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mmoses101 10-14-2012 08:02 PM

Calling GBR.... moisture management siding
While I welcome any and all comments I do hope GBR will provide his/her helpful hints.

The situation:
- Full replacement of all siding on my 100 year old house.
- There is actually two lays of wood siding with the bottom layer being nailed directly to the structural members of the house.
- I live in Alexandria VA (DOE climate zone- 4A)

Doing my research the following is what I believe to be the best solution:

from inside out:
1- OSB
2- Tyvek Wrap (with the self adhesive strips around door and windows and z- flashing)
3- 1" XPS foam insulation
4- air gap (rainscreen)
5- siding (I am using a mix hardie board on the back and side and wood on the front....the wood is a historic district thing

I read over this post:
(which is in large part why I ask for GBR by name) but to be honest I started getting a little lost.

1) should I be worried about moisture between the XPS and the Tyvek
2) Are there different types of Tyvek... I started reading into what class Vapor Retarder Requirements...
2) I did the back of the envelope calculations and 1" seems to be sufficient with regards to Dew point (three coldest months ave temp 31*F with 39 delta T). My reluctance to just do 2" has to do with building out the windows too much.
3) Can the rainscreen simply be plastic slats running vertically? I see all kinds of new "system" that look like you're putting up another vapor retarder but it seems theoretically straight forward.
4) the rainscreen should go under both the wood and the hardie board, right?

Sorry for all the questions. I've been research this for a while and the more I learn the more complex and confusing it gets. There is a lot to consider here. I'm a weekend worrier not a pro and when your about to drop this much cash you want it right!


Windows on Wash 10-15-2012 07:31 AM

Stand alone home or row home?

While rainscreen walls are great during a new construction formulation, they pose a specific set of difficulties in retrofit that some folks are not comfortable with.

What I would do is this:

  1. Remove both layers of existing siding
  2. Spray foam/seal (only for sealing not for insulation value) inside wall and all penetrations.
    • Be sure to spray foam/seal all the top plates and install blockers if the home is balloon framing
    • Install batting if there isn't any and use High Density FG or Roxul
  3. Figure out if you need let in bracing or if the home requires some sort of plywood to lock the structure together
  4. Install rigid foam
    • I prefer ISO but XPS of proper thickness (1" is fine here for a 2x4 wall) is just fine.
    • See what the price delta is to go to 2" and if you are already having to do the trim and fur out work, go with 2"
  5. Install Tyvek or other WRB
  6. Install rain-screen furring strips
  7. Whether or not you will require solid sheathing to the outside depends on wall stud spacing but in most cases you do not require a full exterior sheathing as long as you meet bracing guidelines.

Your questions:
  1. Not if you seal the interior and make sure your air barrier is complete. If the moisture is from the outside, it should be largely avoided via proper flashing, Z-bar detailing, and moisture management technique.
  2. The Tyvek in this case should be vapor open (Class III) as it is not serving as the vapor control level. The XPS is the vapor control in this case and the Tyvek is but the bulk moisture preventative.
  3. Yes. There are several manufacturers of furring strips out there. Most people use wood as they are easier to get and the wood should not be getting, and certainly not staying, wet.
  4. Yes, if you use a plywood deck.

Gary in WA 10-15-2012 10:59 PM

Welcome to the forums! Thanks for the "page" though there are many members who could help you. (Such as WoW).

1. Is there a vapor barrier under the drywall?

2. Will you be replacing the cavity insulation if you remove the wood (diagonal?) sheathing? If not, you don't need the OSB unless the sheathing is horizontally run only.

3. 2x4 or 2x6 thickness?

4. What is the type/R-value of the cavity insulation?


AndyGump 10-16-2012 01:57 AM

1 Attachment(s)
I think some along these lines is probably good for your area, seeing as how you are in a 'mixed-humid' area.
Just leave an air gap about 1/4" or maybe a little more between the WRB and the siding. Furring strips might do nicely.
Your exterior siding will replace the stucco.


Windows on Wash 10-16-2012 07:29 AM

+1 on the questions.

Need to really know what the interior wall construction and type is.

