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Old 06-07-2015, 07:08 PM   #1
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Make Up Air Duct Surround Moisture Problem


Hoping for your input. In summer 2013, a make up air duct (active system, not passive) was installed from exterior brick through crawl space, connecting to air return. LV connection opens damper when vent hood is turned in kitchen.

This weekend I was in the crawl space as part of ongoing work, and took the photos below. The first photo shows the integrated rim joist on brick (no sill plate) just next to the mua duct. I scratched on the white, it comes off somewhat, and with my screw driver noted hard intact joist wood. Got to deal with this now and I'm glad I went in to crawl. The second photo shows a "tenting outward" mua duct at one of its seams which is just proximate to the joist. The screw driver was able to go in the tent along the seam to the duct interior. Most all ducts have mastic applied to joints, but this mua had insulation covering it that was recently removed.

Lastly, I can see daylight when looking through the space on the right exterior brick. It appears the duct sitting in the circle does not completely fit the circle. It will be hard to get my hands in there to fix this, but I am determined to cut down on exterior air intake into the crawl space which is currently in the process of being conditioned.

I'd appreciate your help on how to remedy these problems? Thank you very much.

Last edited by ProGreen; 06-10-2015 at 08:33 PM.
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Old 06-08-2015, 06:36 AM   #2
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Crawlspace is vented?

Probably the overall best thing would be to condition the crawl. Whats the water situation like in the crawl? Any issues?
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Old 06-08-2015, 07:28 AM   #3
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Quote:
Originally Posted by Windows on Wash View Post
Crawlspace is vented?

Probably the overall best thing would be to condition the crawl. Whats the water situation like in the crawl? Any issues?
The crawl space is in the process of being fully conditioned. The exterior vents were removed, as was floor fiberglass insulation above, all ducts had mastic to joints and of course, water proof membrane is on ground up 6 inches to sides.

There is no water entry. Footer exterior tile drain to interior sump to daylight, all downspouts out 10 to 15' and 6" drop in grade 10 feet forward from foundation.

Rim joists have as yet not been foam insulated. That's my next job.

This crawl space area lacks a rim joist. What I think is happening is the make up air entry appears not tight around duct in the brick, much to my surprise. So now in summer, hot humid air can come in through the wire laterally since there is daylight seen, as well as come out of the duct through that open tent area since it's open and into that area.

It's just fortuitous I investigated this based on my remote humidity readings and saw this. I will call the company who did the job but still wish for suggestions here to know how to soundly get that MUA area so little infiltration from the area except into the duct itself.

Hope this helps. Thank you for your contribution on this matter.
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Old 06-08-2015, 10:50 AM   #4
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Sorry, of course there is a rim joist-it's the joist in the first photo. I meant " this crawl space lacks a sill plate."

What do you make of that white on the rim joist? My impression is it's moisture related as I said above.

What are your thoughts? Input very appreciated.
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Old 06-08-2015, 11:08 AM   #5
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Looks like it might be efflorescence from or water detailing the exterior sheeting meets foundation wall.
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Old 06-08-2015, 11:35 AM   #6
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Rim joist>cinderblock foundation (exterior tarred)>dimple membrane>dirt.

Will go back in and scout around some more for answers, particularly if white is on joist side facing interior foundation. I do remain concern about source being the hole in the brick for the MUA (needs better insulation at the point duct meets brick) and escape of untreated MUA through tented duct directly onto that proximate joist next to it.

Here's a close up of the joist:

Last edited by ProGreen; 06-10-2015 at 08:33 PM.
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Old 06-08-2015, 02:31 PM   #7
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Ok, took a few more photo's of this area. The first is simply a larger view of the area. Duct is above the galvanized steel support and rim joist on left. Second photo is helpful, to me at least, as it shows white area of joist on brick is thicker on area next to duct and less so closer to foundation wall. Third photo is looking head on to the rim joist up the foundation wall: that is concrete on the left next to the rim joist sitting on the brick. I see no signs of concrete foundation efflorescence of the wall itself (except a little in for front of photo that's old).

It's really the rim joist abutting the brick which was cut to allow the duct that I'm concerned about. The question is is the white efflorescent area new or did it precede the MUA duct installation? That is unanswerable by me. So I will assume it's new and needs reduction/drying/cessation. The exterior vents are gone. The space is without water. There is membrane down.

