Hello pros!

I have done much reading on the subject and am still in need of a final round of feedback on using foil-faced polysio board as a weather-resistive barrier (WRB).

I completed a project over the summer where I used Tyvek (lapped and taped) as the WRB with 3/4" foil-faced (FF) polyiso over the Tyvek, and then installed 1/2" wood sleepers beneath the new siding as a drainage plane and air space.

I did this because I read that FF polyiso (and other closed-cell foam boards) should not be viewed as effective WRBs over the long term.

Well, I really want to try using FF polyiso as a WRB in my next re-siding project. My thought is as follows:

1. Install 1/2" AdvanTech sheathing to wall studs,

2. Install 3/4" Dow Tuff-R FF polyiso to OSB sheathing,

3. Flash windows\doors and tape seams between polyiso boards with Grace Vycor Plus,

4. Install 1/2" AdvanTech sleepers to polyiso and around windows\doors,

5. Install HardiePlank siding & composite trim.

Should I just add a step after #1 and install Tyvek DrainWrap beneath the polyiso?

Thanks a lot!

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What is your climate and how are you determining the 3/4" polyiso?  The well sealed foil is a very good vapor barrier and few locations need that on the outside and none need 2 VBs.  The guidelines for adding the foil faced polyiso (frequently done) usually require a thicker layer.

Not an expert on this, but see it done wrong a lot in my cold climate.

Bud

Bud,

I am in northern Virginia (NOVA). The recommended above-sheathing R-value is an R-5 here, and therefore the 3/4" polyiso.

I definitely do not want to create a double VB.

You see a lot of 1/2" rigid under vinyl-sided houses in my area. I have grown to abhor this approach. They skimp on structural sheathing and just toss 1/2" rigid on top of the studs then install some crappy vinyl.

I am consistently running into cases here in NOVA where stud cavity insulation is poorly installed, walls are not wrapped, and performance is lacking. The idea of FF polyiso as an exterior retrofit appears to be the only cost-effective solution. 

Ahh you warm climate folks, we need about 2" minimum rigid on the outside.  But I see everything from fanfold to 1" with only a few using the 2" FF polyiso.  I will finish this summer (health delays) my long overdue upgrade where I'm adding 3.5" of rigid over the plywood sheathing.  Then osb, house wrap, and vinyl siding.  Besides improving my own home I have been testing different approaches and identifying how-to methods.  The house is just over half done and what a difference from the old 3.5" walls.

Adding to the outside, especially when new siding is needed, is a very good option.  And we should add as much as we can.  I would have to play with your climate numbers, but while you are at it it might not cost that much more to double the foam.  Again, I don't have your numbers.

As for the FF polyiso acting as the WRB, I have heard it both ways.  Leaving the foil exposed should have some benefits and the DOW material has a very thick foil.

Bud

Thanks Bud.

Back to the drawing board for now.

First off - way to go, a rain screed detail is one of the best things you can do

If going with foam, a double layer is best if you offset the seams as Joe Lstiburek has found out - foam expands & contracts. If you don't go that route, using a WRB like Tyvek helps make up for that any shortcomings. With that the Tyvek should be placed where it ties into the windows. You may even consider using a tall wall product like a Zip Wall system

For that & more I am going to point you to a piece I did - To tyvek or not to tyvek http://blog.sls-construction.com/2013/common-sense-weather-resistiv... which should help you out in this regard

You may also wish to check out http://blog.sls-construction.com/2013/common-sense-building-exterio...

Bud: sheathing then foam provides a stronger structure more able to resist snow loads, wind, etc...

Thanks Sean!

By using insulation on the outside of the structure, you reduce conduction losses. These thermal modeling runs were 400-minutes with a 20C temperature differential, by adding 1-1/2" insulation board on the outside of the sheathing and using furring strips to allow condensation drip to the ground, then setting the siding away from the board radiative conduction to a cold wind is reduced.

By using 1-1/2", it turns the mass of the wall itself into thermal-mass for the interior room, cold climates and northern exposure may use more, this is a big deal, it moderates any temperature changes and quiets the rooms, quite noticeable how much less heat intrusion from adding the layer.

Using 1-1/2" of hemp-mortar that can be applied with gunite gear gave a temperature change at the thermometer of only 1.2C, also, the hemp-mortar transmits moisture slowly so far better in damp climates to allow walls to fully dry out when the weather gets dry and fungus can't grow against it.

For building it, you can use a sealant on the outside joints to create a vapor barrier on the outside and remove the vapor barrier on the sheathing except around windows-doors-vents, the board is waterproof if closed-cell, the foil is a radiative layer to reduce those losses.

The top image is the run with a standard wall & batt insulation and gave 13.6C of change, the lower the same wall with the added insulation board showed 4.1C, white is outside temperature:

Wow, thanks Tom.

Pictures (images) really can tell a lot.

I am a couple weeks out before we begin removing the existing siding, trim, etc. 

The most recent idea was to re-insulate the cavities with R-13 un-faced batts, sheath the walls with 7/16" Zip System panels, and then see where we stand through blower door and IR testing.

I really want to add FF polyiso on top of the sheathing and create a drainage plane\air space before installing new siding, but the costs may be prohibitive (relative to the benefits). These images are making me second guess the value of the benefits relative to the costs.

Thanks again.

From a lot of these runs, I'd suggest you can leave the batting alone, my take is to not touch the interior at all for this, remove any vapor barrier from the exterior sheathing, box out for windows and all then add on the insulation & furring, re-apply the siding.

For furring out the siding, rip 4x8 sheets of 1/2" plywood 1-1/2" wide full length. These can be glued down offset from studs to not conduct as much with construction glue & a few screws into the board to hold them till the glue sets and that does show up in less heat transfer on the thermometer that's 1-1/2" from the sheetrock, runs are conduction only, no convection or radiation.

If you have temperature-humidity loggers, using them to document changes is so worth the trouble. Room noise is also part of the improvement ...

I believe PolyISO can absorb moisture if it gets wet... and it would get heavy.  Make sure any punctures in the foil are sealed well.

Have you looked at the DOE's Building America website for notes on projects like this.  I believe Building Sciences Corporation has done testing.., as part of DOE project.

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