I'm new here and am not very informed on insulating and weatherizing homes.
I have a small wall 8'X20' that I want to re-side and install two new windows in.
I want to keep the drywall intact on the inside.
I'm in No Colorado so it does get cold.
Material cost isn't a big issue because the wall is small.
Is there step by step info on detailing this?
I would like to have a foam board or panels on the outside of the studs.
So far I'm thinking of this.

1. Since I'm not taking the drywall off I should put a vapor retarder in the wall bays first. I could us paper faced R-11 with the paper on the interior side. this would slow down the migration of interior moisture to the outside.

The R-11 warms the bay a bit so the temp differencial is moderated somewhat.

2. then I could cut unfaced open cell foam in the bays and foamed in around the sides. I could set the cut foam in a little in from the outside edge of studs so there is a bit of an airspace.

3. I'm not sure if I need sheathing on this wall. I'll see what is on the wall when I open it up.

4. On top of the sheathing or studs I will put an open faced foam. I'm not sure how thick at this point.

5. Nailing strips or a rain screed goes on next.

6. Then a vapor retarder/barrier/house wrap goes on.

Does this sound right? Let the corrections begin!

Since you have an existing wall to insulate you might be interested in Retrofoam: See www.retrofoam.com  I watched a demo of this product recently and noticed that it can also be installed in a wall that already contains fiberglass bats. 

I am going to do all the work. I did search for a dealer installer near me and it came up with no results.
I'm slow at work so I do just about everything myself these days.
I am taking the siding off.
I will have complete acces from the outside of the 2x4 wall.
I just don't want to disturb the drywall on the inside.

Posted By popawheelie on 22 Mar 2010 04:06 PM
I'm new here and am not very informed on insulating and weatherizing homes.
I have a small wall 8'X20' that I want to re-side and install two new windows in.
I want to keep the drywall intact on the inside.
I'm in No Colorado so it does get cold.
Material cost isn't a big issue because the wall is small.
Is there step by step info on detailing this?
I would like to have a foam board or panels on the outside of the studs.
So far I'm thinking of this.

1. Since I'm not taking the drywall off I should put a vapor retarder in the wall bays first. I could us paper faced R-11 with the paper on the interior side. this would slow down the migration of interior moisture to the outside.

The R-11 warms the bay a bit so the temp differencial is moderated somewhat.

2. then I could cut unfaced open cell foam in the bays and foamed in around the sides. I could set the cut foam in a little in from the outside edge of studs so there is a bit of an airspace.

3. I'm not sure if I need sheathing on this wall. I'll see what is on the wall when I open it up.

4. On top of the sheathing or studs I will put an open faced foam. I'm not sure how thick at this point.

5. Nailing strips or a rain screed goes on next.

6. Then a vapor retarder/barrier/house wrap goes on.

Does this sound right? Let the corrections begin!

The more foam you put on the outside of the studs, the less relevant interior side vapor retarders become.  Unless you're already committed to pulling the exterior sheathing off, you can probably save labor and get more water protection by dense-packing cellulose into the cavities from the exterior before putting the foam board up.

If you ARE committed to pulling the sheathing, caulk all seams where wallboard & framing meet, and foam seal around all plumbing & electrical penetrations, then spray on vapor-retardent latex paints (that are semi-impermeable minimum class-II vapor retarders, but still allow SOME inward drying.).  Then, wet-spraying cellulose from the outside will fill all voids & imperfections far better than any batt-job, and will block 95%+ of intra-insulation convection that robs batts of R-value when the temperature differences are high (ie. when you need it most!) 

But if you insist on batts, unfaced high-denstiy "cathedral ceiling" batts have lower convection and will give you ~R15 in a 2x4 studwall, and the extra performance is well worth the slight increase in cost. If you have an micro-cavities narrower than 1" that aren't readily stuffable with insulation, fill them with 1-part foam (Great Stuff, etc.), and trim the excess flush after it's set up.

Foam board isn't very structural- if you don't use OSB or plywood sheathing you'll need angle-bracing planking to stabilize the wall from racking under wind loading.

Unless you're putting up at least R20 of foam on the outside it's better to use unfaced foam so that there is decent outward-drying capacity. But 3" of foil-faced isocyanurate runs ~R20, and may be the worth it.  FSK tape (2" aluminum, designed for this purpose) makes it a near perfect air & vapor barrier.  Foam seal the tops, edges & seams to make it completed.  With R20 on the outside of a 16" oc. batt or cellulose insulated studwall the average winter temperature of the coldest side of the stud will still be above the dewpoint of the 30-35% relative-humidity 68-70F interior air, and condensation/mold conditions are fleeting- a few hours on the coldest days of the year at worst.  But if you go thinner on the exterior foam it needs to be unfaced so that it's more vapor-permeable than the interior side vapor retarders, and dries toward the exterior.  At 2" of thickness XPS (pink or blue rigid foam) is only R10, and has a vapor retardency of about 1 perm- similar to the interior side vapor retarders, and that's about the max you should go unless you design the stackup & R values specifically for inward-drying.  If your plan was to put 1/2-1.5" unfaced foam out there, go for it.

