Cozy Mineral Wool Sweater on the House

John's house has a number of uncommon features that are designed to make the house more efficient and more resilient, better able to withstand weather and time. I'd like to talk about a couple of those details, the mineral wool exterior insulation today and the function of the rainscreen in another post.

Here is our cozy Mineral Wool Sweater before the addition of the rainscreen.

The most time-consuming of these two features is the exterior insulation and all that installation details that entails. So we (I say we, but it is, of course, Carlton and his crew that have done this) covered the entire house with a cozy mineral wool sweater, so to speak. This 2 inch thick sweater of insulation  (total R value of 8) provides an uninterrupted layer over the house that is insect, fire, water resistant and helps maintain the desired interior temperature with much lower energy use. It reduces thermal bridging - the transmission of heat through the parts of the building that are more conductive such as the wood studs and beams. Remember the R-value (defined as resistance to heat flow) for wood is about 1 per inch whereas most insulation ranges from 3 to 6. 

This is a drawing of a thermal scan showing where the heat loss occurs in a normal stud framed house. You can see that more intense heat loss occurs through the windows but a significant amount happens through heat flow across the wood studs. In the average home built with 2' x 4' or 2' x 6' framing, approximately 25% of the exterior is wood which means LOTs of thermal bridging.

Here is an actual thermal image showing the interior of a house where you can clearly see the wood studs and bracing in the wall. The deep purple is probably due to air leaks through the intersection of the studs in the corner. 

From Erik North's Energy Auditing Blog

When I re-sided my house about 3- 4 years ago , I covered the entire house with Polyisocyanurate foam sheets (polyiso for short) to reduce the thermal bridging. I chose polyiso because it has the highest R-value for common insulation materials and was relatively easy to install. Also that was early in my studies on energy efficient design. Disadvantages of polyiso are that it is made from oil, somewhat flammable, insects love to nest in it, and it has "global warming potential" from the blowing agents used in the making of the foam. So at that early stage in my studies, that's what I chose. 

After realizing that there are many other options out there, we chose Roxul Rockboard 80 for John's house. It's much more of a pain to work with because it itches (some people aren't bothered by it, but I try not to touch it without gloves on) and the sheets come as 2' x 4' rather than 4' x 8' for foam sheets. Truthfully, I did not calculate the cost difference because I was set on Roxul and was not going to consider foam. I feel that this is the proper choice for John's house based on fire, insect, and water resistance. 

Later in the construction process, we will do an energy audit so we can see how effective the cozy mineral wool sweater actually is.

Back to Reality - Homebuilding, Work

This week has been a serious back to reality week. I was exhausted after coming back from Oregon last week. I slept like crazy. Going to bed early and everything. I had wonderful days at work, busy, but not overwhelming. I have to take my Pediatric Emergency Medicine recertification boards this week. Pretty scary. I studied some, even in Oregon, but you can't study enough to feel comfortable. I am pretty anxious about that. And then there is the house which is going really well. That's mostly what this post is about.

You may recall the we are using an exterior insulation called Roxul. I looked for Roxul Comfortboard  but was unable to find it easily. I could have gotten it from one of the big box home improvement stores but it would take 6 weeks or so to get it in. I did find Roxul Rockboard online from ATS acoustics, an acoustic supply house. Their lead time was about a week. I posted online at Green Building Advisor (my favorite website) to see if Roxul Rockboard and Roxul Comfortboard were equivalent and Albert Rooks of Small Planet Workshop (where I am getting a lot of my air sealing supplies) said that they were. The compression resistance of both products is 743 psi so the products were very similar. I ordered from ATS despite very high shipping costs, but we were able to get the product in one week.

Carlton and Chris and Myron have been installing this material on top of the very well air-sealed exterior for the past week when I was gone.

Front view of the house with the installed 2 inches of Roxul. Foam was used around the windows so we could make sure that we airsealed well around the windows. The rainscreen  which goes on before the siding has not been applied to this area of the house.

