Planning a new house. We're in zone 5 although at the warmest edge and tempered by ocean so more like a zone 4. Here is the wall stack up. Latex paint (non-vapor retarder) drywall, 7" HD cellulose (horizontal and vertical 2x4's), sheathing (OSB or CDX), 2" XPS, Commercial Tyvek drainage plane, 3/4" furring (forming vented air space), vertical board and batten siding (back primed and painted). Question is this - with a true rain screen where should I try to do the primary air sealing. Seems like making the sheathing to air barrier with caulks and tapes as appropriate is the logical place but then is the drainage plane exterior of the foam also acting as an air barrier? I view the drainage plane as handling the bulk water issues but seems it would be hard to detail as an air barrier. I'd also like to hear thoughts on what materials to use for both the drainage plane and the air barrier. Bear in mind we are in a very moist location so I need to keep the wall drying ability very high. Roof will be similar (unvented cathedral) with drywall, strapping, 2x10 rafters filled with cellulose, sheathing, 2 layers 2" XPS, furring and standing seam metal roof (underside of roofing vented from eave to ridge). Again, drainage plane material will be on top of the XPS - but should there be an air barrier at the sheathing too?

Multiple air barriers can only help. Is the furring something that can't rot?

I guess the concept is air barriers are good as long as they are permeable. That begs the question as to what is the ideal product.

I planned on not using a pt wood for the furring as Building Science has noted that in taking apart their barn the furring showed no signs of rot except in places where there was continuos bulk water leakage. I may try to predict where those spots might occur and use pt there as a precaution - but otherwise the furring would be untreated in both walls and roof.

Yes, but the weather and design of their barn might differ from your house. Maybe untreated wood furring if it is also primed and painted (like the backside of the siding). A metal roof cools way down at night (below ambient) - are you sure the air flow behind it won't cause nightly condensation on the furring strips?

All metal roofs get condensation on the underside for the reasons you note. The venting is to allow that to dry, the furring allows it to drain and the membrane manages the bulk water (like slope shield by Vaproshield).

I guess "barn" is the wrong word. Building Science did a retro fit of a barn into an occupied building as an experiment. They then dismantled it some 12 or so years later to see how it performed. some interesting findings: http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks They seemed pretty confident about the furring.

Softwoods I've seen don't hold up well to a daily soaking, even if it some sides of it can dry out later.

Posted By Birdman on 20 Aug 2012 09:40 AM
I guess the concept is air barriers are good as long as they are permeable. That begs the question as to what is the ideal product.

I planned on not using a pt wood for the furring as Building Science has noted that in taking apart their barn the furring showed no signs of rot except in places where there was continuos bulk water leakage. I may try to predict where those spots might occur and use pt there as a precaution - but otherwise the furring would be untreated in both walls and roof.

The importance of vapor permabilty of the air barrier depends entirely on where it is in the stackup.

Like the siding and sheathing, the furring dries readily into the rainscreen gap so long as the gap is vented both top & bottom. It doesn't need to be anything special.

Detailing the sheathing as the primary air barrier is a good approach, since it's harder to accidentally damage it's integrity.  Using low-expansion gun-foam or construction adhesive on the framing as you install the sheathing can make it VERY air tight. But be sure to put a bead of quality caulking (acoustic sealants work best) under the bottom plate of the studwall along the exterior too. To keep the air-barrier continuous it needs to be sealed at the band-joist/subfloor and band-joist/foundation sill interfaces too, but that can be achieved with spray foam from the interior after the fact.

Detailing housewrap as an air-barrier is possible (and will only help), but it's less robust as a primary air barrier, since it's easy to accidentally knick up  or puncture during construction.

Dana, If I understand what you're saying... you recommend in my stackup, treating the sheathing itself as the primary air barrier by caulking and sealling etc. but not adding an additional material there (between the XPS and the sheathing). Additionally I should do all I can to make my drainage plane as air tight as possible (taped seams and sealing at penetrations for example). This would mean that at windows and doors and penetrations the critical air seal between wall assembly and window/door frame would be at the plane of the sheathing. This makes detailing the window flashing tricky as air barrier and the drainage plane are now 2" apart but both want to connect to the window. I suppose the primary air seal at the sheathing could be caulk or low expansion foam and the drainage plane could be flashed t the window with membrane flashing. Does that make sense?

Posted By Birdman on 20 Aug 2012 12:09 PM
All metal roofs get condensation on the underside for the reasons you note. The venting is to allow that to dry, the furring allows it to drain and the membrane manages the bulk water (like slope shield by Vaproshield).

I guess "barn" is the wrong word. Building Science did a retro fit of a barn into an occupied building as an experiment. They then dismantled it some 12 or so years later to see how it performed. some interesting findings: http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks They seemed pretty confident about the furring.

The roof looks like a wood SIP roof (EPS). They experienced issues the first year with it because of a common issue discussed by many researches; the SIP roof rot. As seen in the above BuildingScience test home, the location where the EPS roof panels meet causes a bad spot for heat transfer as the EPS insulation is lacking where the 2 panels meet.

SIP ROOF ROT


BUILDINGSCIENCE Test Home Observations:

I was quite surprised to see that the EPS shrunk so much as to create gaps where the panels met. This is new news to me. Why would the EPS have shrunk so much?

My other surprise was that Bituthene membrane (peel and stick) sagged around the window areas.

The window issue they had with the "heat mirror" film (as found in Serious Windows) began to fail after 16 years.

In the redo, the homeowner went with XPS/polyiso because he believes EPS is not up to the task like XPS is.

