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Building a new house, looking for opinion on insulation/exterior foam combinations

Brian Ducharme | Posted in Energy Efficiency and Durability on

I am currently in the process of designing a new home. I built one four years ago that we have decided is too big and inefficient for what we need. This time around I am looking to build small, simple and efficient. The house will most likely be framed with 2×6 walls, however I would like to maximize R value of that wall.

The house will be a 28 x 36 cape style home. no bump-outs or dormers. There will be 3′ knee walls around the exterior of the second story to allow for more living space upstairs (to fit 3 bedrooms and a bathroom comfortably). There will be zip system for exterior sheathing.

I am looking for opinions on how you would choose to insulate these walls efficiently while on a budget, Would you use roxul? (and if so would you need any type of barrier or would the zip system and the painted drywall be sufficient enough for moisture control?) I haven’t checked the cost of blown insulation in the walls yet but I know this can be a favorable option.

On the exterior I was considering installing a rigid foam for the thermal break however I am wondering how you guys typically handle this at the point of the windows and doors. What are you doing to pad out the windows? If for example there is 1″ foam are you ripping 1″ wood material and just being okay with not having the insulation factor of the foam behind the nail flanges? It seems as though this would cause an issue of not being able to properly flash your windows to the sheathing as well as not having tyvek to put your top piece of flashing tape underneath. And then as far as siding would you recommend longer fasteners or would you use furring strips? We are between using clapboard or vertical siding at the moment.

Thank you for your help.

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Replies

  1. Brian Ducharme | | #1

    I forgot to mention this house is in Connecticut, and I am hoping to install PV panels with mini split units.

  2. Steve Knapp CZ 3A Georgia | | #2

    There is a wealth of information on this site. It sounds like you want to build a "pretty good house" and that's a great objective to aim for. I would also consider hiring a builder who has experience in green building strategies.

    One suggestion, consider using reclaimed rigid foam on the exterior.

  3. GBA Editor
    Martin Holladay | | #3

    A few points:

    1. If you are building a Cape-style house, deciding how to insulate your walls is the least of your problems. The toughest problem is deciding how to insulate your roof. Capes are notoriously tricky to detail, because it's hard to define the thermal boundary for the second floor. If you are willing to install a thick layer of rigid foam above your roof sheathing, this problem can be solved -- but it's essential that you figure out these details before you start building.

    Here is a link to an article with more information: Insulating a Cape Cod House.

    2. Your plan to build with 2x6 walls and exterior rigid foam will work. In your climate zone (Zone 5), the minimum R-value for your exterior rigid foam is R-7.5. If you decide to use polyiso, the foam will need to be at least 1.5 inch thick. If you decide to use EPS, the foam will need to be at least 2 inches thick. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    The best insulation to use between your studs is dense-packed cellulose, but carefully installed mineral wool batts will also work.

    3. Here is a link to an article about installing windows in this type of wall: Installing Windows In a Foam-Sheathed Wall.

    4. Here is a link to an article about furring strips and rainscreen gaps: All About Rainscreens.

    5. Here are links to several other articles you may want to read:

    Nailing Window Flanges Through Foam

    How to Install Rigid Foam Sheathing

    Fastening Furring Strips to a Foam-Sheathed Wall

    6. Here is a link to an overview of the topic under discussion: How to Design a Wall.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    Knee walls usually refers to walls that are set back from the main exterior walls, leaving a triangular cross sectioned attic space behind them (which can be difficult to air seal.) But from the description "...around the exterior of the second story..." it sounds more like the exterior walls are going to be just 3' above the floor level, with a cathedralized section of wall between the exterior wall and the full-height ceiling (?).

    You can save a ton of money on the exterior foam if you use reclaimed roofing polyiso. The cost of reclaimed roofing foam is usually no more than 1/3 the cost of virgin-stock foam, and in CT you're in easy trucking distance of some of the larger reclaimers, such as Nationwide Foam in Framingham, MA, or Green Insulation Group in Worcester, MA, but there are others who may be closer.

    A continuous layer of 3" of reclaimed polyiso (assume R16 minimum when derated for climate & application, but it could be as high as R19 depending on vintage) on the exterior of a 2x6/R23 wall doubles the wall performance, and only 1.5" of exterior foam would be needed for dew point control on a 2x6/R23 wall, eliminating the need for an interior side vapor retarders any tighter than standard latex paint.

  5. Brian Ducharme | | #5

    I have been reviewing articles on the site that Martin has suggested and I now realize how difficult it can be to get a cape-style roof insulated properly. I read the different ways this can be achieved, however, rather than installing rigid foam on top of the roof sheathing (due to the difficulty / labor costs and material costs associated with this method) is there just as good of a method in your opinion that can be done from the inside the house?

