So in the winter you will be adding moist house air to a space where it could find a cold surface to condense on.

 

Well, is that any different from a basement? I thought the building science consensus nowadays was generally to seal and insulate and then condition a crawlspace.

I'm wondering more about the effects this could potentially have on my furnace, stressing it out or causing other problems. Or if for a situation like this (see diagram) there would be little to no effect.

 

Well, is that any different from a basement? I thought the building science consensus nowadays was generally to seal and insulate and then condition a crawlspace.

I'm wondering more about the effects this could potentially have on my furnace, stressing it out or causing other problems. Or if for a situation like this (see diagram) there would be little to no effect.

Even in a heated basement the floor is hard to get to the mid 60s so heat would have to be distributed well.

 

Adding runs will not stress the furnace, but will change distribution.

You shouldn't splice into existing branch lines as those runs will be disproportionately affected.

Add a new run off of the trunk line with a damper.
You may need to damper down other short runs to get enough flow elsewhere.

*

A passive return between floors may pose a fire safety risk and may not be allowed by code in your area.

Add a small return to the crawl, down low near floor of crawl -> a single 6" would do.

Keep in mind crawl-spaces do not need to be warm, 60 to 65f is fine.

*

Sealing and insulating the crawl is a good idea.
There won't be condensation problems as the foundation will be warmed up by the higher air temperature in the crawlspace.

Uninsulated foundations in basements do not get condensation in winter. What causes condensation low surface temperatures the air can get in contact with, like using batts without a vapor barrier.

...because vapor barriers do not belong in basements or crawls (trap moisture from foundation in insulation), rigid insulation is the best choice, it creates a thermal break and the air doesn't come in contact with the cold surface.
The foundation surface being made cold by the insulation stopping most of the heat from getting to it.

 

Add a new run off of the trunk line with a damper.

A passive return between floors may pose a fire safety risk and may not be allowed by code in your area.

Add a small return to the crawl, down low near floor of crawl -> a single 6" would do.

Thank you for the reply. I can definitely tap into the trunk for the supply and run a few feet of 6" duct with a damper to a single register that just blows down into the crawl.

I'm not sure how I would add a full return down near the floor of the crawlspace (or anywhere down there) because this is a down flow system where the return is in the first floor ceiling, runs through the attic, and then down into the top of the furnace which is in a closet on the first floor. So it would have to go through the living space somehow and into the attic, and that would be difficult.

Do you know which building code the passive return would be referenced in? Or do you have any other suggestions?

 

250 sq, ft crawl space you need about 5 cfm from the hvac system. You might have that already with your unsealed duct. You definitely don't need a couple of 6" supply taps.

 

Talking ventilation - would need more for heating.

Wise to heat the crawlspace a little which would need more than 5 cfm.

Hence my suggestion to put a single vent down there with a damper and return.

The return is also helpful in summer for dehumidifying that space when a/c is on.

 

I don't know the codes - they vary a lot by area.

A passive return may be okay with a fire damper.

An option is to run a 6" from the return just before it goes into the furnace and have it go down to the crawl and terminate near floor - would need to make a hole.

An option is to not add any supply/return, with the heat from the uninsulated duct heating the crawlspace - and put a small dehumidifier down there for summer/spring/fall.
I would suggest an old one with a mechanical humidistat off of kijiji or facebook marketplace because new ones are chinese crap that are notorious for failing per-maturaly as well as starting fires.
The old ones with mechanical humidistats and no electronics just work!

 

Thanks, I really appreciate these different options to explore.

Would the fire damper be similar to the image below, just installed on the underside of the subfloor with a register on top of it at floor level?

And it would be very difficult (for me at least) to get a return above the filter, possibly in the attic space but if I had to open the ceiling I would probably lean toward no supply or return and using the uninsulated ducts to warm the space.

Would an option be to add one 6" branch and supply in the crawlspace with a damper to help dehumidify in the summer, but no return? And rely on the probably-leaky multiple floor registers to serve as a kind of return?

 

A fire damper closes in response to heat, not that well versed on them so wouldn't know which product to use.

Best to not pressurize the crawlspace much as it increases leakage to outside.


