Buffer spaces are (almost always) evil

Amy Musser

I encounter a lot of “semi-conditioned” spaces in my work. Homeowners inexplicably seem to love them, and builders and architects don’t do enough to talk them out of it.  Please, just say NO to the buffer space.

What I’m trying to help people do in green building is actually quite simple. We need to figure out what’s “inside” and what’s “outside” and then we need to build a functional separation between the inside and the outside. The functional separation, or “building envelope” needs to be able to keep heat and moisture where we want (or don’t want) it. Buffer spaces mess this up, because we’re not sure if they’re in our out. We kind of want them to be conditioned, but we’re trying to get a free lunch. There’s no such thing.  Let’s explore the various types of buffer space, and how these go wrong.

The basement buffer space:
In this scenario, most of your basement is finished and conditioned, but there’s a mechanical/utility room that’s not. It’s located underneath conditioned space, and it has your forced air system in it. Is this inside or outside?
• Homeowner reaction: I don’t want to pay to finish this, and I don’t care if it’s cold in there. But I’m going to get upset later if its moldy.
• Builder reaction: It’s going to be cheaper to put batt insulation in the floor above this than to insulate the walls. I can’t get the homeowner to spend even a small amount of extra money on this space.
• Architect reaction: I don’t want anyone looking at this room, ever. Can we make it smaller?
• My take: By the time you insulate the floor above and the walls between this and the adjacent spaces, you’ve used at least as much insulation as you would need to insulate the exterior walls. The ceiling is probably full of ductwork, and I doubt if you can get someone to install the insulation with any sort of functional quality. The band joist between this space and the house is probably wide open, and would be very difficult to block and insulate it because there are a bunch of ducts and pipes going through it. Insulate the walls and call it conditioned – it has almost no heating/cooling load anyway. And by keeping it a bit warmer, you reduce the chances that it you develop a moisture problem in there. Yes, there are smart and dumb insulation choices for the walls, but it’s a problem worth solving.

The walk-in attic buffer space:
My least favorite, because it’s done SO badly every single time. In this case you have a full-size door that allows you to walk from conditioned space into an unfinished attic area, where there is probably an air handler. A wood frame wall separates the two spaces. This area might be blocked off from the rest of the attic by another wood frame wall or by a suspension of kraft-faced batts hanging out in space, or by a cube built out of rigid foam insulation.
• Homeowner reaction: I don’t want to pay a dime to put my air handler in conditioned or finished space, but let’s throw up some cheap batt insulation to try to keep my Christmas ornaments and furnace from freezing.
• Architect reaction: Practically free storage space. I’m totally in favor of this. I don’t care how it works or if it works. If it’s even a little better than fully unconditioned attic, I’m all for it.
• Builder reaction: This is incredibly cheap and no one cares about this space. I want to do the bare minimum I think is necessary to keep these pipes from freezing.
• My take: I need you to build one decent wall that has insulation, is air sealed, and has proper air barriers on both sides. If you want to build a second wall, I don’t recognize it as a functional wall, and I don’t think you should rely on it to keep anything important from freezing or melting. Additionally, by building these two walls, you are almost guaranteed to confuse the code official (who will likely see these two walls at different inspections several months apart) about which one is the true exterior wall. Make a closet for this stuff that is inside your true conditioned space. Or better yet, seal your attic.

The sealed attic/crawlspace:
These have good intentions and usually work out well. The sealed attic is unvented and has spray foam or rigid insulation at the roof. The sealed crawlspace is also unvented and usually has insulation on the walls (although it can be at the floor, which is a bit more complicated, but can work great).
• Homeowner reaction: Great! Extra storage space!
• Builder’s and architect’s reaction: (varies) On the plus side, these things will probably reduce the chances of my getting a callback. On the down side, they both add cost.
• My take: If you want to store stuff, it’s a bit of a fire risk, but in general, fine. Just don’t put your gas lawnmower or your lawn pesticides in there. This is part of your indoor air. If you wouldn’t put it on your bedside table, don’t put it there. These spaces can have some additional dehumidification needs, depending on your specific climate. In WNC, most crawlspaces work best with a dedicated dehumidifier, and attics work best with a small amount of supply air from the air conditioning system. The latter strategy is still working its way through the building codes, unfortunately.

The garage:
Sometimes the hardest thing is deciding if it’s actually a garage or not. If it has garage doors, I have to assume that at some point someone will drive a car into it. Cars spew a bunch of stuff that you don’t want in your indoor air.
• Homeowner reaction #1: I’m not going to park a car in here and don’t care about the next person that owns the house, so how do we get around this code thing that says we have to separate it from the rest of the house?
• Homeowner reaction #2: If we insulate the garage, isn’t that helping the house stay warmer?
• Builder/architect/insulation installer reaction: Totally forgets that the wall between the house and garage is an exterior wall of the house.
• My take: If you can drive a car into it, someday someone will. If you have to condition it, use a wall mounted minisplit. Regardless, imagine that its doors will be open all winter and air seal/insulate the walls between garage and house accordingly.

The takeaway:  If you’re building a house, get good, detailed (and local!) guidance about how to best handle your specific buffer spaces. Most mold and building envelope problems that we see are related to buffer spaces. Don’t let the cheapest space in your house cause of problems for the rest of it.

Copyright 2015.  Amy Musser