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Cool/Damp Basement. Will Redesigning HVAC help?

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  • Jun 19th, 2015 4:14 pm
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[OP]
Sr. Member
Sep 27, 2009
764 posts
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Sask, Canada

Cool/Damp Basement. Will Redesigning HVAC help?

So my basement is normally 3-4 degrees colder than the upstairs. It also has excess humidity. The basement is about 1000sq/ft and only has one supply air and now one return air in the hallway (which I recently added). The current supply air is located at floor level, and is actually a larger duct with two registers side by side on the wall. My idea is to take over this floor level supply air and make it another air return for the main basement area. Then add two supply air at the ceiling for each side of main basement area. I would then run my furnace fan for X amount of time an hour to help circulate the cool damp air around the house. I figured that would be cheaper than running my dehumidifier. I'm not 100 percent sure where all my humidity is coming from but that will be another project. Here are some diagrams I made of my HVAC.

Current Setup:
[IMG]http://i417.photobucket.com/albums/pp25 ... z3gmme.jpg[/IMG]

Modified:
[IMG]http://i417.photobucket.com/albums/pp25 ... kb8ocq.jpg[/IMG]

Will this help or should I just stick with running my dehumidifier?
10 replies
Sr. Member
Jul 19, 2014
999 posts
195 upvotes
Snow Road Station, O…
Dump the upper levels return air into one end of the basement and have the supply air to the furnace at the other end. That way the excess moisture will drop out when being cooled for the supply air for the upper levels . And the dry conditioned warm return air will mix with the basement air and lower the basement humidity levels even more.

But a dehumidifier would probably control the humidity levels better and more efficiently .
Deal Expert
Mar 23, 2009
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Toronto
1. Are your walls (and floor) insulated?

2. Supply vents are supposed to be at floor level.
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It likely won't be enough, basement humidity typically comes from humid air condensing on uninsulated walls/floor, air leaks with outside leading to more condensation, water diffusion through concrete and uncovered crawlspaces (and water from other sources, leaks, drains, spills, mopping, showers, etc).
I have tried using the central air to remove basement humidity, with the basement vents putting maybe 40% of the central air output downstairs, its helps but does not bring it to the 60% mold preventing threshold.

I ended up having to buy a dehumidifier, and to save you time the most efficient you can buy for less then 4 figures is 1.85L/kWh which is the current energystar standard, many units meet this, its an easy threshold to meet because the components used are not the most efficient available (they don't seem to need ECM motors to meet it), you won't get this efficiency as an actual output probably because the test is at high temps and high humidity your basement won't see (its meant to maintain lower humidity then the test and your basement won't often get to 30C) and oversizing has a huge energy efficiency penalty so try to use the smallest size you can get away with (the constant fan and low duty cycle being the likely culprits).
Also many units come with a 1 year warranty, so expect them to die in year two (window A/Cs are notorious for this, a fridge, dehumidifier and A/C are the same device used for different purposes). I bought a fridgidaire since it comes with a 1 year regular/5 year sealed system warranty.

[IMG]http://bestdehumidifier.co/wp-content/u ... chart1.jpg[/IMG]
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Banned
Jan 16, 2015
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Toronto, ON
[QUOTE]I would then run my furnace fan for X amount of time an hour to help circulate the cool damp air around the house.[/QUOTE]

omg, no. Probably mold down there. Should think about water proofing the exterior foundation walls. Run your dehumidy and a fan!
[OP]
Sr. Member
Sep 27, 2009
764 posts
519 upvotes
Sask, Canada
Thanks for the replies. Yes it's a finished basement with insulated walls. How well insulated I'm not sure as it was done when we moved in. Well I'll hold off on the project for now and do more investigating on where the moisture is coming from.
Deal Expert
Mar 23, 2009
20367 posts
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Quentin5 wrote: It likely won't be enough, basement humidity typically comes from humid air condensing on uninsulated walls/floor, air leaks with outside leading to more condensation, water diffusion through concrete and uncovered crawlspaces (and water from other sources, leaks, drains, spills, mopping, showers, etc).
I have tried using the central air to remove basement humidity, with the basement vents putting maybe 40% of the central air output downstairs, its helps but does not bring it to the 60% mold preventing threshold.

I ended up having to buy a dehumidifier, and to save you time the most efficient you can buy for less then 4 figures is 1.85L/kWh which is the current energystar standard, many units meet this, its an easy threshold to meet because the components used are not the most efficient available (they don't seem to need ECM motors to meet it), you won't get this efficiency as an actual output probably because the test is at high temps and high humidity your basement won't see (its meant to maintain lower humidity then the test and your basement won't often get to 30C) and oversizing has a huge energy efficiency penalty so try to use the smallest size you can get away with (the constant fan and low duty cycle being the likely culprits).
Also many units come with a 1 year warranty, so expect them to die in year two (window A/Cs are notorious for this, a fridge, dehumidifier and A/C are the same device used for different purposes). I bought a fridgidaire since it comes with a 1 year regular/5 year sealed system warranty.

[IMG]http://bestdehumidifier.co/wp-content/u ... chart1.jpg[/IMG]
I had a basement that was humid and moldy, with only cheap broadloom on the floor and no wall insulation. We wasted a ton of electricity on a dehumidifier. It was also as much as 5 degrees colder at times.

