How tight to seal garage?

[dragoncar]

I've got a semi-insulated garage under my master bedroom (at least I assume it's insulated because there is drywall but maybe there's nothing behind the drywall, and the door is insulated).  With cold weather coming, I'm planning to foam a number of gaps and openings.  However, my furnace is in the garage so I know it needs some kind of fresh oxygen to burn.  Is there any change I'll seal this too tight?  I know there will always be some gaps around the garage door, at the bottom, etc. but I don't know anything about how much air the furnace needs.  Any tips appreciated.

[arebelspy]

Have you tried googling it?

[ncornilsen]

Look at your furnace. Are there two PVC pipes going through the roof/wall? Or just one?

If just one, how large is the pipe? If it's 4" in diameter, it's probably a concentric air exhaust/intake system, and it gets supply and exhaust from outdoors, and there is no concerns about what air it needs from inside.

If it has two PVC pipes, the same applies - no worries about inside air.

If it has 1 smaller pipe, or uses a single sheetmetal B-vent pipe, then you need to pay attention to it, but I don't think you can practically seal it up well enough to cause issues.

What kind of water heater do you have?

[Telecaster]

According to Krigger and Dorsi, the rule of thumb is 50 cubic feet of air for every 1000 btu per hour of appliance input.   

However, before you start is you need to define the thermal boundary of the house.  In other words, what part is insulated and conditioned?   For example, think about the attic insulation.  The part above the insulation is outside the thermal boundary.   If the garage is outside the thermal boundary, then it probably won't do much good to do crack sealing and such in there.

One way you can test to see if the walls are insulted behind the dryway is take a metal insulation support, or any strong, thin piece of wire, and work it through the drywall.  When you pull it out, it should pull out a little fiberglass with it.   

[dragoncar]

Have you tried googling it?

Ha!  Yes of course what do you think I'm some kinda newb?  Go nuts: http://lmgtfy.com/?q=how+tightly+to+seal+garage+furnace

Look at your furnace. Are there two PVC pipes going through the roof/wall? Or just one?

If just one, how large is the pipe? If it's 4" in diameter, it's probably a concentric air exhaust/intake system, and it gets supply and exhaust from outdoors, and there is no concerns about what air it needs from inside.

If it has two PVC pipes, the same applies - no worries about inside air.

If it has 1 smaller pipe, or uses a single sheetmetal B-vent pipe, then you need to pay attention to it, but I don't think you can practically seal it up well enough to cause issues.

What kind of water heater do you have?

I've got a gas water heater, and both appliances have metal exhausts that y into a single metal exhaust that goes through the ceiling.  Looks like the "b vent pipe" you reference"

There are no pvc pipes

According to Krigger and Dorsi, the rule of thumb is 50 cubic feet of air for every 1000 btu per hour of appliance input.   

However, before you start is you need to define the thermal boundary of the house.  In other words, what part is insulated and conditioned?   For example, think about the attic insulation.  The part above the insulation is outside the thermal boundary.   If the garage is outside the thermal boundary, then it probably won't do much good to do crack sealing and such in there.

One way you can test to see if the walls are insulted behind the dryway is take a metal insulation support, or any strong, thin piece of wire, and work it through the drywall.  When you pull it out, it should pull out a little fiberglass with it.   

Well, like I said it's semi-insulated.  There's drywall on most of the ceiling and the walls, the external wall has plywood, the last 4 feet of roofing has nothing, and I have no idea what if any insulation is behind of plywood/drywall.  It stays pretty warm in the winter, but is not heated.

There's one vent in the foundation, but it was drywalled over on the inside.  But then someone made a hole in the drywall to run the sprinkler control system.  That's one area I'd like to seal up as it's spider central and a pretty big hole.

Hell, I might even add some fiberglass batts to the uninsulated roof portion someday.

I'll try the wire/insulation trick later

thanks all

[lthenderson]

I've never seen a garage door that sealed tight enough for one to be concerned with makeup air for a furnace or water heater out in the garage.

[BudgetSlasher]

I'm going to second telecaster, if your intention in sealing the garage is to improve heating costs it will do you no good if the garage is not inside of the conditioned space envelope. If it is not, you would be better off insulting and ducts that run in the garage and potentially building and insulated utility room around the furnace.

On the other hand if your goal is simply to keep the critters out of the garage, seal away in all honesty it is unlike you will be able to seal a garage so tightly as to impact your furnace, especially just using a can of great stuff after it was build, still a CO detector couldn't hurt.

[Greg]

In my experience, you can insulate your garage and experience more comfort in there even if you leave ventilation for your furnace and heater.  You should set aside two vents, one high and one low is recommended, and insulate/air seal everything else.  Even better is to build a cold room around them with ducted combustion air supply.

[dragoncar]

I'm going to second telecaster, if your intention in sealing the garage is to improve heating costs it will do you no good if the garage is not inside of the conditioned space envelope. If it is not, you would be better off insulting and ducts that run in the garage and potentially building and insulated utility room around the furnace.

On the other hand if your goal is simply to keep the critters out of the garage, seal away in all honesty it is unlike you will be able to seal a garage so tightly as to impact your furnace, especially just using a can of great stuff after it was build, still a CO detector couldn't hurt.

Sounds good, I'll probably go nuts at the holes that are bothering me

I don't agree that insulating a garage will do "no good if the garage is not inside of the conditioned space envelope."  Heat transfer is a function of temperature differential.  My living area will lose heat to my garage at a faster rate, the colder my garage is.  My garage will lose heat to the outdoors at a lower rate the more it's insulated.  Thus, any insulation of the garage is effectively insulation of my living space.  It might be more effective to add insulation to the attic, but it's extremely low effort and cost for me to seal off the garage than any alternative.

There are no uninsulated ducts in the garage.

In my experience, you can insulate your garage and experience more comfort in there even if you leave ventilation for your furnace and heater.  You should set aside two vents, one high and one low is recommended, and insulate/air seal everything else.  Even better is to build a cold room around them with ducted combustion air supply.

This is probably my eventual next step -- the furnace and water heater are already in a recessed area that I could put doors in front of, and then open an intake vent to the crawlspace.  But I don't know if this meets code and haven't investigated at all since it's not a super high priority

[Greg]

This is probably my eventual next step -- the furnace and water heater are already in a recessed area that I could put doors in front of, and then open an intake vent to the crawlspace.  But I don't know if this meets code and haven't investigated at all since it's not a super high priority

I'd ask your local permitting office about that, it sounds like a bad idea. Pretty sure it wouldn't pass inspection now that I think about it, it would create a path from furnace/garage into the understructure of the house, breaching the firewall.  Ducting would have to be to the outside, on the garage side of the firewall (wall with drywall between house and garage).

[dragoncar]

This is probably my eventual next step -- the furnace and water heater are already in a recessed area that I could put doors in front of, and then open an intake vent to the crawlspace.  But I don't know if this meets code and haven't investigated at all since it's not a super high priority

I'd ask your local permitting office about that, it sounds like a bad idea. Pretty sure it wouldn't pass inspection now that I think about it, it would create a path from furnace/garage into the understructure of the house, breaching the firewall.  Ducting would have to be to the outside, on the garage side of the firewall (wall with drywall between house and garage).

Seems likely... I could also convert the exhaust into a coaxial intake:exhaust if that would help.

Hopefully I can at least put on vented/slatted doors for purely aesthetic reasons

[Fishindude]

I've never seen a garage door that sealed tight enough for one to be concerned with makeup air for a furnace or water heater out in the garage.

+1