Foam Shrinks, and Other Lessons

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Foam Shrinks, and Other Lessons

What we learned from updating a 16-year-old deep-energy retrofit

Posted on Feb 23 2015 by Joe Lstiburek

I did a deep-energy retrofit on my barn 16 years ago. Building Science Corp. was young and growing, and we needed a bigger office. The barn would be that office for the next 10 years. In fact, Betsy Pettit wrote about it in “Remodeling for Energy Efficiency” (FHB #194).

The first thing we did was to cut all the overhangs off the barn roof and wrap the outside of the building with plastic. Next, we applied 8-in.-thick expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) foam on the walls and 10 in. of EPS on the roof. On top of the roof, we installed a layer of plywood as a nailing base for the shingles. The plywood cantilevered past the edge of the barn’s roof to form the new overhang.

Our first lesson came about four months after the retrofit was complete: When the first frost hit the roof, I discovered an obvious melting pattern. At the seams between the foam panels, warm air was escaping and melting the frost. Rather than installing one thick layer of foam, I should have installed multiple layers, offsetting the joints and staggering the seams. I decided to fix that when I replaced the shingles.

The first sign of trouble. Melted frost on the new roof pointed to a problem during the first winter: Heat was escaping between the seams of the foam sheets. The author had used mastic and mesh tape to seal the seams, but that wasn’t enough to keep in the heat. Turns out that the actual problem was a little more interesting. See below for the solution.

I was curious about a few other things:

  • We had carpenter ants, which indicated a water problem. I couldn’t figure out where they came from exactly, but I knew that I wanted to hunt them down and find out what was going on.
  • The air-leakage test revealed that the barn wasn’t as tight as I thought it should be. I wanted 1.5 air changes per hour (ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.) at 50 pascals, but what I got was 3 ACH. We haven’t retested with a blower door yet, but testing on similar buildings with this same retrofit system have netted excellent results: 0.1 ACH at 50 pascals.
  • I wanted to know whether I had any other bugs living in the furring-strip space between the siding and the foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. .
  • I wanted to know how my water management actually worked. You can’t really tell with computer simulations.

Clearly, there were more issues with this building than just the shingles and the foam, and they have been nagging at me for more than a decade. I decided to look at the project like an experiment—take the barn apart, see how the components performed, and then put it back together again using the best off-the-shelf technology available now.

Beginning at the outside, I’ll take you through what we learned.

Paint can last a very long time

There was nothing special about the siding we had chosen for the barn except that it was installed over a vented rain screen and we were meticulous about sealing cut ends. The siding is lodgepole pine from Alberta. It was shipped to New Brunswick, where it was kiln-dried and then coated on all six sides with a penetrating wood preservative. Next, it was coated on all six sides with an oil-based primer and painted with acrylic-based latex paint. It was then shipped to Massachusetts as “Cape Cod” siding. We installed it over we 3⁄4-in. furring strips, which allowed air circulation behind it. This meant that the siding could dry evenly in front and in back.

When we took the siding off the barn, it was pristine. The trim, also lodgepole pine treated the same way, did not hold up as well in some places. The trim was not backvented like the siding. Instead, it was nailed directly to 3⁄4-in. plywood strips.

The bug screen worked perfectly

Installing siding over furring strips is a water-management strategy that keeps siding and walls dry and free of rot. I can’t tell you how many times over the past 16 years that someone has said to me, “You can’t do that. There will be animals and bugs up there, and they’ll eat your house.” When we took the wall assembly apart, however, there was absolutely no evidence of bugs, bees, or other critters living in the space between the 1x4s.

We kept critters out by wrapping a 3⁄4-in. strip of attic ridge vent called Cobra Vent ( at the bottom of the siding. We’re rebuilding it in much the same way, but with slightly more advanced materials.

Epoxy-coated screws perform as well as stainless steel

We used two kinds of screws to fasten the thick foam panels to the barn boards: stainless steel and epoxy-coated steel. When we opened the walls, all the screws looked brand new except for two of the epoxy-coated screws. Those two screws were in a place where we had some water-management problems, and the heads had rusted out.

In the past 16 years, the technology of epoxy coatings has improved significantly, so I have no reservations about using epoxy-coated screws on the rebuild. At about 35¢ per screw versus $1.50 for stainless steel, this is an easy choice. I have absolutely no hesitation in recommending epoxy-coated steel screws and non-pressure-treated furring strips.

