Everything you wanted to know about insulation, but were afraid to ask

In residential construction, the most common wall types include wood-frame, masonry, and steel-frame walls. Wood-frame walls, the most commonly used in residential construction, are appreciated for their versatility and ease of insulation, but they are less resilient. Masonry walls, composed of materials like brick, concrete, or stone, offer durability and better resistance to fire and pests while also providing a classic aesthetic appeal, but they are harder to insulate. Steel-frame walls are increasingly popular due to their strength, lightweight nature, and resistance to warping or settling, making them ideal for modern designs, however, they struggle with thermal bridging and they are difficult to isolate and insulate. Each wall type presents unique benefits that cater to various building needs and preferences, but the rules for insulation are generally the same.

Insulation plays a critical role in enhancing energy efficiency and ensuring comfort in residential buildings. By acting as a resistance against heat transfer, insulation helps maintain a stable indoor temperature, reducing the reliance on heating and cooling systems. This not only lowers energy bills and minimizes environmental impact, but it also creates a more comfortable living environment. As homeowners increasingly seek sustainable living solutions, insulation is at the top of the list for improvement.

While insulation is vital for energy efficiency, it can trap moisture in walls, and thus can pose dangers if not properly managed. The more insulation in a wall, the harder it is to dry it out when it gets wet. Pushing that delta where condensation can occur, the ‘danger moisture zone’ or DMZ, out and away from the structure is the trick. Understanding the risks associated with where we place insulation (the thermal control layer) in the wall assembly, and the importance of air- and vapor-control layers, their performance and location, is critical. As builders and remodelers, we want to minimize risk and maximize resilience. 

Thermal Bridging

Following the second law of thermodynamics, heat energy naturally flows from areas of higher temperature to those of lower temperature to seek balance. In other words, your warm conditioned space is losing energy to cold materials to warm them. Framing members that span the width of the wall without interruption can be thought of as bridges, on which energy is traveling from inside our homes to the great outdoors, which has an infinite capacity (seriously infinite if you consider the cold void of space). The colder the climate, the greater the concern. Think about what happens to a cold soda when it meets hot air, it sweats. What makes those Yeti coolers work so well? They are air sealed, insulated, and have minimal thermal bridging. The inside of the cooler is cold, but the outside surface is similar to the environment it is placed in. The point where condensation could occur is in the middle of the foam insulation, but with no air flow and a vapor tight skin on both sides, there’s no moisture and no issues.

A home is not as simple as a camping cooler made of molded plastics, so our strategy must be different. With the caveat that some climate zones exaggerate the risks, and we’re keeping this simple, here are our choices, sorted from highest to lowest risk. 

In-Wall Insulation Types

Batt insulation—This is a very common and traditional way to insulate walls. R-values for batt insulation vary, but they are generally less energy efficient than others. There can be some additional potential drawbacks. Batt must be installed properly, and training is required to get it right. In addition, it is more difficult to fit batts around wires and pipes. This can create many potential air-loss pathways. Batts are tried and true and they are perhaps the most cost-effective way to reliably insulate a home. That is why so many remodelers and builders make it their go-to type of insulation.

As a remodeler who focuses on energy efficiency, here are some of the potential drawbacks I see with batt insulation. A fluffy batt insulation in the wall cavity with a sheet of 6-mil poly on the warm side of the wall can lead to condensation issues. It’s important to use a fluffy batt insulation in the wall cavity with an ‘intelligent’ vapor-control layer. These include, Rothoblaas Clima Control, CertainTeed MemBrain, or even old-fashioned kraft-paper faced batts.

Blown-in insulation—Blown-in insulation is often a loose product with many potential air pockets. Thicker walls are required to achieve higher R-values. Blown-in products control air flow well. They also overcome the challenges pipes and wires present. These types of insulation require the use of a vapor-control layer on the warm side. A wall filled with a dense-pack cellulose or blown-in batt system like Spider from Johns Manville and fitted with an intelligent vapor control layer or vapor retarding primer on the drywall can be very effective.