Assuming it's less than 2" of XPS, do NOT use vapor retarders on the exterior, only air-barriers (like housewrap or classic felt products, both of which are vapor-permeable).  But any wraps should be next to the foam, not the siding, if you're using furring to form a rainscreen gap behind the siding (which is an excellent plan, BTW.)

Thank you for the information. I'm slowly getting the gist of this.

Mayhaps I'm a bit too wordy, but the difference between an air barrier and a vapor retarder can be the difference between rot/mold or a long-lived structure that's healthy to live in. A lot of people just don't get the vapor-thing, and how R-value affects where you may, should or shouldn't put a vapor retarder. Most people get that you shouldn't use two vapor retarders. But the basic deal with vapor control is this:


Average mid-winter outdoor temp| siding | R|R|R| point in R where the average temp is the dew point of interior air |R|R|R| interior wallboard

The dew point of 68F 30-35% relative humidity air is ~35-40F (at sea level). If you put a vapor retarder in the stackup, the humidity of the air on the interior side of the stackup will be at the same level as the interior air, even when it's air (but not vapor) tight. But the temperature as you move toward the exterior drops the further out you go, so the RELATIVE humidity rises the further out you go. If the vapor retarder is at a point where the average midwinter temp is at dew point of the interior air or lower, condensation occurs, and water accumulates in the assembly all winter. If it's even a little-bit toward the interior from that point, it may have hours of condensing conditions at a time on cold nights, but it re-evaporates before it soaks into anything where it can do real damage. If a strong vapor retarder (like foils or poly sheeting) is on the exterior, the buildup of moisture int the assembly over the winter can be HUGE, and eventual rot is a near certainty over the years.

Obviously the local climate make a huge difference of where in the assembly the vapor retarder can live, so the generic prescription for cold climates is to put it on the interior surface, inside all of the insulation. Unfortunately, that's where it's more likely to have holes punched in it- everything from plumbing penetrations to picture hanger nails. And a single square inch of hole with air movement is worth a whole wall of wallbaord permeated water. Controlling air movement within the cavity with highest-density fiber or using spray-foam is critical with or without an interior vapor retarder.

But if you're willing to put the bulk of the R value outside the studs, the vapor retarder can be on the exterior side of the studs, or skipped entirely- if the dew-point hazard region occurs within a foam layer, it doesn't need a vapor retarder at all since there's no interior air in that foam to condense. A combination vapor permeable interior & exterior air-barriers, and the more air-resistant the fiber (or spray foam) insulation the better the insulation performs. Vapor permeable air-barriers allow water to escape as well as enter- bulk water incursions can dry! Cellulose insulation can be useful for humidity control, since unlike mineral or glass fibers, it can absorb a significant amount of any water that gets in there and redistribute it, without losing R-value (until it's approaching saturation). Being alos more resistant to internal air movement helps too, but by no means should it be considered an air-barrier on it's own. Spray foam, even the open-cell stuff IS a good air barrier. Closed cell spary foam becomes a significant vapor retarder at 2" thickness or more, as does XPS.

It's possible to build without vapor retarders at all, with a bit of good design work, but it takes some forethought, and good practices on the air-sealing front to work well. It's often better to have no vapor retarder at all than one in the wrong place within the R-value, and just concentrate on good air-sealing.

Posted By popawheelie on 22 Mar 2010 04:06 PM
I'm new here and am not very informed on insulating and weatherizing homes.
I have a small wall 8'X20' that I want to re-side and install two new windows in.
I want to keep the drywall intact on the inside.
I'm in No Colorado so it does get cold.
Material cost isn't a big issue because the wall is small.
Is there step by step info on detailing this?
I would like to have a foam board or panels on the outside of the studs.
So far I'm thinking of this.

1. Since I'm not taking the drywall off I should put a vapor retarder in the wall bays first. I could us paper faced R-11 with the paper on the interior side. this would slow down the migration of interior moisture to the outside.

The R-11 warms the bay a bit so the temp differencial is moderated somewhat.

2. then I could cut unfaced open cell foam in the bays and foamed in around the sides. I could set the cut foam in a little in from the outside edge of studs so there is a bit of an airspace.

3. I'm not sure if I need sheathing on this wall. I'll see what is on the wall when I open it up.

4. On top of the sheathing or studs I will put an open faced foam. I'm not sure how thick at this point.

5. Nailing strips or a rain screed goes on next.

6. Then a vapor retarder/barrier/house wrap goes on.

Does this sound right? Let the corrections begin!

Didn't you say before that there was an 8' soffit overhang or similar directly above the wall?  That would effect the need for a rainscreen.  Also re the need for a structural sheathing there are metal strips that can be substituted for that.

Thanks for the reminder about metal diagonal braces. They will keep the wall thickness down and keep less layers on the wall that aren't needed.

I want to treat this wall just like it will get full exposure. I want to brush up on all my skills in this area. I consider it fun, really.

From what I have gathered, there are two reasons for a rain screed. One is to have a place for wind driven rain to escape and drain out from behind the siding.

The other is to keep the siding in more of an equalibrium between the outside and the inside. The air gap behind the siding helps the siding. Especially since this will solid wood.