PM
How old are your furnace and water heater? if they are more than 10-15 years old I would say that a third option would be to replace both the furnace and water heater with new direct vented appliances. In this way you could close off the outside combustion air, insulate the exterior walls of your mechanical/storage room and enjoy lower utility bills and significantly greater comfort.

Thanks for the input. The HVAC system is right at about 8-10 years old and the water heater is less than a year. I don't have that big of an expense in the budget yet but would like to do that at some point when funds allow so I'll keep that in mind. In thinking about that for the future, it does seem like #2 would be the way to go now though, doesn't it...

We need some more info to give you good advice. Where do you live? What is the fuel source for the furnace and water heater? Do they share the same flue? what do you pay per KWH for electricity? What are your annual fuel costs? cost per unit of fuel? What is the size in Sf of the room that the water heater and furnace are in? What is the btu rating of the water heater and furnace? how much of your ducts run in the unheated portion of the basement and are those ducts insulated? Are the seams of the ducts sealed? The answers to these questions will be helpful. Cheers, Eric Anderson

Option 3: Do nothing.
If you take combustion air from inside the home (your louvered door idea) it will be air that you paid to condition and it will be replaced by infiltration air that you have to pay to condition (heat or cool).
Code required combustion air openings if properly sized (generally 2 openings (high and low) of 1"sq/4,000 btu's of appliances combined) you likely won't see a huge temp difference in that room. Closing them off likely will add to draftiness of house.

Option #2 is the right way to go, taking combustion air from conditioned through a louvered door or floor grates (if needed). The absolute volume of combustion air is pretty small, and will only be drawn during active heating. But an uninsulated foundation is a parasitic load 24/365, with both thermal & humidity consequences. Insulating between the joists just means the standby & some of the distribution losses accrue to the basement rather than the conditioned space. By insulating the foundation walls instead, you bring it all inside of conditioned space.

The details & R-values of foundation insulation that makes sense will vary by climate. Where are you? (If US, got a zip code?) Poured concrete foundation walls, or CMU?

Insulate the furnace room exterior walls and install a barometric relief damper so that the equipment gets air only as needed and yet doesn't pull it through the house walls (ie, create excessive drafts).

Thanks everyone for the responses so far. I've thought through a lot more options with the comments that have been made and that's been very helpful. I apologize for not including the details that I should have about the house. Here they are for reference:

Home location: Salt Lake City, Utah just East of downtown a few miles.
Furnace room sq footage (currently unfinished): 160 sq ft.
Furnace fuel source: Natural Gas
AC fuel source: Grid electricity
Water heater fuel source: Natural Gas
The HVAC and the water heater do share the same exhaust plumbing up a dedicated flue in the chimney (i.e. separate from the two fireplace flues)
KW-h price for electriciy: $0.08-$0.11
Per decatherm price for natural gas: $7.73529
Duct location: Only the ducts in that furnace/mechanical room are in the unheated part of the basement. All other ducts are in heated areas of the basement. The seams that I can see in the mechanical room are mostly sealed but I do see some that aren't...hmm.
Duct insulation: None of the ducts themselves are insulated.

I don't know the exact BTU of the water heater, I forgot to look before I left but it's a standard Rheem 40-gallon water heater so probably 40,000 BTU. I'll have to find the BTU rating for the furnace somewhere.

Posted By jonr on 04 Oct 2013 11:10 AM
Insulate the furnace room exterior walls and install a barometric relief damper so that the equipment gets air only as needed and yet doesn't pull it through the house walls (ie, create drafts).

This sounds interesting. I'll have to do some research on the barometric relief damper. Would I install this on the external vent in that room?

Yes, it goes between the furnace room and the exterior. As always, check what code requires (high/low room interconnects also?) and install a CO alarm.

I'm with jonr on this one but I would use a simple air trap.
http://furnacefactorydirect.ca/html/installation.html
The pink bucket in the pic makes up the trap but there are many commercial ones available.
With a trap in place, the room needs to go into a slight negative pressure to pull air in through the combustion air pipe, therefore you only get cold air when one of the appliances is exhausting air out the chimney. The cold incoming air will go up the chimney with very little impact on the rest of the house as only enough is drawn in to supply the vent capacities of the appliances.

Closing off the combustion air is not an option. It is required.
Canadian codes have used this setup for a number of decades already and it works quite well. With out it any other air loss (exhaust fans, vacu flow) to the building might put the chimney in back draft.

Many codes require that the furnace room be sealed from the sleeping/living areas to prevent combustion by-products from entering these areas. Putting louvers in the door would bypass this safety plan.

My understanding is that with a damper, you can adjust it to allow -5 pascals of pressure before it opens. This allows appliances to draft properly but still provides resistance to stack effect induced flow. Any lower level vent that allows flow at ~0 pascals will allow outside air to flow in, up and then out any cracks in the upper floor(s). This is bad for energy usage and puts moisture into the walls.

OK based on the info so far, I think you would be safe putting the combustion appliances in conditioned space and insulating the exterior walls. You will need more than just a louvered door though. What you want is a high vent and a low vent, to the rest of the house each having a net free area that is equal to 1 square inch per 1000 btus of the combined input rating of the water heater and furnace. This complies with the national fuel gas code. Before I did this I would close off the outside air connection in the furnace room, open the door to the room completely and ask your gas service tech to measure the draft of both appliances and use a combustion analyzer to measure CO output. One thing to look out for is a Large return duct register near the room as this could negatively pressurize the space (bad) The other thing that would make me worry was if you had a large kitchen stove vent ie one of those “professional ranges” with huge rangehoods, that would change my answer, or at leastrequire some testing. Install a CO detector in the furnace room if you don’t have one there already. When you get ready to upgrade your systems, you would be wise to consider a sealed combustion replacement furnace and a power vented water heater. Cheers Eric

Dampers fail and at least with the ICC are not permited on combustion air openings. You can duct it to a 5 gallon pail which will impede un needed infiltration into the furnace room.
Drawing combustion air from within the house is permissable but only if you have X feet of space that freely communicates.........
CO detector is great advice to anyone with gas appliances. Put one in your bedroom as well.