Spray polyurethane foams (open/closed)—These provide excellent air control, vapor control (closed cell), more resistance to energy loss, sound attenuation, and can handle complex geometry found in modern home designs. The downsides are cost, limitations with spraying temperatures, and the requirements of cleanliness of surface for adhesion. 

I commonly use a wall filled with closed-cell spray polyurethane foam with zero global warming potential (0 GWP CCSPF). They offer air sealing, vapor retarding, and high R-values per inch. I’m intentionally leaving out some newer products that I haven’t had much experience using. These include foil-faced bubble wrap, insulating paint, and foam products that are injected into wall cavities. Some of the claims made by products in this category suggest that they will compress away all other insulations and fill voids.

Bridge Breaking 101

When you take any of those assemblies and you add one inch of insulation to the exterior of the building, you effectively create a thermal break. You get a significant bump in performance and your risk begins to drop. Products like Henry by Carlisle’s Blueskin VPTech offers an integrated water-resistant barrier (WRB) with an R-5 insulation board. This is a vapor-permeable insulation, which is especially important if you’re using any of the above wall assemblies, except closed-cell spray foam (CCSPF). You can fasten cladding through the board to the framing or add furring strips. It’s important to note that most nails are made of metal, which can be a fantastic conductor of energy. Any metal nail that is exposed to air in your fluffy insulation, can serve as a condensation campground.

When you add two inches to the exterior of your wall assembly (not just one inch), something magical happens—the need for a vapor retarder on the warm side of the assembly dissipates. This should be reason enough for everyone in the construction industry who is trying to build energy-efficient wall systems to smile. This eliminates the need for warm-side versus cold-side considerations. There’s no more cut plastic to patch up. In summary, with a two-inch exterior insulation solution, there are fewer steps, fewer steps for moisture management, and there’s an increase in resilience.

One product that offers two inches of exterior rigid foam, with two inches of CCSPF on the inside is Huber’s ZIP R12, which gives you sheathing and insulation. We’ve also made our own panels using Georgia Pacific’s ForceField and two inches of DuPont’s near-zero GWP XPS (extruded polystyrene), which you can place on either side of the sheathing. Another solution is two inches of BASF’s Zero GWP CCSPF installed in the wall cavity. This will seal up all the imperfections in the assembly and leave plenty of space for mechanicals without an ultra-thick wall. The result is an effective R-28 wall assembly.

This is the sweet spot for high-performance walls. The framing process remains almost the same. The window installation and cladding installation is the same. The risk of air migrating into the assembly is very low, and the wood products stay warm and dry.

Not everything is rainbows and sunshine, however. We must remember to keep our wall balanced. Too much insulation inside the wall will draw that DMZ right back into the wall cavity. In climate zones 5 and 6, we shoot for 50 percent of the insulation on the exterior and 50 percent of the insulation on the interior. For zones 2 and 3 you may theoretically remove all the cavity insulation with R-12 continuous exterior, but you shouldn’t.

In the Weeds

Everything is simple until it’s complicated. Thin material is easy. Thick material is less easy. The material we currently use for exterior insulation is a rigid foam board. The seams do not always butt together tightly in the field. Air pathways run between those connections. There are two primary exterior insulation and wall sheathing configurations. The first has wall sheathing tight to framing with rigid insulation over it. The second uses rigid insulation tight to the framing with wall sheathing over that as the outer-most layer.

Let’s call configuration No. 1 the ‘really good wall’. You have a decent air-control layer with the sheathing, and you’re keeping the sheathing warm. This is smart. If you don’t tape all the seams of your foam, you have outside air knocking on the sheathing. This is not good. You cannot hang siding on foam, so you have to add strips of wood on the outside, and buck out your windows, which is not attractive. Those strips of wood require long screws or long nails. They will not all bury themselves in the framing. You could hire someone to search for these shiners, but this is residential construction, so you won’t, and they will get missed. This is bad. If you are using fluffy insulation types, you will have problems. If you are using an air sealing insulation product on the interior, they are encapsulated, and the risk is minimized.