Furring strips will not rot

I can’t tell you how many times I have heard, “You’ve got to use pressure-treated furring strips,” but it is absolutely untrue. When we took the walls apart, the furring strips looked brand new. Why does this detail work? Because the vent space is designed to dry. That is its whole job.

Why not use pressure-treated furring strips as an added measure of protection? One reason is that it means having to use stainless-steel siding nails, which has proven to be an unjustified extra expense.

Stagger the sheets and choose the right foam

When I took the photo of my frosty roof above, I thought the heat loss was the result of using a single layer of foam, which meant that I couldn’t stagger the sheets and offset the seams. That is what I’ve been telling people all these years in my confessional speeches about deep-energy retrofits. That, however, was only part of the problem. Now I know the rest of the story.

When we installed this foam, we covered the joints with mesh tape and mastic. Over time, the panels shrunk, and the tape/mastic combination cracked, opening the seams. This is where the heat came through to melt the frost on my roof.

Because we used XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. on parts of the main house, we dug into that siding to compare the EPS with the XPS, which was sealed at the seams with Tyvek tape. After 16 years of service, the XPS and Tyvek tape were in perfect condition.

MULTIPLE LAYERS OF FOAM OUTPERFORM A SINGLE LAYER The second line of defense against air leaks was staggering and taping the seams of the many layers of polyisocyanurate foam.

Peel-and-stick membrane sags

Interestingly, we also learned something about Bituthene membrane (peel and stick). Some of the peel-and-stick membrane that we used to flash windows wrinkled over time. Specifically, the pieces running horizontally above window head flashings sagged along their top edge. Anywhere that we didn’t seal the top edge with contractor’s tape, the peel-and-stick membrane sagged. This meant that water could get in and cause problems. Wherever we taped the top of the
membrane, water couldn’t get in. Some folks in commercial construction call this a termination layer and use glue, sealant, or some type of tape. Now we specify that all the tops of this membrane should be taped.

Plastic is a dumb air barrier

In the 1980s, I was preaching all over Canada about plastic being a dumb air barrier. What did I choose to wrap my barn with? Plastic. It was cheap, and I was overconfident. The blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. years later didn’t specify where, but it told me that there were a lot of holes. Rather than try to figure out where all the holes were and detail the plastic layer better, I switched to covering the barn in peel-and-stick membrane as the first line of defense in my air-barrier system.

Note: This plastic air barrier is different from a plastic vapor barrier that is commonly (and often wrongly) installed in roof and wall assemblies.

Window film is a poor substitute for the right glazing

At the time of the deep-energy retrofit, we used the best windows we could buy: triple-glazed Heat Mirror windows from Hurd. The third layer of glazing is a film suspended between the two layers of glass. The performance was pretty good, but the longevity of the inner layer of film was not so great. After 16 years, I can see fish eyes on the film—not enough to make me want to replace the windows, but enough to keep me from specifying window film in the future.

Flashing matters

One of my missions was to hunt down the carpenter ants that I knew were chewing up my foam. I found ant galleries in three distinct locations. Each place was adjacent to an area with a water-management problem. In two places, there was reverse-lapped flashing. The third place was up on the roof: The cupola’s windows are at the same height as the roof peak. This is a dumb design detail. The windows should have been placed higher than that, and they are now.

How did the ants get to these areas? They crawled up. Ants crawl all over the place on reconnaissance missions. When they find a water source, they dig in. As irritated as I was that the ants were eating my foam, I was glad to find the ants localized in places with water problems. I am absolutely embarrassed about the water-management details. Two of the spots were reverse-flashed on a day when neither my architect nor I was on the job site.

Bottom line: If you don’t screw up your water management, then you’re not going to have ants.

Ants can tunnel easily through foam.

Cantilevered window boxes won’t sag over time

Here’s another question that I have been bombarded with for the past 16 years: Won’t those window boxes sag? When I talk about this window detail (called outies by the energy nerds), I often get questions about whether the boxes will sag under the constant weight of the window. I have always answered this question by saying that the window boxes on my house hadn’t seemed to sag so far. Now, after having had the barn stripped of its siding and foam, which gave me the opportunity to measure the deflection, I can say unequivocally, “No, they will not sag.” 

CANTILEVERED PLYWOOD BOX DID NOT SAG When thickening walls by 8 in., one of the difficult design details is how to install and trim windows. The author chose to cantilever plywood boxes past the frame and to install the windows with masonry straps. Naysayers cautioned that the boxes would sag under the weight of the windows, but time told a different story.