Dampers fail and at least with the ICC are not permited on combustion air openings

On required openings. If you are allowed and set up to take combustion air from the house and also add an exterior damper, then there is no problem.

a 5 gallon pail which will impede un needed infiltration

Unless the room is not well sealed from the house and there is stack effect negative pressure pulling on it (which there will be). Seal the room well if you go this route (but ducts, furnaces and water heaters in unconditioned space are still a bad idea).

"Dampers fail and at least with the ICC are not permited on combustion air openings"
"On required openings. If you are allowed and set up to take combustion air from the house and also add an exterior damper, then there is no problem."

Sure you could, but what's your point? Do both? Now you have a louvered door between the house and the cold air inlet. Not the best plan, but codeworthy at least.

Jon - if you put the required combustion air inside a pipe with at least twice its volume, you can slide the internal pipe up or down to hit the best spot for enough combustion air and little spill over. The point is to keep the room sealed and to have the c.a. inlet as close to the furnace burners as possible. The only cooling effect will be from the short distance the air travels as it spills out of the top of the secondary and gets pulled to the burner. This will cost less then, as Joe mentioned, using air you have already paid to heat.
Even if it didn't the five bucks or so it cost in heat loss is the value of you life.

OP's house is cinder block with plaster so despite its age, it will be reasonably tight construction but have a large heat loss. If he is worried about the heat loss in the furnace room, he probably will tighten up the window etc. as well. This means there will be little redundant air in the house but the furnace will be running no stop during a cold snap.

Thanks again for the replies everyone. Given the discussion here and also taking into consideration costs, here's what my current plan is:

Leave the external air source (after reading up on the IMC 2012, it seems like that is best) and then insulate the ceiling of the mechanical room with Roxul R30 (rock wool batts) and insulate the two interior walls with Roxul R23 batts - 2x6 walls for that room. Because I'm leaving the external vent open to the outside, there is no need in my mind to insulate the exterior two walls of that room (remember it's a basement room and is 75% below grade).

Once all of that insulation is in place and that room is effectively blocked off from the living spaces in the house, I'll look more into doing a cold air trap if I feel I need more than just the insulation.

SO, now one more question... since there is no insulation on the interior walls of that room currently (the walls that connect to the rest of the basement living space), there is obviously no vapor barrier either. But in reading about the insulation I obviously see a lot of talk about vapor barriers. The problem is that everything that I've read says the vapor barrier should be on the living space side of the insulation/wall. Since the living space side of these interior walls are already finished there is no reasonable way to put up a vapor barrier on that side. So... do I need to worry about a vapor barrier in this case or just forget about it since I'm using the mold/mildew-resistant rock wool and I live in Utah where the air is usually pretty dry??

Thanks for the help!

PS, lastly I will also insulate all the duct-work in that room since I'll be leaving the external combustion air source.

As a fellow Utahan living in the Salt Lake City area I would say you probably have little to be concerned about. If you live in the Park City area or other areas above 7000 ft. it might be a different story - more temperature extremes. I my opinion it would in large part be dependent on how tight the rest of the house is (infiltration and vapor wise), whether you are adding extra humidity by running a humidifier or minimal bath and cooking ventilation , if you have other mechanical exhaust or fresh air systems that will be running for prolonged periods of time. Running bath and kitchen vent fans and fireplaces create a negative pressure in the house and unless the mechanical room is sealed that is where much of your makeup air will infiltrate to the rest of the house. Do you have a tight seals around the the plumbing, flue stack space, supply and return ducts entering and leaving the mechanical room or do you have several inches space between the duct work and the build-down. Are all the ducts sealed with mastic in the rest of the house to prevent back flow or pressure differentials between the conditioned living space and the mechanical room? Are all of the ducts in the mechanical room well insulated?

I'm assuming that your water heater will also be located in the same room so you will want to make sure to have all of the water pipes well insulated, both cold water supply lines and heated water out. Your water heater should have at least an extra 2-3 inches of insulation added around the jacket - without blocking the burner supply area or the draft diverter. With a lower temperature in the room the tank losses will be relatively high. With most conventional gas water heaters the heat exchanger in a cold space like you describe acts as a *heat exchanger* when it is not firing and you end up with a nice cold draft going into the cold burner up the heat exchanger, being warmed and then drafting right up the flue. Just put you hand of the single wall vent 10-15 minutes after the water-heater has fired and you will find it is warm. The draft diverter may alleviate some of that that but it still causes your water heater to fire more frequently in the winter to maintain a tank full of warm water.

Most of these fixes are simply labor intensive and require minimal expense and materials but can make a very significant difference. If the mechanical room is in reality isolated from the rest of the house then some sort of vapor barrier on the warm side (back or your finished wall) might be worth the effort. Though far from a perfect solution 6 mil poly cut, fitted and taped into the stud space might be the only economical solution. Such a situation would come close to emulating direct vented systems but as you indicated that was not in the budget.

...leaving the external vent open to the outside...I will also insulate all the duct-work in that room since I'll be leaving the external combustion air source.

You will have created the rough equivalent of putting the furnace and water heater in an unheated attic. I suggest some google searches on that. The pipes will probably freeze when there is no flow through them.

On the other hand, I will admit that soil is a limited form of insulation.