Let’s call configuration No. 2 the ‘awesome wall.’ If you have an inconsistent air-control layer at the framing—most of us do—your sheathing is in contact with the outside air. This is potentially problematic. You’re using long nails to attach the sheathing to the framing. In this configuration CCSPF is required on the interior. See above.

In both configurations, you must mitigate against energy traveling by both air and conduction. The ‘awesome wall’ configuration offers an ease of window and siding attachment. The risk at the face of the sheathing can be mitigated with a simple entangled-matrix style ventilated rainscreen like Benjamin Obdyke’s Slicker MAX.

Remember, the ideal scenario is 100 percent exterior, which brings us to the ‘best assembly’, which places four inches of rigid insulation on the outside of the home with strapping to hold it all together. Maximum performance. Maximum resilience. Maximum difficulty. Maximum expense.

Critical Details

Two final notes. You can build a resilient high-
performance wall, but the system requires you to consider more. You will need windows with a high condensation-resistance factor. An energy recovery ventilator (ERV) appliance is a necessity, and it should be ducted independent of the heating and cooling system. Acoustical caulking or DuPont’s Drywall Gasket (not sticky) should be used at all wood-to-wood connections. 

While insulation is vital for energy efficiency, it reduces a wall’s drying potential, and can pose dangers if not properly managed.

Finally, the extra safe, airbag-and-seatbelt approach, requires the inclusion of a ventilated rainscreen like Benjamin Obdyke’s Slicker MAX between your cladding and your WRB or foam, depending on which system you choose. This ensures you have both drainage (gravity) and drying (air pulled up the backside), which will safeguard against the inevitable imperfection.

Mason Dixon

Remodelers in warm, hot and humid climates might think they don’t need to worry about wall systems. Sorry. While the warm side is on the outside, the same rules of the universe apply. As you cool your interiors, the condensation point is pushed to inside the home. While cold-climate remodelers must worry about rotting OSB and other wood substrates, warm-climate remodelers often must address mold on gypsum board.

Wallpaper is back, and it is sexier than ever. Interior designers are putting it everywhere, on walls, on ceilings, in bathrooms. The perm rating of a standard Type 2 (20-ounce) wall covering is 0.68. Here’s the rub. Wallpaper is also an ad hoc vapor barrier. And if you’re running the AC, that is the cold side of the wall. The condensation point is now on the surface of your drywall paper covered in glue, which is excellent mold food. Cue the biological dance party.

‘Danger Will Robinson’

Equipment and ducting in and around walls is an important consideration. I watched someone spend hundreds of thousands of dollars to rebuild their walls with an elaborate set of layers and rainscreen strategies to solve a problem rather than move the ductwork that was the source of the entire problem. The topic is the subject of many ASHRAE articles, but in short, the issues go beyond sweating ducts, and include the pressure systems and environmental exchanges. Equipment stored in unconditioned attics, run through unconditioned crawl spaces, run in soffits connected to exterior walls, and run using framing members as the chase, creates a world of problems that no wall assembly can overcome. 

It bears repeating. Keep your mechanicals inside the conditioned parts of the home. If you insist on running soffits on exterior walls, you must place an air control layer between the framing and the ducts. And, seal up those penetrations, which function like super highways between inside and outside, with something that will remain flexible.

Final Thoughts

In summary, constructing a high-performance R-28 wall assembly involves careful consideration of insulation placement, moisture management, and ventilation to achieve optimal resilience and efficiency. While a balance of insulation is crucial to prevent moisture accumulation within the wall cavity, an ideal scenario features robust external insulation, ensuring maximum performance. 

Additionally, selecting windows with high condensation resistance, integrating an energy recovery ventilator, and employing acoustical caulking will contribute to the integrity of the assembly. The inclusion of a ventilated rainscreen is essential to facilitate effective drainage and drying, ultimately safeguarding against potential issues. When these elements come together, they create a resilient wall system that stands the test of time. QR