Fine Homebuilding Project Guides

Insulation

Guide Home

Installing Rockwool for Continuous Exterior Insulation

Careful cutting and fitting plus smart flashing details make the job of wrapping the walls and roof of the 2018 FHB House in mineral wool go according to plan.

By David Good

On top of the air barrier and WRB we have a thick wool coat. Continuous insulation provides comfort which is why Rockwool calls it ComfortBoard.  On the walls we used the ComfortBoard 110 formulation–which is a bit more dense than the ComfortBoard 80. We installed two layers of 2-in.-thick, 24-in. x 48-in. sheets with the joints offset. They are milled to high tolerance and easily fit tight. We used appropriate length washer head nails to place it. All it takes is 2 or 3 fasteners per sheet as the siding will ultimately strap the insulation to the building.  The ComfortBoard cuts nicely with handsaws.

The roof has one layer of 5-in. Toprock DD which we got in 48-in. x 48-in. sheets. The TopRock is confined to the thermal boundary and was placed flush with the patio and breezeway roofs that are thermally broken and elevated with Maine Deck Brackets. On top of the exterior insulation is a redundant WRB under the siding and roofing. The impressive thing to me is that the Rockwool plane came out so true that our siding and roofing was not fussy to install true. There is no ply over the Rockwool either–we fastened right onto it into the framing.

TopRock DD is a dense, rigid Rockwool panel that doesn't compress underfoot. It's intended for low slope roofs like this.
TopRock DD is a dense, rigid Rockwool panel that doesn’t compress underfoot. It’s intended for low slope roofs like this.

 

The insulation planes with the roof overhang on the left. An underlayment will be installed over the insulation and overhang and then the metal roof is installed directly on top of the underlayment.
The insulation planes with the roof overhang on the left. An underlayment will be installed over the insulation and overhang and then the metal roof is installed directly on top of the underlayment.
A few washer nails hold the ComfortBoard in place. The siding will also secure it. The walls are covered in two layers of 2-in.-thick Rockwool.
A few washer nails hold the ComfortBoard in place. The siding will also secure it. The walls are covered in two layers of 2-in.-thick Rockwool.

 

Fabricating some flashing for the windows to accommodate 4 in. of exterior insulation. Fabricating some flashing for the windows to accommodate 4 in. of exterior insulation.
The two layers of Rockwool will now fit behind the window trim. The two layers of Rockwool will now fit behind the window trim.
The rigid board cuts nicely to fit around windows and doors. The rigid board cuts nicely to fit around windows and doors.
The Rockwool was cut with bread knifes and handsaws depending on the carpenter's preference. The Rockwool was cut with bread knifes and handsaws depending on the carpenter’s preference.

 

All of the conditioned space has exterior insulation. The second layer on the walls flushes out with the windows.
All of the conditioned space has exterior insulation. The second layer on the walls flushes out with the windows.

More from FineHomebuilding.com

Fine Homebuilding Recommended Products

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

Smart String Line

String lines — sometimes called dry lines — are invaluable for straightening walls and a million other things. The reel makes it easy to stretch and wind the string.
Buy at Amazon

Plate Level

A plate level allows to you plumb walls accurately because it registers off the plates without touching potentially warped or twisted studs. This one can reach the plates on 12-ft. tall walls.
Buy at Amazon

Reliable Crimp Connectors

These reliable, high-quality connectors from Wirefy work on 10-22-gauge wire have heat-shrink insulation to keep out water and road salt.
Buy at Amazon
Previous: What to Consider When Choosing Blown-In Mineral-Wool Insulation Next: “Warm and Fluffy” Insulation

View Comments

  1. Palantine | | #1

    It appears to me that 2 X 4 construction is adequate to provide the code required R value for walls using mineral wool bats and the outer wrap in this article. Can anyone confirm or dispel this for me?

  2. qofmiwok | | #2

    In my Passivehouse training I have learned that the WRB should be outside the exterior insulation (and air barrier inside it). Why did you decide to put the WRB inside of the exterior insulation?

Log in or create an account to post a comment.

Become a member and get full access to FineHomebuilding.com

Insulation

Insulation

Trusted, comprehensive guidance from the pros for a home that is healthy, comfortable, and energy efficient

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Insulation Project Guide.

Start Free Trial

Energy Efficiency
Air-Sealing
Choosing Insulation
Installing Insulation
Water Management

Fine Home Building

Membership & Magazine

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.