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How to attatch rigid insulation to old house - furring out window?

We are building a house in Northern Canada and I am not sure the next step. The house is 2400 sq. feet, 5/8" drywall, vapor barrier, 2x3 horizontal strapping with R-8 batts, 2x6 wall with R-20 batts, OSB sheathing, house wrap, and I want to add 2" rigid to the exterior of the house. The house has a HRV system throughout, electric baseboard heating, and the roof is R60 blow in. My question is in regards to how to add the rigid to the exterior of the house. Do I glue the rigid on, and then strap vertically through the insulation and to the house studs?? How do I fur out the existing windows to match the new level of rigid and vinyl siding that we will attach? I am all new to this, so any advice is welcome. Thank you in advance.

Asked by Anonymous
Posted Tue, 08/10/2010 - 12:33

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11 Answers

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1.
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Randi,
Attach the foam temporarily with long cap nails; then screw vertical 1x3 strapping through the foam to the studs to secure the foam permanently and create a rainscreen air gap.

Windows can be done many ways. Most builders build a box out of 3/4-in plywood. In your case the plywood would be cut into 8-inch-wide strips. The plywood box fits in the window rough opening (which has, of course, been framed 1.5 inches oversized in both directions) and projects 2 inches out from the OSB,

Your rigid foam is installed tight to the plywood box. Then install a "picture frame" of 1x3 strapping, installed on the flat, around each window. The picture frame is screwed to the end-grain of the plywood (and through the foam to the OSB as well). Window flanges are nailed or screwed to the picture frame.

This is an "outie" window installation. You can also do "innie" windows. Read more here:
'Innie' Windows or 'Outie' Windows?

Answered by Martin Holladay, GBA Advisor
Posted Wed, 08/11/2010 - 04:04

2.
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I would reconsider the vapor barrier under the drywall. Combined with the rigid foam on the outside you will lock in any moisture that gets in the wall. Standard procedure these days is to leave out the vapor barrier and let it dry to the inside of the house.

Answered by jhoin
Posted Wed, 08/11/2010 - 10:14

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How do I fur out the existing windows?

I'm confused. This is new construction? Are windows already installed at this point and you have decided to add exterior foam as an afterthought?

You already have a reasonable thermal break with the cross-hatched inside framing. If you were to install dense-pack cellulose insulation or even cotton batts rather than (I assume) fiberglass batts, you'd have a reasonably efficient wall system without the exterior foam. You can add a layer of rigid foam on the interior, which would improve the thermal break, add additional R-value and serve as the vapor retarder. It would be far easier to extend the window jambs inward than outward if they are already installed, and this approach would allow drying to the exterior which is important in a cold-climate house.

Answered by Riversong
Posted Wed, 08/11/2010 - 10:26

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We bought the house partially built, so yes, the rigid is an afterthought in the planning.

Thank you for your answer Martin, so we will not need to use glue to seal the rigid on, only nails and then the strapping.
Jhoin - We did not think that the layer of rigid would cause moisture problems. It is a common practice up here to add the rigid on the exterior and keep the vapor barrier inside. I believe we would need the vapor barrier inside, as part of a house inspection up here is a blower test, and if we only had the outside rigid I do not think that it would be as "tight" and pass. The rigid is only on the exterior walls, not not joining to the roof vapor barrier. But now you have me totally worried, and what you are suggestion does make sense, I wonder why so many people do it this way? What to do now?
Robert-The fiberglass batts are already installed, and unfortunately I do not think adding the rigid to the interior is an option as the inside walls are already framed up.

Answered by Randi
Posted Wed, 08/11/2010 - 12:27

5.
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Randi,
When you install your exterior rigid foam, use caulk around the perimeter of each sheet. This is an air sealing measure; it has nothing to do with attachment (which is accomplished with cap nails).

Even in Northern Canada, where poly is often used, I would hesitate to install poly if you have exterior rigid foam. A better option for an interior vapor retarder is MemBrain.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 08/11/2010 - 12:45

6.
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Are the windows already installed? If so you'll need another detail than Martin's.

Answered by Dan Kolbert
Posted Wed, 08/11/2010 - 14:58

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Yes the windows are installed

Answered by Randi
Posted Wed, 08/11/2010 - 16:41

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We were thinking of not sealing the pieces of rigid together, with tape or caulk, to allow it to breathe if necessary.

Answered by Randi
Posted Wed, 08/11/2010 - 16:43

9.
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Randi,
I'm a designer who has been working through these issues the last couple of years and am currently working on my own home thinking through some of the same details. I would repeat the concerns already voiced that adding 2" rigid insulation to the exterior along with an existing interior poly vapor barrier does not allow much drying potential in either direction. Vapor diffusion through the wall assembly is not my biggest concern, my biggest concern is that all your flashing details are designed and executed well because if you get rain infiltration back to your sheathing OSB is relatively more prone to moisture damage than other sheathing options.
Is the added R value to the walls worth the added risk of poor flashing or poor air tightness that may lead to damage to the OSB sheathing? In terms of making your overall house more energy efficient focusing on air-tightness is a top priority (have you done a blower door test?) Also a significant amount of heat loss can occur through the rim joist of the home, also through the basement walls and basement slab. Some things to consider if you budgeted money to improve the energy efficiency of your house, other measures may be as effective and hold less risk than others.

Answered by j chesnut
Posted Wed, 08/11/2010 - 19:07

10.
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Randi,
we work in a more moderate climate than yours (Zone 5) and I cannot claim practical experience with what you are facing but I would absolutely agree with every comment made about the need to avoid double vapor barriers. With no drying potential you are asking for trouble. Also, if an interior air barrier is needed to achieve the tightness you need, it seems to me that you are missing out - if exterior air is allowed into the wall cavity and able to penetrate to the inside of the house, that air can circulate inside the wall cavities which reduces the effectiveness of your batt insulation. I would concentrate on getting the most perfect exterior air barrier possible - check out this wall section (http://www.buildingscience.com/documents/information-sheets/high-r-value...), which shows closed cell spray foam applied to the outside of the wall. That can give you the best of both worlds - a perfect air barrier and what ever insulation level you want. (In the interest of full disclosure, my company installs spray foam and I freely admit to a bias)

Answered by Torsten Hansen
Posted Wed, 08/11/2010 - 20:37

11.
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From Torsten Hansen:

I would absolutely agree with every comment made about the need to avoid double vapor barriers. With no drying potential you are asking for trouble.

Good so far.

check out this wall section, which shows closed cell spray foam applied to the outside of the wall

So, you're absolutely opposed to double vapor barriers unless they are composed of spray foam rather than rigid foam.

Also, if an interior air barrier is needed to achieve the tightness you need, it seems to me that you are missing out - if exterior air is allowed into the wall cavity and able to penetrate to the inside of the house, that air can circulate inside the wall cavities which reduces the effectiveness of your batt insulation.

With a complete interior air barrier, of course, no air can "penetrate to the inside of the house". And with no exit for air movement, outside air cannot penetrate into the thermal envelope (this, of course, requires no channel flow within the thermal envelope).

(In the interest of full disclosure, my company installs spray foam and I freely admit to a bias)

Would you also admit to ignorance of the hygro-thermal principles of building construction, since you have not only demonstrated such but have also completely contradicted your own "absolute" conviction.

Unfortunately, such ignorance is endemic to the spray foam industry, which is much more about hype and marketing than about building science.

Answered by Riversong
Posted Wed, 08/11/2010 - 20:55

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