GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

80% efficiency furnace in a foamed / sealed attic space

MattJF | Posted in General Questions on

We have an 80% furnace that is in a walk up attic space we are planning on foaming, insulating and sealing up. I am trying to determine what steps we to take to determine if there is sufficient combustion air or if we will need to provide outside air. The space will be a conditioned storage space. We are planning to replace the furnace in a year or two.

The furnace is an American Standard Silver 80 100,000 BTU.

From the manual and ANSI Z223.1 the furnace is a class 1 fan assisted combustion device.

From ANSI Z223.1 with an unknown ACH (I assume ACH50, but the standard copy I have doesn’t clarify), you need 50ft^3 per 1000BTU. The space contained within the drywall is 5370 ft^3, so we technically have enough space. Should I worry any further? This formula assumes a .33 ACH (from ft^3=15ft^3/ACH*(BTU Device/1000 BTU)), which seems unlikely to achieve with interior spray foam, caulking, and drywall.

Should we have a CAZ test performed? I can DIY this to know I we are safe, but the inspector is going to want someone professional. Who is most cost effective to perform this test?

I can have a blower door test done, but that is generally whole house.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Here's the advice I gave in my 2010 article on the topic ("Creating a Conditioned Attic"):

    "You have to come up with a plan to provide your appliance burners with combustion air. The best solution is to install only sealed-combustion appliances in a conditioned attic. ... If you hope to convert an existing unconditioned attic to a conditioned attic, the presence of any atmospherically vented appliances (for example, a gas water heater or a gas furnace) complicates the retrofit work. If you can’t afford to buy new sealed-combustion appliances, you’ll probably be better off leaving your attic unconditioned."

    If you are planning to replace your old furnace "in a year or two," the obvious solution is to get this job over with now, and do it right. It's time to install a sealed combustion furnace.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |