Over ten years ago I removed the wooden lap siding from my 1932 28/40 bungalow. I caulked/fixed all gaps, put 15lb felt on, added 1 inch polyso ( r-board without foil either side), typar wrap, and vinyl siding. I had previously blew cellulose insulation in the walls. there is only paint on the lath/plaster walls. I did not tape the seams of the 1 inch rigid insulation. Thought it would be better to let it breathe. Anyway that's the way I was thinking. I have noticed at various times of the year that the plastic headed nail fasteners that I used actually show as moisture on the outside of my siding. I used a lot of these so I'm sure I punctured the polyso/typar many times. The house is weather 5(central iowa) Now I want to add more rigid insulation to the exterior. How should I do this? Can I put more rigid insulation over the untapped first layer? Do I have to remove the typar air barrier before I do this? What type of insulation board and how much would you recommend? All windows and doors are furred out 2 inches. My big concern is that I don't want any moisture problems by not taping the original polyso. Thank you for your insight and expert advice. c williams

remove the tyvek, tape the seams, install and tape your next layer. Then strap the wall to minimize nail penatrations, and fasten the siding to the strapping.

Bob thanks for your reply. Since I will be doing the labor anyway, would you recommend more than 1 inch foam? Also, what type do you recommend? Thanks c williams

2" polyiso would be excellent and the foil face is easy to tape.

You only need 1.25in of polyiso in zone 5, 1.5in of XPS. If you go 1.5in thick that is more than enough to mitigate dewpoint.

Posted By Surfsup on 27 Jan 2014 12:40 PM
You only need 1.25in of polyiso in zone 5, 1.5in of XPS. If you go 1.5in thick that is more than enough to mitigate dewpoint.

In cold climates you have to derate the polyiso for exterior sheathing apps- figure on ~R5.5/inch if applied on fiber-insulated 2x4 construction if the iso is 3" thick, R5/inch at 1.5" or less.  In zone 5 you need R5 min for 2x4 construction, R7.5 for 2x6.

So while 1.25" iso would meet code from an ASTM C 518 labeled rating point of view, it's actual performance at zone 5 outdoor temps would be a bit shy of what's needed for 2x6 (but 1.5" would be OK.)  The folks at Building Science Corp recommend installing EPS as the outer layer of stackups when using iso in colder climates, to get the best average performance. The peak performance of iso is when the center-foam-thickness temp is about 50F, but if falls off performance cliff when the center-depth of the foam is below 25F.  The performance of EPS keeps rising with falling temps to well below 0F, so if the outer inch is EPS in a 3-inch stackup it'll perform better than if that last inch were iso in cold/very-cold climates.

Do you believe moisture will be a problem at all considering the interior walls have been painted many times in 81 years. Also, lath/plaster interior walls. Thanks

Dana, So you think for 1.5 total thickness...zone 5...1in of Polyiso then 0.5in of EPS on top of that or 0.75/0.75?

Since I'm doing the work myself I could put two layers (though I'd prefer to save that time)...I could put the housewrap on the sheathing, then put the two layers of rigid foam (taped and sealed), then furring, mount the windows to the furring framed out around windows, then siding...

Why put the housewrap on the sheathing?
Put it on the foam so it can overlap the flashings.

I'm thinking now I will put on the foam, then the furring strips, THEN the wrap (over the furring). The windows/doors will be furred out too so I can fold it into the openings then mount the windows/doors.

I would not use Tyvek or a similar product under wood siding; leave the wood siding open to the strapping so it can breathe. Tarpaper is fine if you want to use that. If you're using one of the plastics (tyvek, etc) you can apply it to your sheathing to help curtain any air movement. We also sometimes put a rainscreen fabric outside the foam under horizontal strapping if we are using cedar shingling.

Foam then flashings and house-wrap to direct bulk water to the flashings, then strapping then siding. You defeat the purpose of the air gap if you move the house-wrap to behind the siding.

Water will get by the siding, so it will hit the house-wrap and run down to base flashings to be pushed to the outside. If you don't put the h.w. over the foam then the foam has to sit in front of the flashings. Dew point control is very important but more buildings are lost to bulk water ingress then to internal moisture. Of course if you have a tight stack-up and then allow bulk-water the situation is that much worse.