Most of the details worked well, but some could have worked better. Three main changes were incorporated: adding plywood for structural stability, switching to a better air-barrier layer, and adding multiple layers of foam.
(click illustration to enlarge)

Joseph Lstiburek, Ph.D., P.Eng, is an ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. fellow and a principal of Building Science Corp. in Somerville, Mass. Photos by Daniel Morrison, except where noted.

From Fine Homebuilding No. 225 — Download a PDF of this article.

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  1. Daniel Morrison

Feb 23, 2015 5:00 PM ET

Edited Feb 23, 2015 6:53 PM ET.

Trim not back-vented... any ideas?
by Kent Jeffery

Joe - I've read with interest a great many of your articles for the science behind my design choices as I build my CZ6 house. I would like to know more about your experience with the trim.

I used 1x3 furring strips to create my rainscreen, and fiber cement lap siding. At the structure's corners, and to create nailing surfaces around my windows, I used 1x6 pine. This provided ample nailing space for my MiraTec 5/4" engineered lumber trim boards, as well as a lip for nailing the lap siding ends, which abutted the trim boards. With no air gap behind the tim, as they were attached directly to the 1x6 pine, I've been concerned about how well this assembly will hold up over time. It sounds as though you discovered perhaps your trim did not hold up as well as you expected? Do you feel that the absence of a gap, or venting, behind the trim was the sole reason?

I remember reading an article of yours from several years ago where you debated the merits of priming the 1-by furring strips, and decided it wasn't necessary. Do you think that would've made a difference? Would kerf cuts spread across the back of the furring to allow some air movement make much of a difference? I ask, because I debated both, but wound up doing neither. Hmmm....

Feb 24, 2015 2:19 AM ET

by Lloyd Dinkelspiel

It seems that EPS shrinks but not XPS.
Is that correct?
Is it due to the roof temperatures vs. the sidewall temperatures?
Is it just in the nature of the different material fabrication?
Would one need to layer a XPS assembly or would just taping the seams be sufficient (for sidewall use)?

Feb 24, 2015 8:33 AM ET

Response to Lloyd Dinkelspiel
by Martin Holladay

Q. "It seems that EPS shrinks but not XPS. Is that correct?"

A. No. There are plenty of reports of shrinking XPS. To learn more, see this article: Using Rigid Foam As a Water-Resistive Barrier. (Scroll down to the paragraphs below the bold heading "Do rigid foam panels shrink?")

Feb 24, 2015 11:46 AM ET

Kent, the answer is to use
by aj builder, Upstate NY Zone 6a

Kent, the answer is to use pressure treated materials to bump out corner trim. Corner Trim makes Full continuous contact with the under lying material which sets up capillary water entry hence rot and Carpenter ants etc.

No one should build with eight inches of exterior foam.

For high levels of insulation the PH people can do so with out foam. The wall condensation issue is solved by having one low perm air barrier centered in the wall where it will not condensate. High perm air barriers to the exterior and Interior. Add high levels of cellulose with venting for the roof. Do use foam underneath the slab eps or possibly perlite bagged will catch on and get formal install acceptance.

One last note as to foam roofs. Ventilate always above the foam in cold regions such as my zone 6a. You too Joe.

Feb 24, 2015 12:40 PM ET

by Malcolm Taylor

I think I've posted this here before, but my experience is that although Carpenter Ants may prefer damp environments, they are quite happy to tunnel in dry foam which perfectly mimics the rotten stumps they like to build nests in. I don't think good water management is an adequate strategy to prevent ant damage.

Mar 2, 2015 11:15 PM ET

Edited Mar 2, 2015 11:18 PM ET.

Corner Boards, Ants and Extr. foam
by Brad Hardie

AJ - Why do you say folks shouldn't build with 8" of foam or other insulation on the exterior? There are plenty of studies to prove otherwise and thousands of houses with lots of insulation on the exterior, and I don't mean SIPS. The PERSIST and REMOTE wall assembly techniques have been used successfully for years. Cellulose doesn't work when you're trying to preserve and show off the interior of an old barn or home either. Plus you lose so much Sq. Ft..

Condensation issues can't simply be resolved "by one low perm air barrier centered in the wall" - its related to interior RH or exterior RH, difference in exterior & interior temps, along with the ratio of insulation on the exterior to interior.

In my experience ants are seen in foam where there is water nearby....I remember being told that they won't travel/burrow further than 10' from a water source (I can't cite any factual data though to substantiate that though).

You can use sign board ( available in different thicknesses) and rigid enough behind your corner boards. Just rip the sheets on the table saw to desired widths.You can also rip 3" thick Roxul Comfortboard IS and use it behind the corner boards too.

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