Video: How to Install Rigid Foam Insulation Outside a House
GET THESE FIVE TRICKY SPOTS RIGHT, AND YOU’LL GET A SUPERINSULATED, ENERGY-EFFICIENT HOME
An experienced builder explains the ins and outs of airtight exterior insulation
BY DAVID JOYCE
Installing rigid foam on the outside of a house is a great way to double the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. and eliminate air leaks. Large sheets of foam insulation, screwed and sealed to the walls or roof, can cover or replace conventional 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. and create a continuous barrier to heat loss or gain—something you don't get with cavity insulation alone. We wrapped the house in this video with 4 inches of polyisocyanurate in two staggered 2-inch layers. This thickness strikes a balance between ease of installation and maximum R-value. If the foam is any thicker, it's hard to hit studs when driving screws. Plus, combined with 5-1/2 inches of cellulose in the stud bays, 4 inches of rigid foam brings the wall up to a respectable R-40.
Foam sheathing needs to be tight and sturdy
No matter what type of house you're insulating, how well the rigid foam is connected to the rest of the wall makes all the difference in performance and durability. Strong screws should extend from wood strapping on the outer surface through the foam sheathing and into the framing below. The foam boards should fit tight, and all edges and seams must be well sealed to keep out air and water. The key is to identify all gaps and choose the best way to seal them; a few key details should cover most of the bases for a job well done.
1. Use bug screens and flashing below the foam
Many builders protect foam sheathing from insects by wrapping the bottom edge with strips of window screen. We take a belt-and-suspenders approach to air-sealing by backing up the screen with a sheet-metal drip edge. This directs water away from the foundation surface and adds extra protection from bugs and wind.
My crew and I bend aluminum or galvanized steel sheeting so that it extends up behind the foam and out past the bottom edge far enough to nail to the bottom of the wood strapping. Then we nail a strip of window screen several inches wider than the flashing to the vertical part of the drip edge, leaving enough length to wrap under the foam and over the bottom of the strapping that holds the foam in place.
2. Build plywood boxes to span deep window openings
To compensate for the thick foam, we extend all rough openings with site-built boxes made of 3/8-inch plywood. The boxes are not structural; their main purpose is to provide a straight, solid surface to air-seal to and to align the windows. Strips of plywood nailed to all four surfaces of the rough opening extend from the inner edge out to where the outer edge of the foam will be. Don't forget to leave room for the plywood when framing rough openings.
3. Fasten the foam with plywood scraps before the strapping goes on
It's virtually impossible to install the strapping that supports the foam sheathing as the foam is going up. Instead we drive long screws through a couple of small scraps of plywood to temporarily support each piece of foam. The screws go through the foam into the framing below, which comes in handy when laying out the permanent strapping.
Solid-wood strapping screwed through the foam into every stud will pull the sheathing tight to the framing and provide a solid nailing surface for siding and trim. The 3/4-inch space this leaves behind the clapboards does two things: it leaves plenty of room for any water that gets in to escape; and it allows the foil facing on the surface of the polyisocyanurate foam to act as a radiant barrier, boosting the insulating value of the wall.
4. Stagger all layers of foam, caulk all outer edges, tape all seams
Another belt-and-suspenders approach of ours is using two layers of 2-inch foam rather than one 4-inch layer of foam. We offset the pieces so that nowhere is there a continuous seam penetrating straight through to the framing. Even the corners are "woven" together with alternating pieces. We install one layer at a time, taping or caulking all gaps as we go.
We use construction adhesive or silicone sealant where the foam laps wall plates and outside corners. Housewrap tape goes over all seams and inside corners, and where foam meets windows and doors.
5 Use the right tape for the job
Different tapes stick well to different materials. Some are designed for a single purpose, while others are good multitaskers. After trying several kinds of tape, we've been happy with Dow Weathermate for most foam seams; it sticks to everything and always stays put. For wider gaps and outside corners, we use strips of ice-and-water or peel-and-stick window flashing.
—David Joyce is a partner at Synergy Companies Construction. The home in this video was designed by Building Science Corporation in partnership with the U.S. Department of Energy's Building America Program.
13.
Mon, 01/23/2012 - 15:02
Mon, 01/23/2012 - 15:02
Edited Mon, 01/23/2012 - 15:03.
Response to Joshua Riley
by Martin Holladay, GBA Advisor
Joshua,
There is no single answer to your question. Whether or not you decide to use wider vertical strapping at the corners depends o the thickness of the rigid foam, the type of siding you are installing, and whether your siding extends to the corner or you have vertical trim boards at the corner of your house.
Some builders switch from 1x4 furring strips to 1x6s or 1x8s at the outside corners; others don't.
Here are some links to photos found on the Web to give you some ideas about how other builders do it:
http://web.me.com/jgreening/Site/HG_H_Shed_Project/Entries/2010/1/5_sidi...
http://www.cor-a-vent.com/userfiles/image/Flywheelers%202%20pic.jpg?1269...
http://i373.photobucket.com/albums/oo171/mcyr/DSCN0798.jpg
http://aconcordcarpenter.com/wp-content/uploads/image-import/_0vuJZhliRZ...
12.
Mon, 01/23/2012 - 14:40
Mon, 01/23/2012 - 14:40
corner boards
by joshua riley
How does the strapping work at corners (i.e. how do you connect corner boards)? Do you have an image/detail of this? Thanks-
11.
Thu, 01/19/2012 - 08:52
Thu, 01/19/2012 - 08:52
Edited Thu, 01/19/2012 - 08:53.
Response to Steve Wilson
by Martin Holladay, GBA Advisor
Steve,
Q. "When retrofitting an existing home with cellulose/tar sheathing, would it be best to add the foam in two layers, as described in the video, to meet the min R value for moisture control in my zone or will one layer suffice?"