Look up a product called Cor-a vent to close off the area between the strapping.

well put.

The placement of the depth of windows in the framing determines whether the housewrap goes between the foam & sheathing vs. the exterior side of the foam, since the window flashing needs to lap properly with the housewrap:

An "outie" window mount puts the glass roughly co-planar with the exterior of the foam, and the wrap goes on the exterior, but tight to the foam, not the siding- leave a rainscreen gap between the wrap & siding.)

An "innie" mount puts the glass roughly co-planar with the structural sheathing, and the housewrap goes between the foam & sheathing, tight to both. For innie-mounts it's worth using a crinkle-type housewrap, since that provides a capillary break and a micro-gap to manage any bulk water injected by the flashing.

Either are easy to do with only 1.5-2" of foam.

For 2x6 construction in zone 5 you need R7.5 minimum outside the sheathing to be able to get by with just the interior latex paint as the interior vapor retarder. 1.5" of polyiso is typically rated R9-10, but would perform right at about the minimum during the colder months when it counts. 2" of EPS is rated R8.2, but delivers close to R10 at mid-winter temps.

The January mean temp in Chicago is +25F, and the goal is to keep the sheathing average a 40F (typical indoor air dew point) or above with a 70F interior. With R20 center cavity insulation on the interior that means the delta-T per R you're looking for is (70F-25F)/R20 = 2.25F/R. The difference between the 40F sheathing average temp and the 25F outdoor average temp is 15F, so the absolute minimum exterior R performance you can get away with in your very specific climate is 15F/(2.25F/R)= R6.7

The average center-depth temp of the exterior foam if you go for the minimum is 25F + (40F-25F)/2 = 33F. At that mid-foam temp even 1.5" of EPS makes it (if barely,) and you'd also be OK with 1.5" of polyiso despite derating it to ~R7.3-R7.5 at that temp. With an inch of iso and a half inch of EPS you'd be at R7.5 or a hair over at that temp, but perform more poorly than 1.5" of iso when it's 50F outside.

With foam that thin the EPS isn't doing a whole lot for the iso performance, the way it does when the foam is much thicker. If you went with 2" of EPS (or an inch of each) you'd meet code with margin by the ASTM rated numbers an exterior inch of of EPS would outperform an exterior layer inch of iso when it hits 0F or lower.

An alternative to FBBP's very good recommendation for using Cor-A-Vent, is using a ~6" wide strip of the spun plastic roll mesh ridge venting, which keeps most the smaller insects out as well. Some people install both, since Cor-A-Vent is more rodent-resistant than the mesh products, but has corrugations wide enough for some ant species.

Dana - if you put the h.w. between the foam and the sheathing, would you not need to use a flashing deep enough to cover the siding , strapping and foam in order to drop the bulk water back out to the front? Or am I missing something in what you said?

Cor-A-Vent now glues insect screen to the face of their corrugations. Big improvement!

Posted By FBBP on 29 Jan 2014 11:48 AM
Dana - if you put the h.w. between the foam and the sheathing, would you not need to use a flashing deep enough to cover the siding , strapping and foam in order to drop the bulk water back out to the front? Or am I missing something in what you said?

Cor-A-Vent now glues insect screen to the face of their corrugations. Big improvement!

If you put the flashing 2" out from the housewrap you create a discontinuity in the drain plane are are more prone to having bulk water getting behind the flashing at the edges of the flashing.  This was one of the insights of Canadian building scientists  looking at bulk water leakage paths in different assemblies back in the 1990s. (IIRC it came out building engineering folks at University of Waterloo, but I cood b rong.) 

If you extend the flashing to the exterior, the housewrap MUST come with it to form a continuous weather-resistant drainage layer. But it's fine if the drain-plane is behind the exterior foam (which is moisture tolerant), as long as it's continuous with the flashing. It's common to do it either way, and with thin foam you could mount it innie and put the WRB & flashing at the exterior of the foam if you wanted to. With thicker foam innie-mounts it's generally easier & more reliable to put the drain plane/flashing at the sheathing rather than the exterior side of the foam.

