Return on an outside wall! Keep it, or shut it off and insulate the bay? Help?

[Hube]

To whom it may concern; Wooden joist spaces, wall stud cavities, etc are used for the Return air passages in millions of homes in N. America without any problems.

[Dana]

I forgot to mention that part of it.

There is no insulation, and having this all open destroys the continuous vapor barrier that you SHOULD have.

My biggest concern here is the lack of vapor/air barrier, which means you'll eventually start growing mold on the inside of your sheathing.

What you could do, but would be quite expensive, is have a spray foam guy put an inch of foam on the inside of the sheathing and the inside of the two studs so that it will tie into the existing (I hope) vapor barrier.

You'd still have your duct, albeit a smaller one, but it wouldn't be a breeding grounds for heat loss and mold.

More important than thermal performance of the insulation is AIR sealing the building envelope.

Without knowing what climate zone this building is in you can't make any blanket statements about whether it should have a vapor barrier at all, or where it should be placed. What's good practice regarding vapor control north of the 48th parallel isn't necessarily true south of the 38th parallel.

But for sure air sealing is important everywhere, and this looks like a disaster.

Where is this place? (This duct routing approach wouldn't have been allowed a building permit or passed an inspection in any place I've ever lived/worked.) Got a zip code?

But independent of climate zone, blowing it full of dense-packed celllulose and cutting in a floor return grill seems like the right solution. Using the great outdoors as your return plenum never works in your favor. Have an insulation contractor assess the rest of it too, for both insulation and air-tighness.

[Dana]

To whom it may concern; Wooden joist spaces, wall stud cavities, etc are used for the Return air passages in millions of homes in N. America without any problems.

True, but many/most would fail the duct teakage standards in CA Title 24 (unless great care is taken to caulk every seam and penetration) and...

...using an EXTERIOR wall cavity for the return creates a huge breach in the thermal envelope of the structure, and in some climate zones will create mold conditions several months out of the year.

Interior stud & joist cavities have far fewer issues and are quite common, if not always best practice.

I imagine only in areas with relatively low heating loads would anybody try to get away with using an exterior wall cavity as a heating system duct. 10-15 square feet of uninsulated wall without even the cavity as a thermal break represents a significant heat loss in 7000+ heating degree day climates, but in 2000HDD climates, not so much.

[dlarrivee]

So wait a minute, you think that a vapor barrier isn't required just because it's warm outside a lot of the time?

[Hube]

Vapor barrier is always needed but as to where exactly depends on the climate of the area of where it is to be installed.
A vapor barrier always goes to the heated or warm side of the insulation.In a cold weather climate, it will be on the face of the studs just before the drywall.
In a warm weather climate, it will be on the exterior side of the stud wall.

[Hube]

.

A return serves BESTwhen installed on an interior wall, but an outside wall is generally ok as long as the passage is fairly air tight or insulated.In your case it is not the best because of this leakage to the outside. area.

Dana; The above 'quote' is from an earlier posting of mine to the poster" lithnights". Again,INTERIOR walls areBEST but if it really comes down to the last resort an exterior wall is ok as long as it is weather tight/insulated.

Note; the proper installation of a Return air passage via studs, joists, etc, can be done quite sufficiently air-tight if a little care is taken by the installer. And this can also be accomplished without the use of any caulking or tape,etc, by just a bit of good workmanship on the part of the installer. Panning and blocking of the joists using metal can be just as air-tight as any supply ductwork if it is done by a skilled tradesperson.
Been there , done that.

[lithnights]

For those that are curious, the house in question here is in the Philadelphia area.. southeast PA.

[Dana]

So wait a minute, you think that a vapor barrier isn't required just because it's warm outside a lot of the time?

An interior vapor retarder is required if the dew point of the interior air is found for extended periods of time within a moisture-susceptible layer of the wall structure. In most of Canada that will be for weeks/months on end, but in Tennesee, a few hours on the coldest days of the year, in which case air sealing is sufficient.

In hot humid cooling dominated zones (Florida, and most of the gulf coast), air conditioned buildings need a vapor retarder on the exterior of the structure to keep water vapor from EXTERIOR air from condensing on susceptible materials within the wall structure, and placing vapor retardent materials on the interior keep the wall from being dried out by the mechanical cooling systems.