A. Two layers is better, but one layer will work. In all cases, pay attention to air sealing the perimeter of each sheet of rigid foam.
Q. "In a two layer system, can you use unfaced foam for the first layer?"
A. Yes.
Q. "Even if you meet the minimum R value for your zone for sorption protection, is moisture drainage a potential issue at the bottom of the foam when sealed with caulk/adhesive?"
A. No. The important drainage area is at the bottom of the air gap between the rigid foam and the siding.
Q. "Is there any problems you see with caulking the bottom at angles, much like a w with a couple of inch gap at the bottom of the v portion of the w?"
A. This suggested caulk pattern won't be as airtight as a pattern that provides a seal around the perimeter of each sheet of foam.
10.
Thu, 01/19/2012 - 08:30
Thu, 01/19/2012 - 08:30
retrofit installation
by Steve Wilson
When retrofitting an existing home with cellulose/tar sheathing, would it be best to add the foam in two layers, as described in the video, to meet the min R value for moisture control in my zone or will one layer suffice? Properly taped and sealed at the outer edges.
In a two layer system, can you use unfaced foam for the first layer? Foil faced is 15-20% more expensive.
Even if you meet the minimum R value for your zone for sorption protection, is moisture drainage a potential issue at the bottom of the foam when sealed with caulk/adhesive?
Is there any problems you see with caulking the bottom at angles, much like a w with a couple of inch gap at the bottom of the v portion of the w. So it would look something like \ / \ / \ / . Only shallower angles.
9.
Thu, 01/19/2012 - 07:58
Thu, 01/19/2012 - 07:58
Installation Questions
by Steve Wilson
Really great video. My house has the 1/2" or so cellulose/tar sheathing from the 70's, brick on the first floor and siding on the second. It only has 4" studs and I was thinking of adding 2" of foam outside and replacing the bad vinyl siding job that split the good cedar siding, when I reroofed.
Do you use any J or Z flashing up top?
I have a Cape Cod with three dormers. I am thinking I would have to extend the roof deck since the sides are pretty much flush with the roof. Would you add J flashing here?
Do you use any flashing under the eaves?
How is the screen attatched to the z flashing?
How is the screen finished off at the bottom of the foam? Is it wrapped up around the outside of the foam before siding?
How much of a gap is left between the z flashing and the foam?
I have offset roofs for the house and garage. How much space would you leave between the foam and shingle step flashing/roof deck?
I assume I will have to pull the windows and adjust to match the foam/stringer/siding.
8.
Sat, 03/26/2011 - 21:19
Sat, 03/26/2011 - 21:19
Edited Sat, 03/26/2011 - 21:20.
Response to Darrell
by Martin Holladay, GBA Advisor
Darrell,
We need more information to answer your questions.
1. "I looked under fiberglass insulation in the interior walls and some plastic screws holding the board leak in the winter." Why do you have fiberglass insulation in your interior walls? Why is the fiberglass insulation exposed instead of covered? What is "the board"? What are the plastic screws?
2. "And I have a damp spot in the area and blackened the wood board." What area? What is "the wood board"?
3. "I looked under one of the plastic siding outside and it looks like the siding is attached right to the foam." Are you talking about vinyl siding? If so, that's not unusual.
4. "How can I fix it now?" How can you fix what? What is broken?
7.
Sat, 03/26/2011 - 16:19
Sat, 03/26/2011 - 16:19
foam leaking
by darrell fuglseth
I have a bilevel house about 6years old.I have one room not finished in the lower level or basement.I looked under fiberglass insulation in the interior walls and some plastic screws holding the board leak in the winter.And I have a damp spot in the are and blackend the wood board.I looked under one of the plastic siding outside and it looks like the siding is attached right to the foam.How can I fix it now?
6.
Sat, 12/11/2010 - 16:41
Sat, 12/11/2010 - 16:41
Response to Mum
by Martin Holladay, GBA Advisor
Mum,
Q. "What do I do now?"
A. Repair it. Barring that option, remove it and replace it.
5.
Sat, 12/11/2010 - 16:38
Sat, 12/11/2010 - 16:38
Insulation
by Mum
My house has this rigid insulation two layers of 2" offset - installed 30 years ago. Birds and other critters have gotten into it. Also water damage. What do I do now? The house loses more heat at night than it should so there is obviously a problem.
4.
Sun, 08/15/2010 - 20:42
Sun, 08/15/2010 - 20:42
Boxing window openings
by Dan
Can 1/2" OSB be substituted for framing window openings? It is about half the cost of plywood in my area.
3.
Wed, 06/23/2010 - 16:42
Wed, 06/23/2010 - 16:42
Timberlok screws
by Rob Wotzak, GBA Advisor
Paul,
I believe the guys at Synergy are using Timberlok
screws. I'm sure there are other types of long screws out there, but Timberloks are very strong, are easy to install, and are available at most building supply stores.
2.
Wed, 06/23/2010 - 16:10
Wed, 06/23/2010 - 16:10
strapping 4 inch foam
by Paul Myers
looking for 6 inch screws for this application. any suggestions Thx.
1.
Thu, 02/25/2010 - 14:51
Thu, 02/25/2010 - 14:51
Great Video!
by Marshall Durrett
Thanks for the helpful video and information. This is definitely the RIGHT way to install rigid foam insulation. I should point out that your method also allows for a thermal break between the exterior siding and the interior sheetrock. We used this on a LEED Platinum home in Austin, Texas but didn't go as far with double-layering the rigid foam and using the rain-screen strapping. I'll know better next time.
Comments