Is there a wall assembly that shows the innie/outie window/foam/wrap assembly? I am losing you guys in the discussion. I am putting 1.5in rigid on. So I was going to furr out around the windows/doors and mount the windows to that. So you're saying I should have sheathing, then foam, then tyvek, then the furring, and the flashing goes where? Questions:

1) How do I find the studs easily with housewrap over the foam? I was thinking with foam up, I could just screw the furring at the 4x8 seams and down the middle "thirds" of the panel to easily find the studs. With housewrap over the foam, are there markings on the housewrap? Seems like a pain.
2) Regarding the flashing... with sheathing, then foam, then tyvek, then furring, the flashing should be attached to the outside of the foam, and the tyvek should continue over the top of the flashing? Or should the flashing be attached to the sheathing behind the foam, long enough to come out past the foam face?
3) I see the cor-a-vent site with lots of products, I assume we are referring to the "siding vents"?

http://www.cor-a-vent.com/pdf/cor-sv5.pdf

Also, Dana you said PI is derated to ~R7.3-R7.5, the article shows it at R6.5:

http://www.finehomebuilding.com/item/4822/which-rigid-insulation-should-i-choose

Is the article incorrect or is polyiso getting better?

Posted By Surfsup on 31 Jan 2014 06:07 PM
Is there a wall assembly that shows the innie/outie window/foam/wrap assembly? I am losing you guys in the discussion. I am putting 1.5in rigid on. So I was going to furr out around the windows/doors and mount the windows to that. So you're saying I should have sheathing, then foam, then tyvek, then the furring, and the flashing goes where? Questions:

1) How do I find the studs easily with housewrap over the foam? I was thinking with foam up, I could just screw the furring at the 4x8 seams and down the middle "thirds" of the panel to easily find the studs. With housewrap over the foam, are there markings on the housewrap? Seems like a pain.
2) Regarding the flashing... with sheathing, then foam, then tyvek, then furring, the flashing should be attached to the outside of the foam, and the tyvek should continue over the top of the flashing? Or should the flashing be attached to the sheathing behind the foam, long enough to come out past the foam face?
3) I see the cor-a-vent site with lots of products, I assume we are referring to the "siding vents"?

http://www.cor-a-vent.com/pdf/cor-sv5.pdf

Also, Dana you said PI is derated to ~R7.3-R7.5, the article shows it at R6.5:

http://www.finehomebuilding.com/item/4822/which-rigid-insulation-should-i-choose

Is the article incorrect or is polyiso getting better?

No, it's not getting better. The ASTM C 518 test is at 75F mid-foam temp.  The R-value of polyiso is extremely non-linear-  as the mid-foam temp falls it's performance improves for awhile, peaking at about 7.5/inch, then starts getting worse- MUCH worse when the mid foam temp is below freezing.

See this bit o' bloggery, which has this pic:



The brownish trace is the thermal conductivity of the R6/inch polyiso sample.  The blue vertical line is 75F, the center foam temp at which the stuff is tested. If you look at the right-edge scale the minimus of ~0.15 occurs at about 15C which is about 60F.  A conductivity of 0.15 is an R value of (1/conductivity=R) of R6.7.  (It's a bit difficult to estimate the english-units conductivity, since the grid is correlated with the left side scale.)  Newer polyiso tests out at about 6.5/inch at the ASTM C 518 mid-foam temp, so a reasonable guesstimate is that it'll be R7.5-ish/inch at the knee.

But at -5C/+23F center-foam the thermal conductivity has climbed like crazy to about 0.46, or R2.2.  That's a  performance DISASTER for the last inch of foam in an climate that averages +23F in January (sorta like Chicago.)

But note that anywhere above 7.5C/45F center-foam temp it's still outperforming high density EPS, so in thicker stackups you can optimize the performance by using EPS in part of the stackup where the average mid-winter temp is above that 45F point, and polyiso inside that point.

Similar derating curves from other sources show similar effects, if  not quite as severe, but it's still bad enough to be of concern.  If you're designing the stackup with the cavity fiber so that the mid-winter mean temp at the sheathing is at 40F (pretty much the IRC prescriptive minimums) you're better off with all-EPS.  But if half the total-R is exterior foam, using EPS on the exterior with iso under that can deliver the performance in a thinner stackup.