    If you were building a cape style home and preferred not to do the added work on top of the roof, how would you insulate this portion of your house from inside to create an efficient home if you were in the process of designing it?

    Thanks for all of the help, this website has been extremely informative.

  6. GBA Editor
    Martin Holladay | | #6

    If you are forced to insulate a Cape style house from the interior rather than the exterior, here are the rules:

    1. The insulation should extend from the eaves (near the soffit) all the way to the attic above the second floor. This method will ensure that the triangular attics behind the kneewalls are inside the home's thermal envelope.

    2. Methods that incorporate a ventilation channel immediately under the roof sheathing will perform better than unvented assemblies.

    3. It's impossible to have a vented assembly if your design includes skylights, dormers, valleys, or hips. Plan accordingly.

    3. The biggest mistake you will be tempted to make is to skimp on R-value. It's essential that you choose very deep rafters (for example, I-joist rafters or open-web trusses) so that you have enough space for insulation that meets or exceeds minimum code requirements. Aim for R-49 or more.

    4. Read this article to find out about methods that work: How to Build an Insulated Cathedral Ceiling.

    -- Martin Holladay

  7. Robert Opaluch | | #7

    In addition to the GBA articles already mentioned, you might find useful information the construction details shown in the GBA "Strategies and Details" tab, "Construction-detail Drawing Library". Even if you don't follow the details exactly, it can give you ideas and how you could handle your own preferred framing details. Or show and discuss these to your builder or framer if you aren't an owner-builder.
    For example:
    https://www.greenbuildingadvisor.com/cad/detail/vented-attic-siding-mixed-climate-raised-plate-metal-roofing-wood-shingle-siding-over-rig

  8. Expert Member
    Dana Dorsett | | #8

    To hit R49 with a fully vented cathedral ceiling takes I-joists, which are a cost adder.

    To hit comparable performance with a combination of exterior foam and fluff-in-rafters can be cheaper, if using reclaimed foam. eg:

    Putting 4" of reclaimed polyiso, runs 60-75 cents per square foot for the foam, $1-1.25 per square foot after adding the cost of a half-inch OSB nailer deck, ( including the timber screws) , and that would allow you to use 2x10 rafters with R30 batts or blown cellulose in the rafter bays without interior side vapor barriers. You would be at a comparable or slightly higher center-cavity R value as a vented 16" TJI solution, but it would outperform it due to the R22-ish thermal break of the 4" foam over the structural timbers.

    With 6" of reclaimed polyiso foam you could get to code min performance with 2x4/R13 rafters, and beat it's performance soundly with 2x6/R20 rafters. (There's a learning curve to hitting the rafters with 7.5" timber screws though, and the screws aren't so cheap.

  9. Brian Ducharme | | #9

    I would agree that I joists seem like a major additional cost for rafters. I would also be curious as to how it would look with the exterior trim out having 16+"fascias with a square-cut rafter, Although I guess I would be battling the same problem with the foam and additional layer of OSB on the roof. Martin mentioned above using deep web trusses. Is this the best approach to gain the decent look on the exterior and high r value in the rafters? I would probably build my own ventilation channels I haven't looked into this truss design yet so I am not sure how this would leave me set up for the exterior trim out either. I also saw in an article on this site showing rigid foam on the inside of the building fastened to the rafters with furring strips over it, in your opinion is that not as efficient of a design? I am new to all of these insulation ideas and creating high-performance homes, I see the value in it and I want to be sure to get it right. it seems there are so many different ways to achieve an efficient end result that it is a bit overwhelming. All of the information is greatly appreciated.

  10. Expert Member
    Malcolm Taylor | | #10

    NA/ NA,

    When people use I joists for rafters they usually terminate them at the face of the exterior wall and use much smaller applied rafters to form the overhangs to avoid deep fascias.

  11. GBA Editor
    Martin Holladay | | #11

    Consider building a two-story home with a vented unconditioned attic.

    -- Martin Holladay

  12. Expert Member
    Dana Dorsett | | #12

    "...would also be curious as to how it would look with the exterior trim out having 16+"fascias with a square-cut rafter"

    You don't need a 16" fascia board for the rafters, since the bottom of the rafter or I-joist they doesn't need to extend beyond the top plate of the structural stud wall at the bottom edge of the rafter.

    Malcolm's fake-rafter ends approach works, or you can notch the rafters, and continue the wall sheathing and wall-foam up to however high it needs to be to meet your cosmetic standard for the depth of fascia.

    An addition to my 1920s vintage bungalow with 2 foot overhangs & exposed rafter ends used the notched-rafter approach, giving the exterior appearance of a 2x6 rafter, whereas the true rafters are 2x12s. It's pretty subtle. Capes typically have minimal over hang depth and fully soffitted eaves, which should make it even easier to hide the true rafter depth, if that's your aesthetic goal.

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