I could use round closed cell backer rod to push into the hole at the junction of the exterior brick to close the daylight I see. I can call back the hvac installer and ask them to close up that seam that is tented and open, in case the mua is blowing on that joist region. I need to insulate this entire crawl space at the rim joist area, yet understand you can't if the joist sits on brick (integrated, I think is the term). I also believe one nearby hvac duct needs some insulation because I felt a draft from it when in there today.

What do you think and thanks.

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Old 06-09-2015, 07:54 AM   #8
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A hot humid day here. Humidity in crawlspace was 68%, dew point 56% until I put on house air conditioner, then dropped to H 64, DP 52% . Any thoughts here?

Would my removing the insulation in-betweeen the floor joists let humidity into this crawl space from above? Cool, moist air sinks right?

Really appreciate your help on this.

Last edited by ProGreen; 06-09-2015 at 08:24 AM.
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Old 07-02-2015, 10:17 AM   #9
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Wish to offer an update on above posed problem. The GC who installed the MUA system came with the HVAC boss. Three steps were taken:

a) rim joist evaluation reveals it is fine. not efflorescence but mortar. brick seems okay, and is no different they feel from at installation

b) foam was added around periphery of exterior brick aperture where MUA duct draws. Silicone had been placed at installation before this new foam.

c.) tenting of MUA duct next to rim joist near exterior: confirmed. closed with combination of screws to union the metal, then wrapped with silver duct tape, then foamed at union (or maybe it was vice versa).

d.) length of mua duct suspended along its line with new metal suspension to keep angle to exterior correct

I am very pleased. Within 8 hours of these fixes, the formerly higher relative humidity in this crawl space dropped 10%, almost to the same as the opposite side crawl. To me, this means exterior air was entering the affected crawl and now is not. Progress!
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Old 07-03-2015, 05:38 PM   #10
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You are aware when conditioning a crawlspace, it requires air from/to the house above to reduce any moisture present; Page 5/19;http://www.google.com/url?sa=t&rct=j...59026428,d.cGU

Where are you located?

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Old 07-03-2015, 07:21 PM   #11
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Yes, I am aware. I am following Martin Holladay's "Musings of an Energy Nerd" Building an Unvented Crawl Space as well as current state/local residential codes for Virginia.


Creating an unvented crawl space
If you live in a humid climate, and you still want to build a crawl space — or if you are trying to correct problems in an existing moldy crawl space — here’s how to go about it.
To help keep the crawl space dry, correct any grading problems on the exterior so that the grade slopes away from the foundation.
Remove all rocks and debris from the crawl space floor, and rake the dirt smooth. Ideally, the crawl space floor will be higher than the exterior grade, although keeping the grade high on the interior of a crawl space is not always possible.
If the home is located in an area where radon is common, install a passive radon collection system in the crawl space floor.
If the crawl space is subject to water entry, be sure to slope the floor to a sump equipped with a drain or a sump pump.
Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall.
Attach the top of the vapor barrier to the wall with horizontal battens, secured to the wall with masonry fasteners.
Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in fiberglass mesh tape.
Consider installing a 2 in. or 3 in. thick concrete slab (a “rat slab”) to protect the vapor barrier.
If this is a new-construction crawl space, and you can’t afford a rat slab, you may want to install a temporary (sacrificial) second vapor barrier — usually a layer of 6-mil poly — on top of the permanent vapor barrier; once construction is complete, this temporary poly is rolled up and discarded.
Insulate the interior of the walls and rim-joists with rigid foam — many builders use Thermax, a polyisocyanurate foam that does not require a thermal barrier or ignition barrier — or spray polyurethane foam. Another option: insulate the exterior of the foundation walls. If your crawl space has stone-and-mortar walls, you can’t insulate the walls with rigid foam; the only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam. Install at least as much insulation as required by the 2012 IRC for basement walls, namely R-5 for climate zone 3, R-10 for climate zone 4 (except Marine Zone 4), and R-15 for Marine Zone 4 and climate zones 5, 6, 7, and 8.
Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below.
Install an exhaust fan or a forced-air register to meet code requirements for conditioning the crawl space. Be sure that the fan does not exceed air flow requirements for the size of the crawl space, since exhaust fans carry an energy penalty.
Install good lighting — most crawl spaces will benefit from at least six fixtures, spaced evenly across the crawl space ceiling — controlled by a switch located near the entry door.

Read more: http://www.greenbuildingadvisor.com/...#ixzz3esPxL5Pu
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