questions about detailing residential wd frame w/exterior foam


So, as I've mentioned to a few of you, I'm starting my own (small) office after years of doing large project work. This means that residential detailing is new (again) to me. The last time I was doing residential detailing, the building code didn't require exterior continuous insulation. Now it does. In my area this means 1" of exterior foam. 

I'd like some tips on properly detailing at openings (windows & doors) and trim (corner boards, rake/eaves, etc).

It looks to me that I have a couple  of choices / questions:

1) tyvek/air barrier/weather barrier can go between the sheathing and insulation, or over the insulation. Regardless, the air barrier must be the primary drainage plane and any flashing (flexible or rigid) must be detailed into the weather barrier plane. Correct? Which location do you prefer for the weather barrier - under or over the insulation? Why?

2) Same sort of question at the openings. For example, a window can be located such that the window fin is attached directly to the sheathing. OR, it could be pushed out to attach through the foam. OR, there can be a nailer installed to push the fin out to the thickness of the foam. Regardless, the opening needs to be flashed / sealed to the weather barrier.  (I've seen all these sorts of details when searching online, would like some help choosing...)

If the fin is attached to the sheathing, it would seem to me that you need to install some trim to cover the edge of the foam between the window and window trim. 

How do you detail this condition, especially considering getting maximum coverage of the foam, for best energy efficiency? 

3) Same question for trim - such as at corner boards. Do you install a nailer behind the trim or just attach the trim over the foam? 

Ideally, I'd like to have energy efficient and simple details. Thanks in advance. 

Oct 15, 13 9:35 am

Here are a couple of examples


Oct 15, 13 9:40 am

There are numerous problems with exterior rigid panel insulation ranging from inability to effectively seal joints and penetrations to attracting insects (isocyanurate) and difficulty attaching siding. In some conditions there is an issue with the dew point and condensation causing rot in sheathing and framing. Tyvek degrades in contact with tanin in wood and alkali in cement.

The best practice is to avoid exterior insulation (and tyvek) altogether.

I use a combination of 2" of closed cell polystyrene foam and encapsulated fiberglass batts to fill the cavity. The foam is an excellent infiltration barrier (infiltration is about 50% of losses) and is impermeable to water (a vapor barrier). Windows and doors are foamed in place.

The spray foam is about R6 per inch and costs about $2 / sq.ft. installed. It's non-toxic when cured but nasty when sprayed. Installers wear hazmat suits with air supply. The cost is negligible when you subtract what it replaces and the simplification of labor not to mention energy savings.

Oct 15, 13 10:50 am

Miles, fair enough. How to you make this code legal? My reading of the Energy Conservation Code is that continuous exterior insulation is required without energy modeling. 

Oct 15, 13 10:53 am

In NYS it's "Alternate Building Design by Energy Analysis" made using heat loss calcs for various building components based on area and infiltration loss for the entire structure. Don't know about RI code.

Oct 15, 13 11:08 am

What do you use for the analysis? ResCheck? I think ResCheck works here if you aren't doing the prescriptive assemblies. 

Oct 15, 13 11:16 am

Miles is absolutely right.  The requirement for 1" foam on exterior walls makes no sense.  Everywhere you nail through it for siding, trim boards, etc. is another place for rust, water penetration, etc.  Not to mention what do you do around dryer vents, light fixtures, etc.  Also, you want to attach windows to something solid, not to spongy foam board.

I don't see foam done around my area so I wonder how they are getting around it.  Government get dumber every year.

Oct 15, 13 11:27 am

Honestly, we have the exact same problem with commercial construction too - just try attaching metal siding over 2 or 3" rigid foam. Suddenly you have continuous steel zee purlins going through the foam, pulling tons of energy straight through the assembly. 

Oct 15, 13 11:53 am

gruen:  Not to mention that the longer the attachments get, the more you have to make sure they don't start sagging from the weight of the exterior panels.  Do they ever think about this stuff when they pass these requirements?  As if I don't know the answer.

Oct 15, 13 1:38 pm

No, they don't  They are fucking clueless and driven by corporate vendors.

Locally the HERS rating system is applied to residential construction but ignores commercial. It also highly rewards geothermal systems despite their proven inefficiency, high electricity use and enormous installation and maintenance costs. In fact most builders plumb their houses for a gas-fired boiler that is installed immediately upon receipt of the c/o.

Aside from that, HVAC contractors do their own ratings, no conflict of interest there ...

Oct 15, 13 1:46 pm
Upstream thinking, in either a time crunch or out of ignorance, negates the best intentions. Decade after decade.
Oct 15, 13 1:50 pm

Along these same lines, I came across this interesting spreadsheet from Owens Corning that lists R-values for batt insulation that's been compressed into different depth stud cavities. The general rule of thumb is that compressing batt insulation reduces it's R-value. This is true, however, some thicknesses/compressions result in higher R-values than could be achieved with the 'standard' thickness batt.

Oct 16, 13 10:28 am

Sad to see so many designers scared shitless of simple rainscreen assembly.

" Suddenly you have continuous steel zee purlins going through the foam, pulling tons of energy straight through the assembly"

Thermally broken purlins do exist. 

Oct 16, 13 11:03 am

^ Long time no hear. Been on sabbatical?

Oct 16, 13 11:15 am

Rusty, can you give me a link to a product? I'd love to specify it on future projects. I adore rainscreens but am trying with each iteration to design more thermally efficient assemblies. 

Recently, I'd specified some high compressive strength insulation under the bottom course of brick (we'd estimated that it would compress only 1/32") to tie wall cavity insulation directly to foundation insulation. Of course, the contractor just built it without the insulation....sigh...

I find that top of wall (parapet) and bottom of wall details are often the most tricky to get good wrapping of insulation and weather barriers. Best practice for insulation often is negated by the need to securely attach building elements - which is really the topic of this thread. 

Oct 16, 13 11:15 am

gruen, I am not sure off the shelf products exist, but these are simple to make assemblies. We have used two part extruded aluminum Z with thermal break in the long leg. 

There is an additional play at hand though. With new code NFPA requires fully tested wall assemblies, and foam-plastic board insulation is toxic and burns. We tend to use mineral wool boards now (like Thermafiber's Rainbarrier). There are other alternates to foam board as well.

Hi Miles. I missed you this many.

Oct 16, 13 11:58 am
Rusty, can you send me a link to this product? I've often thought a fiberglass extrusion might work too. I've had hat channels screwed through the insulation before but it is true that can be difficult if you are hanging a lot of panel, might compress the insulation and bend the screws.

Regarding testing wall assemblies, is this a NFPA 101 reqmt or ?? 101 often only crops up in healthcare projects.
Oct 16, 13 6:16 pm

I believe it is NFPA 285 that Rusty is referencing...

Oct 16, 13 6:36 pm
Yes, I googled it NFPA 285 requirement now incorporated into IBC 2012
Oct 16, 13 7:13 pm

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