An "outie" window mount puts the glass roughly co-planar with the exterior of the foam, and the wrap goes on the exterior, but tight to the foam, not the siding- leave a rainscreen gap between the wrap & siding.)

if the wrap is under the furring, how do I seal the windows? It is my impression the wrap should wrap around into the window rough opening. Then be taped sealed. So if I put furring on top and mount the window to that, there is a seam between the furring and wrap. I suppose I should caulk and screw the window box furring so the caulk seals this area?

Posted By Surfsup on 31 Jan 2014 06:07 PM
Is there a wall assembly that shows the innie/outie window/foam/wrap assembly? I am losing you guys in the discussion. I am putting 1.5in rigid on. So I was going to furr out around the windows/doors and mount the windows to that. So you're saying I should have sheathing, then foam, then tyvek, then the furring, and the flashing goes where? Questions:

1) How do I find the studs easily with housewrap over the foam? I was thinking with foam up, I could just screw the furring at the 4x8 seams and down the middle "thirds" of the panel to easily find the studs. With housewrap over the foam, are there markings on the housewrap? Seems like a pain.
2) Regarding the flashing... with sheathing, then foam, then tyvek, then furring, the flashing should be attached to the outside of the foam, and the tyvek should continue over the top of the flashing? Or should the flashing be attached to the sheathing behind the foam, long enough to come out past the foam face?
3) I see the cor-a-vent site with lots of products, I assume we are referring to the "siding vents"?

http://www.cor-a-vent.com/pdf/cor-sv5.pdf

The lip of the flashing always goes behind the wrap. If water hits the wrap, the wrap must direct it over the flashing.

Most h.w. have 8" marks on them. If you line up the first mark with a stud, the rest will self align. Alternatively, and it sometimes works better if you are by yourself, use 4 foot high rolls. Put on the bottom course, mark the stud locations about 4" down and then put on the next course. Of course this will give you an extra seam to tape but it might be worth it.

Either way works for the flashing. If the flashing is behind the foam, the h.w. can still be in front of the foam. Pay attention at windows and doors.

sv5 is for 3/4" gap. sv3 works for 1/2" gap. Contrary to the cor-a-vent instructions, put the screen at the bottom to prevent insects like mud daubers from plugging the holes. Try to get the siding to just cover the cor-a-vent to protect it but not so much that there is a nesting/hiding space for insects.

Windows are a pita no matter which way you do them. Some people wrap the openings with all sorts of products like peel and stick or flexible flashings. Other just turn the house wrap into the opening tape up the lower inside corners, mount the windows with caulking and call it a day. You can also tape the flange back to the h.w. The thing is, most sill rot is from condensation on the window or air exfiltration carrying moisture with it to condense near the flange. It is seldom from bulk water intrusion. If you wrap everything in high grade products and some moisture still gets through, how will it dry? There was a time when we would wrap the outside of the window jamb with framers poly, sealing it with acoustic caulk and then when we went to install the poly on the inside of the walls, we would seal the framers poly back to the wall poly with acoustic. We had pretty good results with this but it meant for bulky corners and a pain to install trim. Now mostly we just install the bottom window flange over the h.w. and the h.w. over the side and top flanges. Tape the side and tops to the flange and fill the gap from the inside with enerbond foam.

If you mount the windows to the foam, with no strapping under the flange, you can still use this method. Use a wider piece of sheathing around the window so that it sits over the flange and leaves room for any additional trim you might put around the window. some people use L.P.'s Smartside trim http://www.lpcorp.com/trim_fascia/resources/ around the windows and then butt the cement board to this. Put another strip of h.w.under the trim and out so that it will also be under the siding. The problem remains with the drip edge of the lower window moulding. If you bevel the top of the smartside and caulk the upper corner, it will help drain the water to the front. In any case, with this method, the main house wrap will catch any water getting into the trim joints and direct it out over the flashings and out to daylight.

If you feel you need the strapping under the nailing flanges, install the h.w., install the strapping, put a house wrap or other drain pan in the window sill opening and over the lower strapping, mount the window and install a strip of house wrap over the side and top nailing flanges and over the strapping and tape it back to the original house wrap and to the nailing flanges. If your windows are all within inches of your soffits, you are done. If the windows sit lower on the wall in that they have no protection over them, you will need to install another flashing above the window to turn the water out to daylite.

Or you could just build an ICF house and not worry about all this nonsense!