You can see the rainscreen has been applied to the back side of the house. The fixed transom window is above the stairway. I know it looks a little funny, but it is about all that would fit there. We were going to put a window in the bathroom, but decided against it because that can cause all sorts of problems with mold and rot. Since we are trying to build a highly efficient, long-lasting, resilient home, it does not make sense to put a window in the bath/shower area to catch water and cause rot in the wall. So it looks a little strange, but it is on the back of the house. To bring light into the bathroom, we will install a sun tunnel on the roof of the dormer.

Driveway, gable end side of the house. Again, the rainscreen has not yet been installed on this side. You can think of exterior insulation like a sweater for your house. There are essentially no thermal bridges on the outside of the house to conduct heat in or out of the house. So the insulation inside can do a much better job maintaining the desired temperature. The comfort level of the house is going to be amazing, I think.

See what a great job they have done installing Roxul Rockboard 80.

You can see the rainscreen details a little bit better in this photo. There is a small air gap around the windows to allow any moisture in the siding to evaporate which will significantly prolong the lifespan of the siding, the sheathing below the insulation and even paint on the siding.

Well, that's where we are this weekend. Next week Carlton and Myron will work on the soffits and more rainscreen on the gable ends. Then I don't know what is next. I have to figure out penetrations though the wall. Most electrical and plumbing should be through the floor (which brings its own set of problems), but there are still some things that will have to go through the wall. I need to make sure that is planned for now.

Carlton's helper, Chris, was switched to the day shift so he is no longer able to help in the mornings before he goes to work. That is kind of sad, because he has been a very good worker and great help to Carlton. We will miss him.

Challenges in Designing an Energy Efficient Home - Exterior Insulation

I am trying to finalize plans for John’s house. I have to order the kit by January 31^st for delivery in April. I backed up the start date to April just because I’m not ready AND I might be taking boards in March. I do not need to be trying to build a house and studying for boards in the same month. That would absolutely drive me crazy.

Today's topic will be exterior insulation.

Exterior insulation is used to prevent thermal bridging. I learned in my Net Zero energy class that for a standard 2 x 6 wall insulated with dense pack cellulose you lose 25% of the insulation value from thermal bridging through the wood studs because the R-value of the wood is only about 1 per inch. There are a number of strategies to prevent thermal bridging, but exterior insulation outside of the sheathing makes the most sense to me.

Polyiso covering house from the Building Science Case studies

When I installed new siding on the my house, we covered the sheathing with Tyvek housewrap and then  2” polyisocyanurate (polyiso) foam to give me an uninterrupted R-value of 12 around the house. My house looked similar to the house above before the siding was installed. The advantages to using polyiso foam are that it is relatively easy to find, lightweight, relatively easy to install using cap nails, good R-value of about 6 per inch, uses more environmentally benign blowing agents than EPS or XPS foam. There are some serious downsides to polyiso though– ants and termites can tunnel through foam. They don’t eat it but they love to nest in it if it is wet. Although polyiso is less flammable than EPS and XPS, it is still flammable. But the most important disadvantage is that it is derived from petroleum. For those reasons, we’re going to use a different product on John’s house.

I’m voting for Roxul ComfortBoard which is made from volcanic rock. I've talked about Mineral wool insulation before when I talked about insulating John's shop. Below is a photo of exterior insulation with Roxul ComfortBoard.

Roxul ComfortBoard

I’m choosing this because it is non-combustible, does not rot, insects cannot tunnel through it, cannot get moldy, vapor permeable, water resistant. It is probably harder to install the polyiso and it is itchy. I’ve ordered Roxul batt insulation from a local store when I was working on the Lenore house so I think I can get the ComfortBoard which is a compressed version of the Roxul batt. R-value is about 4/inch. Cost is supposedly similar to the foam.

Schematic of Roxul with rainscreen to allow rain to drain from behind the siding. This keeps the wall sheathing drier as well as allows the siding to dry on both sides prolonging the paint and the siding. This photo is from Builder Online  in a slideshow on 10 Hot Products from Greenbuild 2012.

Using exterior insulation solves the thermal bridging problem, but it brings a whole new set of challenges - detailing around the windows and doors, and what to do about the roof overhangs. I hope to cover some of those issues in the next post.