The one-room addition on Emerson W’s home is not what anyone would realistically consider over-insulated: R-11 batts in the walls and R-19 at most in the ceiling. But the immediate issue is the floor. There’s no insulation at all there, and because the addition sits on concrete piers, there’s nothing to stop the wind from blowing freely below.
“As you can see, outdoor air can simply pass through the open space under the floor,” Emerson writes in a post in the Q&A Forum. “When the flat EPDM roof needs to be replaced in 5 to 10 years, we will install exterior foam board outside the roof deck. Heating is primarily warmed air from rest of house or from the propane fireplace. Cooling is AC from rest of house.”
Emerson is weighing a plan to excavate below the room to a depth of 2 feet before installing batt insulation between the joists, adding 1 or 2 inches of rigid polyisocyanurate insulation over the joists, and covering the assembly with OSB or plywood.
“What approach would you take?” Emerson asks. That’s today’s Q&A Spotlight.
Digging out is a good idea
GBA Editor Martin Holladay has some suggested reading for Emerson (see the “Related Articles” sidebar below), but in general suggests that digging out the soil from beneath the room is a good first step.
“Excavating the soil under this addition is obviously a good idea,” Holladay writes. “So is adjusting the grade nearby, if possible. If you can afford to put in a real crawl space foundation (with concrete walls), all the better.”
If Emerson keeps the pier foundation, opting not to spend the money on a full crawl space foundation, there are some sound strategies for insulating the floor. But a key consideration in all of this is how any excavation below the addition will affect drainage.
“Once you have done digging the area out, you have a new problem: drainage,” Holladay says. “Unless the grade around the outside of the building is properly adjusted, the crawl space might become a mosquito-breeding pond. So you have to think this through. (That’s where the advantage of a traditional crawl space with concrete foundation walls starts to shine.)”
Insulation in the floor could be either spray foam or a continuous layer of rigid foam below the joists (with fluffy insulation between the joists), he adds. Spray foam might do a better job with air-sealing the floor, but if the insulation contractor is conscientious, rigid foam insulation also could be just as successful, as long as it is carefully air sealed.
Working in a tight space isn’t easy
It’s one thing to suggest insulation and air sealing below the floor, and another thing to drag tools and materials underneath the addition and actually do it.
“I’ve done what you are contemplating and it’s a lot of work,” says Malcolm Taylor. “To get adequate space to fasten sheet goods to the bottom of joists you need about 2 feet. The work is done on your back. It’s very hard to get the good, tight joints needed to keep pests out, and the language used is generally unsuited for children.”
He suggests Emerson think though each step carefully: add perimeter blocking for the insulation layer early as a guide, make some plywood jigs to hold one end of the sheet in place, and get the right tool. In this case, that’s a short, 12-volt impact driver for fasteners.
Installing batts in the joist bays and then adding rigid foam is in itself pretty easy, he says, providing you install a perimeter of 2-by material. The plywood protecting the insulation can be nailed to this perimeter piece, and screwed to the joists through the foam.
“Also it’s a lot easier if you have marked the joist locations on the perimeter 2-by,” Taylor adds. “I’d think about bedding the plywood in caulk at the edges, and caulking or taping the seams to keep carpenter ants and other pests out.”
Codes in Taylor’s area forbids the use of untreated material close to grade. Just how long the plywood lasts will depend on how well the area beneath the room can be drained. “That may be your biggest challenge,” he says. “The work is just unpleasant. But then a lot of building tasks are.”
Tackling the problem from the inside
“What about installing rigid foam on the interior?”, Jon R asks.
After a week of cold weather, with temperatures in the 20s, the idea has some appeal to Emerson. The addition has been the coldest room in the house — drafty, cold, and uncomfortable in the morning, he says — so “lifting the wood floor seems easier than digging a foundation.” Or, he adds, maybe he just needs a thick rug.