I doubt there is any vapor barrier on the interior as they are few and far between around here.

mmoses101-1 10-16-2012 07:51 AM

While the house is classified as a townhouse it is free standing. I would point out that the clearance on one side is only about a foot between houses. As WoW very smartly pointed out the house is balloon framing with old school 2x4 (or maybe there 4x4 but not 2x6) post running the entire height of the 2-story structure.
1) There is no vapor barrier in the walls. Just drywall, a mixture of fiberglass and flown in insulation, two layers of siding and tar paper in some locations.
2) My expectation is that the cavity insulation will need to be replaced. I had not thought about WoW suggestion of using high density insulation. I don’t know what the extra cost is for high density insulation but that does concern me. and yes I have seen your post on convection loops with low density insulation (
3) 4x4 post that runs the full height of the 2 story structure.
4) I believe that most of the house is fiberglass batts however I have run into spots that have flown in insulation.
Your comment on spray foam as a sealant is a good one. I know for a fact that air from the crawlspace (which has a vapor barrier (20 mil) and dehumidifier) can reach the wall cavity. I didn’t want to muddy the water with this but am glad you brought it up. Sealing the top and bottom plates is important. With regards to the need for solid sheathing, I am not a structural engineer but it would seem to be that the balloon framing would need it to lock the structure in.
Thanks guys for your help.

Windows on Wash 10-16-2012 10:56 AM

Blocker everything out between floors.

High density batts aren't that much more than regular and you only have one shot at this so wait until the resources are there before doing it half way.

Spray foam every gap you see and a 2-part kit will pay dividends here.

Check with the county on code for the structure. They will tell you what they require.

If they let you do let-ins or corners, that is cheaper.

Gary in WA 10-16-2012 09:05 PM

Starting with the map on pp.5, you are in a high exposure area with your annual 43” of rainfall. Perfect for the rainscreen, vented cladding or vented drainage plain; (1)

On page 13, use the housewrap behind the foamboard, in front of the OSB = Wall section #1----- for two reasons; it gives full support and stops (pressure differences) billowing;
OSB (hydroscopic) needs Tyvek “DrainWrap” to give it that air gap; Photo 11; (2)

Page 14, no foil-faced foamboard for your location. An added vapor barrier would stop drying to the exterior; (1)

Your delta T figures are very different than mine for your location; 70*- 47 = 23 x 0.72 = 16.5 – 70 = 53.5* OSB inside face temp. @ 70* = you won’t have condensation until over 55% RH.; (3)

Page 16, the example given are without an inside air-barrier. I would caulk the studs/plates/drywall joint to prevent air exchange to and between insulation cavities, especially at any electrical boxes- inside or exterior (canned foam) ADA; (4)

Notice on page 29, staking the foamboard with poly flashing at horizontal joists; (1).

Page 24, window installation, notice no sticky wrap OVER the sill (bottom) window flange; (1)

The best head flashing drip cap for your application – end dams over the window and back dams under the window, Figs. 3, and 2: (5)


mmoses101-1 10-19-2012 07:19 AM


WRT delta T- I guess it depends on your source. I originally took the ave low from here:
But I guess that was worst case. I recalculated using AVERAGE temp for the coldest three months from here;
(which shows a much different ave low)… The bottom line is that 1” of XPS should do the trick.

This was all great information and I really appreciate all the help from both you and WoW.

Here is what really bothers me. I have now spoke to around 10 +/- contractors for this project and in almost every case I am the one bring all the information to the table. Most of these contractors are “siding professionals” and I’m the one talking dew point calculations and the need for crinkle house wrap. One guy didn’t even want to take off any of the existing siding. He just wanted to nail the new stuff on top, leaving all the underlying problems in place.

This has led me to question why this may be. Why is it that an untrained armature seems to be more knowledgeable than the professionals? As I see it, it is either there are a lot of “professionals” out there that don’t know their trade or I am gold plating the solution (meaning exterior insulation finish system is overkill and I’m overthinking this). The answer is properly somewhere in between.

Be interested to get anyone’s thoughts on this.

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