During a reno we closed up all holes and insulated the walls, albeit only to R12 as that was code at the time. We also realized that the floor slab was only 2-4" thick and was sitting directly upon sandy soil, which would get very wet on rainy days. So, we actually removed the slab, dug down, put down 6" of crushed rock, and then put 6" of cement on top of that for a new slab. Final slab height was the same as before, but now there is a decently thick slab, and proper drainage underneath. Then we put Dri-core over that. And at the same time we revamped the HVAC system, with better and more vents, a nice large return duct, and more air circulation using the "CIRC" function from the thermostat.

Now it isn't humid it all. It's wonderful. No mold and no need for a dehumidifier. Actually we had built a nook with a hole for a hose (to the floor drain) specifically for a dehumidifier, but we have never used it, because it is simply unnecessary.

It's still cooler though, by a couple of degrees, but a couple of degrees is manageable. Plus, just in case we put electric baseboard heaters in the bedrooms down there. Comes in handy for elderly guests who like it warm. So in the bedrooms it can be even warmer than upstairs if desired.

BTW, revamping the HVAC also included enlarging the cold air return from upstairs and enlarging the main supply duct to upstairs. It was a big hassle, but we're so glad we did that. Now during the summer, the AC cools the upstairs much, much better than before, and I attribute that mainly to the better cold air return.
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EugW wrote: I had a basement that was humid and moldy, with only cheap broadloom on the floor and no wall insulation. We wasted a ton of electricity on a dehumidifier. It was also as much as 5 degrees colder at times.

During a reno we closed up all holes and insulated the walls, albeit only to R12 as that was code at the time. We also realized that the floor slab was only 2-4" thick and was sitting directly upon sandy soil, which would get very wet on rainy days. So, we actually removed the slab, dug down, put down 6" of crushed rock, and then put 6" of cement on top of that for a new slab. Final slab height was the same as before, but now there is a decently thick slab, and proper drainage underneath. Then we put Dri-core over that. And at the same time we revamped the HVAC system, with better and more vents, a nice large return duct, and more air circulation using the "CIRC" function from the thermostat.

Now it isn't humid it all. It's wonderful. No mold and no need for a dehumidifier. Actually we had built a nook with a hole for a hose (to the floor drain) specifically for a dehumidifier, but we have never used it, because it is simply unnecessary.

It's still cooler though, by a couple of degrees, but a couple of degrees is manageable. Plus, just in case we put electric baseboard heaters in the bedrooms down there. Comes in handy for elderly guests who like it warm. So in the bedrooms it can be even warmer than upstairs if desired.

BTW, revamping the HVAC also included enlarging the cold air return from upstairs and enlarging the main supply duct to upstairs. It was a big hassle, but we're so glad we did that. Now during the summer, the AC cools the upstairs much, much better than before, and I attribute that mainly to the better cold air return.
You did the big repair, the only thing i would add is rigid foam under the concrete (R10-15) if you were doing it today, and higher R values on the wall. Concrete is porous, it lets water through it, so insulation such as spray foam (ideally) or batt insulation should keep the wall above the dew point meaning less humidity condensing from outdoor air hitting the walls.
How much did your reno cost?
In fact in Rand McNally they wear hats on their feet and hamburgers eat people
Deal Expert
Mar 23, 2009
20367 posts
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Quentin5 wrote: You did the big repair, the only thing i would add is rigid foam under the concrete (R10-15) if you were doing it today, and higher R values on the wall. Concrete is porous, it lets water through it, so insulation such as spray foam (ideally) or batt insulation should keep the wall above the dew point meaning less humidity condensing from outdoor air hitting the walls.
How much did your reno cost?
I will just say it was way too much, probably twice the cost many might spend, but then again this was a full finish with stupid upgrades like giant custom glass-doored and full-tiled thermostatic rainshower, floor heating in that bathroom, full kitchen with granite counter and bamboo cupboards, 14 mm engineered hardwood floors, and complete revamp of the HVAC system. We also knocked down two major support pillars to open up the space, replacing them with one big steel I-beam, and we knocked down another support wall too but that was smaller so we just used an LVL there. The wall insulation was rigid foam actually.

BTW, the basement dig was very labour intensive. It was all done by hand, and all the soil was removed through the window in individual bags. Cement was mixed in the basement and poured by hand. There wasn't enough space to get proper equipment in there, and we were still living on the main floor and second floor.

As mentioned, humidity is no longer an issue at all, even without the additional items you recommended. The main issues are that it's a couple of degrees cooler, and the finished height is only about 6'6". (The house was originally from the 1950s, hence the short basement height.) We didn't try to increase the ceiling height because it would have been uber expensive, and underground rivers are a big problem in that area. Several people in my neighbourhood who dug down hit major water.
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Jan 16, 2015
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No point in doing a 'big repair' without adding weeping tiles.
Mold will grow on the walls and floor needs to be washed away and removed regularly. So when it's covered with insulation how will you do that? That's why most basement are unfinished.
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Rigid foam is also a good choice, fiberglass is a bad choice because its air permeable and cooling humid air causes moisture leading to mold.
The foam under the slab will reduce moisture a bit (you also have your crushed rock which makes a big difference) and will cut the heating bill a bit.

I would love to do what you did but there is no way i could ever afford that, if i can insulate the walls and dig out the crawlspace i'll be happy (partial dig and plastic is the goal, full dig and slab would be amazing)
In fact in Rand McNally they wear hats on their feet and hamburgers eat people

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