Adding a layer of rigid foam over the existing floor is not a bad idea, Taylor tells Emerson, but there are some caveats. “Apart from the insulation, new subfloor, and flooring, you also have to deal with new trim, cutting down doors, and the transition in height between the old and new floors,” he says.
Ceiling height is a concern. Emerson says that because of the low ceiling the room already has, he is considering of a “cut and cobble” approach: cutting pieces of insulation so they fit between the floor joists, with the possibility of adding a single layer above the joists to reduce thermal bridging.
“Depending on the depth of the joist, this would get us to R-20 or so,” Emerson says. “Given the current room for improvement, I think we are seeking a ‘good enough’ solution, and bang for our buck. We can also reduce ceiling penetrations (eight recessed lights in a 12×12 room), and perhaps increase ceiling/attic insulation.”
Consider roof insulation as well
The roof has insulation issues of its own. Emerson describes the roof as flat and covered with an EPDM membrane. He’s met with the roofer who did the work and learned the roof is not vented.
“His visit led me to discovering the ‘decorative soffits’ and also some fungus on the ceiling rafters under the EPDM roof. The fungus was white in color on the wood, with some black in color on the aluminum soffits. There are no leaks, and I am told the EPDM will be OK for 15 more years.”
The fungus is visible on the rafter tails on the exterior of the addition. Emerson had a look by peeling back a section of the aluminum soffit on the roof’s 2-foot overhang. But the bottom line is the roof is not vented, and the situation is complicated by the manner in which the rafters over the addition are connected to the main house.
“I had planned on blocking the joist bays where the low-sloped rafters intersect with the main house (i.e. extend into second floor joists — currently wide open on both sides),” Emerson explains. “This would block the colder air from entering the second-story joist bays.” That may have to do until the EPDM is replaced.
Yes, Holladay replies, if rafters in the flat roof are continuous with floor joists in the main house, he absolutely needs airtight blocking between the rafters where the insulation ends.
Our expert’s opinion
GBA technical director Peter Yost added these thoughts:
The first tough question is whether to move to a crawl space or stick with concrete piers? An important consideration, other than cost, is skill set. Sticking with piers is less expensive and simpler, involving only semi-skilled hard work. Replacing the foundation is going to involve mechanical excavation, temporary support, and casting the perimeter walls. In my book, that sounds expensive and complicated.
The second tough question on this project is whether to work from underneath or from above. I know that working from underneath sounds more like a prison break than a retrofit, but the easiest and best way to get continuous insulation and good air sealing is from below.
I am going to add two details to the pier floor assembly:
First, include an air space between the subfloor and the cavity insulation. See Joe Lstiburek’s on this topic. The space will not significantly change the energy efficiency of the assembly but it will improve thermal comfort for all those walking in that room in bare or stocking feet. I made this mistake on my own SIP kitchen addition, when I did not leave an air space in the pier floor assembly.
Although the need for insulation and airtightness in the floor is no different than walls or ceiling, you don’t walk on the walls or the ceiling. When it gets cold and stays cold, the floor temperature of the addition is at least 5 F° cooler than the kitchen floor over the basement. When I tried to explain the science to my wife one cold morning, she said, “It never ceases to amaze me that you understand so much about buildings and so little about people.”
The thermal images below help explain what I’m talking about.
Second, protect the rigid insulation with galvanized metal lath or stainless-steel mesh. Coincidental to this work, I was just on the phone with Terry Brennan of Camroden Associates doing an interview for an upcoming feature article for BuildingGreen. (The likely title is “How Buildings Fail Their Users.”) He was relating how many times all sorts of buildings get infested with Norway rats, voles, squirrels, and the like. They love finding down-low weak spots to chew their way to warm and cozy nesting sites.
And if you need to protect against termites or carpenter ants, maybe move from metal lath to stainless-steel mesh. I say maybe because Terry is not completely sure that the mammals will be thwarted by the mesh like the insects will. You might think that protecting the whole floor with sealed mesh or lath sounds extreme, but the only thing worse than carefully installing continuous rigid insulation and plywood would be having critters move in.