In much of the moderate zones of the US the wall structures need to be able to dry toward the exterior in winter, toward the interior in summer, and vapor retarders do nothing but creat problems that otherwise would not exist in a decently air-sealed wall.

In sountheast PA where this particular house is located vapor retarders aren't required, but the more highly vapor retardent materials are best placed toward the interior. It's a mixed climate, but still strongly cooling dominated with 5000HDD to ~1000CDD. The average Janary temperature is 30F/-1C but the average daily high is well 37F/5.5C which is over the dew point of 68F room temp with 30% realtive humidity. Some amount of interior vapor retardency is desirable, but not required. It's nothing like the issues you'd find in Ottawa or Calgary. Philadephia's vapor issues are more simliar to what you'd have in Vancouver (which also doesn't really need sub 1-perm interior vapor retarders.) As long as the structure can dry toward the exterior for at least half of the hours in a month, it isn't likely to develop mold issues from simple vapor diffusion through walls. But from air transported moisture, YES. Air tightness, both of the sheathing and the interior wall gypsum counts. A cross-sectional square inch of infiltration into a wall cavity is worth a whole 4x8 sheet of (highly permeable) gypsum.!

And mold potential from a bigass air flow onto uninsulated sheathing from a cavity being used as a return duct, YOU BETt!

[Hube]

[QUOTE=Dana;And mold potential from a bigass air flow onto uninsulated sheathing from a cavity being used as a return duct, YOU BETt![/QUOTE]
So, exactly what I have been saying all along ....."as long as the exterior wall stud cavities are insulated and air-tight then it can be suited for a Return air passage if absolutely necessary.

Ps. you used the word "bigass" ....just what is this? something new on the market, eh?

[hj]

Return air is very "smart". It will find a way back to the furnace. In most homes here, there is only ONE return grill and all the air from the house finds its way back to it. That being the case, reinsulating that stud space and eliminating the direct return would be more efficient. In addition to the heat loss through the uninsulated bay, the return air is being chilled so it adds to the load on the furnace.

[lithnights]

First off, thanks for everyone's comments and suggestions. As always, I learn a thing or two everytime I post a question. I think what I am going to do is what Hube suggested in posts 11 and 14. By installing a return grill on the floor that links into the existing chase going downstairs to the furnace, I keep a return. By then blocking off the old vertical chute (running from the floor to the existing grill) and filling it with insulation, I insulate the wall cavity. I'll then sheetrock over the space where the old grill was, thus cutting off any open air into the room.

I think this will resolve my problem.

Thanks all.

[dlarrivee]

You want more than insulation in that void, you want an air seal as well.

Get something to fill in the holes that make it so you can see your neighbors from inside the house.

[gator37]

I agree with Hube...running return air thru the studs or joist space has been done for years. The only negative in my experience is that you have to have enough cross sectional area to return the air....sometimes you may have to take in more than one stud space to get the area required. This would be most critical if you have a furnace that typically does not have the external static capability.

[lithnights]

You want more than insulation in that void, you want an air seal as well.

Get something to fill in the holes that make it so you can see your neighbors from inside the house.

Agree. I already used mastic foil tape to cover up the tiny holes I saw. I can't see any others from that vantage point (looking through the hole where the current grill is) so short of ripping down the wall, I have to assume there aren't any other air gaps.

[Dana]

Agree. I already used mastic foil tape to cover up the tiny holes I saw. I can't see any others from that vantage point (looking through the hole where the current grill is) so short of ripping down the wall, I have to assume there aren't any other air gaps.

If you use dense-packed cellulose to 3lbs/ft^3 or higher density to insulate it those tiny holes (even some bigger ones) basically go away packed tight with long-lasting cellulose plugs, an even when the plugs give up in a coupla decades, air can't move through the hole at nearly the rate it might with standard-density blown fiberglass or batts. Depending on the joist spacing and the thickness of the wallboard you have you might be able to get away with slow-rise half-pount foam too (with some risks...).

If the stud bays are 24" wide and the gypsum is only 1/2" thick standard-density cellulose would still slow down any air leakage by 90%, but dense-packing might bow out the wall over time. If it's 3/4" gypsum or 16" stud spacing dense packing should be fine.

Success with slow rise foam depends lot on the experience of the installer, but you have a bit more margin against blowouts with narrower bays and thicker wallboard. Foam would air-seal it perfectly, but for the price & risk differences my personal inclination would be on the dense-pack cellulose side. YMMV.