External insulation is a concept that has come on the home building scene recently as homes become more energy efficient. In some areas of the country it is part of building code to have some level of external insulation. External insulation is much more energy efficient because it stops thermal bridging and provides a continual layer of insulation on outside walls.
Thermal bridging happens when the framing stud connects the temperature of the external sheathing to the inside of the house. In a standard wall where only the stud bays are filled with insulation, the studs are a gap in the insulation layer. Wood is a terrible insulator and easily allows outside temperature in. To fix this, external insulation is placed outside of the sheathing which insulates the sheathing and stops thermal bridging. External insulation will be installed after windows and doors are installed.
What is the cheapest way to install external insulation?
In my own building experience the cheapest option is just to use regular 16” wide R-15 mineral wool batts for external insulation. The batts I use are intended for interior studwall installation but I am easily able to use them for external insulation purposes.
| External Insulation Material | R-15 mineral wool batts | Foam Board | Roxul Comfortboard |
| Cost for 500 SqFt wall | $315 | $937 | $1510 |
| Cost per SqFt | $.63 | $1.87 | $3.02 |
Typically when externally insulating, builders use more expensive materials than traditional interior studwall insulation. To more easily attach to the outside sheathing builders use expensive foam board or roxul comfortboard. I prefer to blanket every outside facing surface with a R-15 mineral wool batt to keep everything cozy.
Why use mineral wool batts for external insulation?
Mineral wool batts are the best option because they are the most affordable, are water repellent and have high performance value. The reason builders have traditionally chosen to use foam board and rockwool comforboard is because of their rigidity. They are easy to place up against the outside of the sheathing and wont flop over or fall down. Mineral wool batts are not as rigid and require more care to install but are much less expensive.
Mineral wool is water repellent and bug resistant. Other batt materials like fiberglass and denim batts are a bad choice because they absorb and hold water. In theory the insulation should be fully protected behind the siding but humidity will still find its way in. A vented mineral wool insulation wall will remain dry, keep out pests and retain its R value performance.
How does external insulation save money?
- External insulation is much more energy efficient and will save money on AC and heating costs. This saves you money on the cost of monthly heating natural gas and electricity bills. It also saves you long term by using the furnace or AC unit less frequently allowing them to last longer.
- When you externally insulate it leaves the option for not having to install drywall. Typically drywall will will cover up insulation, wires, pipes to make things look nice. However if the insulation is outside the sheathing there is no need for drywall and that will save money on materials like drywall tape, joint compound and drywall sheets.
- If you use mineral wool batts for external insulation you won’t be spending much more than if you just filled the stud bays with any other insulation. This is leaving out the cost of drywalling. The only additional cost for external insulating is the screen mesh material, plywood for the battens and the 6” lag screws. This will account for about $100 over 1000 SqFt.
How to install mineral wool batts for external insulation
The process for installing mineral wool batts is very similar to the installation process of foam board and comfortboard and can be summarized in 7 steps:
- Attach a bugscreen at the bottom of the house.
- Find where the studs are in the wall and mark their location on the sheathing.
- Lay the mineral wool batt lengthwise and screw 2 holder screws through the batt and into the marked stud.
- Continue to add batts vertically stacked on top of each other.
- Once you are 5 batts up you can install the wood batten over batts to secure them down down.
- Continue to add mineral wool batts and wood battens over them until the wall is full.
- Secure the bugscreen over the bottom and top of the batts and attach to the battens.
How to install the bug screen
I bought a large spool of screen material and cut a foot wide strip for the length and width of the house. This strip will wrap over the insulation and will be attached to the battens. This will keep bugs from entering the gap between the insulation and siding. I used small screws to attach the screen to the rim joist of the house. I cut the same length of bug screen for the top of the wall and attach it to the top stud of the wall.
Finding the studs
I then located the studs and marked their location on the outside of the sheathing. You can do this by looking under the house and seeing where the studs are located. Or you can knock on the sheathing or use a stud finder to find the studs. Marking where the studs are is very important and when done properly will save you a ton of time.
Installing the batts
Now that the studs are marked and bugscreen installed it is now time to start installing mineral wool batts. This is where the process will differ slightly than installing foam board or comfortboard. The batts are applied lengthwise on the house starting at the bottom. You will apply 2 holder screws into the mineral wool batt affixing it to the marked studs to hold it up. I recommend using star screws for the holder screws. These screws are just a temporary way to hold the batt up until the wood battens are installed to secure the batt in place.
Installing the battens
Keep adding batts vertically and once you are 5 batts up you can add a batten to secure them in place. I recommend buying sheets of 4’x8’ plywood and ripping down 8” strips lengthwise to make the battens. I also recommend using 6” lag screws to penetrate through the batten, through the 4” of insulation and secured at least 1” into the stud.
You do not need to take the holder screws out to install the batten, just install the batten over the holder screws. Place the batten vertically and screw it in place. One of the things I struggled when installing the battens is missing the stud. Be patient, when the screw is going into the stud properly you will feel it. Don’t overtighten the lag screws as this will compress the batt. Compressed batts have reduced R value.
The battens hold the mineral wool batts down and also serve as the surface to mount the siding of the house. The battens also provide a gap between the insulation and the siding where air can flow allowing more insulative benefit and for the batts to dry if they get wet.
By now you should have at least one column of batts installed and battened down. Move to the next column and work from the bottom up vertically. Slowly the wall will fill up with insulation all he way to the soffit. When all insulation has been installed to the soffit you can pull over the top bug screen and secure it to the batten.
How to install minerall wool batts for external insulation on a roof
The process for installing mineral wool batts on the roof is the same as installing in the outside facing wall. The batts will lie lengthwise on the roof creating columns that will be battened down in the direction from the ridge to the lowest edge of roof. Since the roof is more horizontal and the batt cannot fall, the holder screws are not nessasary. Start from one end of the roof to the other as to not step on any batts while installing. Dont overtighten the lag screws as this will compress the batt. Compressed batts have reduced R value and you don’t want ruin your good work.
You are now done with the insulation install! One more thing to check is to make sure the battens are level with each other and plum. Making sure your battens are level will make your siding appear straight and not be wavy. This will be very important as you start to install siding.
Conclusion:
External insulation is energy efficient and stops thermal bridging. This will allow the internal stud bays of the wall to be left open and become a design feature of the house. After external insulation is installed the battens are tightened and made level with the house, we are prepared to install siding.
Next Step: Installing Siding
More resources: https://ncma.org/resource/thermal-bridges-in-wall-construction/