I think the roof is in fact a "site built" SIP ie they laid thick EPS on the sheathing then applied another layer of OSB and screwed through the whole sheebang. When the EPS shrank, because it was only one layer it opened up a continuous gap from warm to cold. Very similar to the issue with SIP's but not exactly. I suppose this could be easily mitigated by a.) using XPS that doesn't seem to shrink and b.) building the insulation up in layers so it doesn't have continuous joints through the whole thickness and c.) taping or sealing the joints.

Peel and stick needs to be continuously supported - this is a standard issue and one I hit in commercial work all the time. Even trying to span a gap of 1/4" will result in sagging an eventual failure. In most cases it's usually easy to span the gap with a bit of thin sheet metal then apply the peel and stick over that.

I agree - two layers of EPS with staggered joints is very little extra cost. Anything that allows any air flow from the interior to a cold spot (not necessarily all the way to the outside) is going to cause condensation. An interior side air barrier and no gaps/bridges in the insulation is best.

IMO, room to room pressure differentials are a major cause of within wall/ceiling air flow.

Posted By Birdman on 20 Aug 2012 11:19 PM
Dana, If I understand what you're saying... you recommend in my stackup, treating the sheathing itself as the primary air barrier by caulking and sealling etc. but not adding an additional material there (between the XPS and the sheathing). Additionally I should do all I can to make my drainage plane as air tight as possible (taped seams and sealing at penetrations for example). This would mean that at windows and doors and penetrations the critical air seal between wall assembly and window/door frame would be at the plane of the sheathing. This makes detailing the window flashing tricky as air barrier and the drainage plane are now 2" apart but both want to connect to the window. I suppose the primary air seal at the sheathing could be caulk or low expansion foam and the drainage plane could be flashed t the window with membrane flashing. Does that make sense?

You still need a weather-resistant barrier such as housewrap or #15 felt in the stackup. Whether that gets located between exterior foam and the sheathing or over exterior foam depends on how the windows & doors are mounted & flashed. 

If you have an "outie" window with the drain plane outside a foam layer, you can still use the sheathing as the primary air barrier, using judicious amounts of low expansion foam in the right places.  But yes, window & door air-barrier and flashing details are always important to consider carefully.

Posted By Dana1 on 21 Aug 2012 11:10 AM

You still need a weather-resistant barrier such as housewrap or #15 felt in the stackup. Whether that gets located between exterior foam and the sheathing or over exterior foam depends on how the windows & doors are mounted & flashed. 

If you have an "outie" window with the drain plane outside a foam layer, you can still use the sheathing as the primary air barrier, using judicious amounts of low expansion foam in the right places.  But yes, window & door air-barrier and flashing details are always important to consider carefully.

On an "innie" window like the above, what do you think of the Grave Vycor product?

We've started using the SIGA swiss tapes which are permeable, non toxic and have a better ability to bond with various substrates than tapes like Vycor and Weathermate. Quite a bit more expensive, but worth it it many cases.

I defer to Bob on this.

We're doing a deep energy retrofit in conjunction with another contractor who has worked extensively with Marc Rosenbaum. In late September we'll be installing Serious/Alpen 925 series windows with a 3-1/4" factory supplied exterior extension jamb and will have three layer of tape plus some spray foam for sealing. The exterior extensions place the glass within the insulation plane. When we're done I should have some photos I can share of the window installation. By the way, we're installing them with masonry clips instead of flanges. I've used the clips before with other windows and prefer them for numerous reasons - the windows sit flat and are held in a more permanent manner.

Posted By Bob I on 22 Aug 2012 11:42 AM
  When we're done I should have some photos I can share of the window installation. By the way, we're installing them with masonry clips instead of flanges. I've used the clips before with other windows and prefer them for numerous reasons - the windows sit flat and are held in a more permanent manner.

Looking forward to the photos.

Window clips are common practice in Europe, especially since they recess their windows in the well or "innie". Here in the States the nailing flange is commonly used as the windows are installed flush with the exterior "outie". Like you, I believe the clips offer advantages but they do require getting use to.

OK. Now I'll get really wonkish.... I met with my structural engineer today. I am located in the "wind born debris" region (120 mph winds) which usually requires impact resistant windows. The impact windows are 40% more expensive than non-impact resistant. There is a provision in the code that allows one to forgo the impact resistance provided the building is designed as a "partially enclosed" structure - which essentially assume the windows will be blown out in a storm. The relevant part of this is that to use the partially enclosed provision on has to design the window install to resist the OUTWARD wind pressure on the windows. That is, once a windward window is breached, the leeward windows see a negative pressure on the outside and a positive pressure on the inside which combine into some pretty large loads. Those loads are not easily resisted by nailing flanges but can be met by adding additional clips in a clip system. My question is this: in a clip system how does one flash the window frame to the drainage plane to make a continuous seal that is flexible and durable?

Carefully tape the window to the drainage plane or sheathing - this is where the better tapes come into play. They are flexible, will adhere to most substrates and are long lasting.

Relying on a tape to make the connection between drainage plane and window frame in a location I will never see again in my lifetime makes me a bit squeamish. I hope to use a fiberglass window. On of the reasons for using h the partially enclosed provision is that only Marvin makes an impact resistant fiberglass window. Most of the fiberglass windows install with clips and rely on a bead of sealant to make the primary perimeter seal. Relying on sealant alone also makes me squeamish. Ideally I'd like to see a first rate fiberglass window with a flange I can flash to and a way to install clips to reinforce it structurally to resist wind pressure.