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Old 12-06-2009, 10:37 PM   #1
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Foam Board on exterior of above grade house


I am trying to choose between the foam board 1.5" on the exterior above grade of a new house construction (R-20 6 inch bat insulation between the studs) vs. the new spray foam type of insulation only being applied between the studs.

I realize that the spray foam can add a high R-value to the house and completely seal the house on the inside. Unfortunately with this costly method it does not stop the "Thermal Bridging" of the 2x6 wall construction. The 2 x 6 board only has an R-value of 6 but the foam spray in between the 2 x 6 studs has an R-value of 20. The boards will still allow hot heated air from inside of the house (in winter) to pass through the studs to the exterior of the building.

The spray foam would be easy and quick so I am looking for tips on installing the 4x8 x 1.5" thick sheets on the exterior of the new construction house whithout putting allot of nail holes through the foam board thus causing the thermal bridging anyways via holes from nails to attach the foam board to the house, holes from Tyvek to be attached to the house and then finally holes to attach the siding onto the house. After all of these methods are done the foam board will have allot of holes in it.

Suggested please?????

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Old 12-06-2009, 10:56 PM   #2
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Foam Board on exterior of above grade house


The thermal bridging of the nails is a minor concern in my opinion.
The bigger concern with thick ridged insulation applications is fastening the siding. If you choose this route I would recommend a rain screen wall. For two reasons, One you have firring strips applied over the foam to provide solid nailing and I am a firm believer in the benefits of rain screen walls.
Have you considered a hybrid combination? Spray foam and a thinner ridged foam exterior.
There are many factors to consider, your location, your exterior finish and your budget.

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Old 12-06-2009, 11:06 PM   #3
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Foam Board on exterior of above grade house


Your post is really confusing.

If you are stuck with a lightweight construction of 2x6 studs, you will have thermal short circuiting through the studs, which is unversally known. Whatever you put in the cavity between the studs will still be limited by the stud conductivity.

If you have a R-20 insulation in a 2x6 wall you will have thermal short circuiting that will reduce the wall insulation value to about R14 or R16 depending on the stud spacing. The good thing is that you probably have wood studs instead of steel studs that could get you down to as low a R11.

every exterior wall must have a primary moisture barrier since not siding or veneer is really waterproof with vinyl siding leading the list of the worst in terms of "waterproofing", since it is just loose and hanging on the wall and must be allowed to move with the exterior temperatures.

The idealistic "pink panther" approach of advertising R-values is based on short term lab tests of the insulation material itself without any regard to air infiltration around the insulation fitted into a variable cavity. that is why people spend so much time and money on eliminating air infiltration.

Obviously, a sold sheet of extruded polystyrene on the exterior will be a far better investment in real insulation per advertised "R-value" because it dramatically reduces infiltration and provides real insulation to matter how many nails you pound it.

The question of a sprayed insulation between the studs depends whether you have some sort of a plan for your "vapor barriers" or vapor retarders since there is no definitive measurement of a "vapor barrier" since it is an arbitrary dividing line used to separate different thickness of the same materials for the normal minimal code requirements (the worst you can build to a still be legal).

Dick

Last edited by concretemasonry; 12-06-2009 at 11:09 PM.
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Old 12-07-2009, 12:09 PM   #4
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Foam Board on exterior of above grade house


Let me refresh the question. I am going to be getting a builder to build me a new house. There insulation cost is well over $20K for the spray foam on the inside of the basement wall, above grade exterior walls and cieling in the new house.

I can purchase XPS Owens Corning for the entire exterior and interior basement walls for around $5K and glue it on myself. My question is;

1. If use the 1.5" XPS on the above grade exterior wall and batt insulation on the iterior (2x6 wall construction) will that be a better option than using no XPS on the exterior and just going with the spray foam on the inside of the house in order to retain heat in the extremely cold winter month in Winnipeg, Canada.

2. The layers of the house going from the exterior would be;
a. Vinyla Siding
b. Tyvek
c. 1.5" Owens Corning XPS (with R value of 7.5)
d. 7/16 OSB board
e. 2 x 6 studded walls (with fibre bat insulation of R-20)
f. 6 mil poly vapour barrier
g. 1/2 inch Drywall

3. My second question is with all of the nails going through the XPS from installing all of the exterior amenities, will my XPS foam actually still do the job of a perfect heat/draft seal preventing unwanted air drafts from robbing me of my energy efficiency?
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Old 12-07-2009, 07:48 PM   #5
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Foam Board on exterior of above grade house


This is a subject of great debate in the industry right now and every answer you get will differ.
Energy conservation is a necessity in my mind, but its a matter of degree. For some it has become almost a hobby or a challenge to see how much they can do regardless of the return on investment.

One argument that is a hot button is: Will 1 1/2" foam move the dew point far enough toward the outside of the building envelope to eliminate condensation at the interior side of the foam? This is a question I can't answer for your climate, but it's one I think you should consider researching. My gut feeling is with 6" wall it will.
So off hand I think your system is well planed. I like the idea of minimizing the thermal bridging. That is not going to happen with spray foam alone. Fiberglass is a great insulation, the downfall is installation. A little gap here and a short piece there add up to a lot of area if not done properly. Also I feel that the vapor barrier must be meticulously installed if you have foam on the exterior including receptical gaskets.

As to the nail question. I think the heat loss from nail perforations is so minimal there is no need for concern. The heat loss from one small high quality window will equal much more than all the nails combined. I have no data to back up that assumption, but I still stand by it.
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Old 12-08-2009, 10:20 AM   #6
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Foam Board on exterior of above grade house


The Dew point is definately a concern of mine. We are going to be building a two storey custom home. Our first house I had re-done the insulation in the basement and took out the crap insulation and replaced with foam board. It was only a 9 year old house but the insulation job was horrible.

Now that the basement is done with 2 layers of 1.5", reflectix bubble wrap in between the studs it is nice and toasty warm. Litterly the entire basement of 1450ft2 can be heated with my dehumidifier of 600W. It has a dehumidifier, heater and fan built all in one. It pushes the temperature to 21.9 C. This is a nice comfortable range with little effort.

I would like this in my above grade and below grade in the new house. My other question is below;

1. By using the 1.5" foam board, 7/16" OSB, 2x6 studs and then 1/2" Drywall this gives me a total wall depth of 8 inches. Anyway of getting away from purchases hugely expensive jamb extensions for all doors and windows on the house that can run around $6000 for just them?
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Old 12-08-2009, 05:05 PM   #7
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Foam Board on exterior of above grade house


Look into having your builder make the extension jambs or make them yourself. I have a hunch it may be more economical than buying them from the window manufacture.

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