Sounds like return would be problematic to add, so I think your best option is not adding any supply and using a dehumidifier or adding very little supply, like a 4" pipe.

The radiant heat from the duct alone could be enough to sufficiently warm crawl.

Keep in mind that when it is humid but not hot, a/c will not be enough to dehumidify crawl even with good supply and return - so it may need a small dehumidifier regardless. It depends on the moisture levels in the soil and quality of damp/water proofing the foundation has.

 

Thank you!

I was hoping to not have to run a humidifier, but if I do get one I'll look for an old mechanical humidistat type like you suggested.

For heating, I think I might just leave the crawlspace alone and rely on the passive heating of the existing supply branches. If that's not enough, I will add a 4" supply off the trunk in the future.

For dehumidification, it sounds like the 4" supply and no passive floor return would do nothing to help? So either figure out how to run a return back to the attic, cut a passive return in the floor as long as it has a fire damper, or run a dehumidifier.

The end goal is a stable space for nail down solid hardwood above, but maybe it won't be a problem if I use tar paper or aquabar above the subfloor and under the hardwood like I've seen in some of the install instructions.

Any final thoughts or clarifications before I move on? Really appreciate the input!

 

The supply air from the 4" pipe would have less moisture than ambient, but not necessarily enough to dehumidify sufficiently.

Even with return it can be problematic, because crawlspaces have almost no heat gain, the relative humidity ends up a bit high - cold and clammy.
When cooling load is low, a/c won't run enough to dehumidify - another issue.

So you may need a small dehumidifier regardless - which works better because it has its own humidistat and the air is re-heated by the heat removed to condense moisture out of the air. (plus the energy consumed by the compressor)
The re-heating helps get the relative humidity (percentage saturated) down; the warmer air warms foundation surfaces which helps prevent condensation.

 

4 options under 18 code which as I recall is the same for the 21:
Continuously running exhaust at 5 CFM
The HVAC option as noted above but the walls must be insulated & sealed AND it is based off volume. It says the entire area, not just the floor area as the prior one does. Plus you must remember the area encompasses everything up to the floor sheathing above - so you are probably closer to a minimum of 25 CFM based on the pics.
3 is not an option - treat whole thing as mechanical plenum
The 4th which is a new addition is a dehumidifier sized at 70 pints (as I recall not an option in earlier versions)

 

I appreciate the code breakdown.

So are you saying either of these on their own should be sufficient?

1. Exhaust fan in rim joist area continuously blowing 5CFM outside year round
2. Open a 4" supply pipe to the space by itself? or something different?
3. Not an option
4. 70 pint dehumidifier that pumps outside through the rim (no sump pit down there)

 

To supply any heat into the crawlspace. Just install a flat vent with a damper already in it. You didn't mention anything about make up air for the furnace. A fresh air supply from outside. Generally a couple louvered dampers in the rim joists. This also allows moist air out and balances out to internal pressures of the air in the crawlspace. The generally warmer air in the crawlspace will move outer and carry extra moisture with it. Fresh air is also needed for the burner in the furnace.
Since you have air ducts to each room connected into the main plenum. A fresh air supply from outside will keep fresh air going into your home. Making it more healthy. A 4" pipe from outside with a grill over it. Ran into the air return plenum just in front of the blower fan will allow fresh air to be blown into each room when the furnace is running. You can purchase a scuttle that has a bimetal spring in it that can close it off in cold weather automatically. I prefer to just have it open. Natural convection will draw fresh air in from outside and into each room automatically. This will also give your home a slightly higher internal air pressure so that old air exits when doors open. A bathroom vent with a humidistat in it will automatically balance your internal moisture and expell any excess moisture.
Just having the advantage of naturally accruing fresh air into your home is a good thing.
Just thought I would share some different ideas to your discussion. Good luck n have fun, looks like a great project

 

Thanks for that input and those ideas as well. I'm hesitant to bring in air from outside, as I'm more focused on sealing and conditioning the air than I am ventilating. In fact, my project so far has been the opposite - to eliminate outside air.

To clarify further, this is a fairly open floorplan with a couple of closets and a laundry room. It's a separate part of the house, but has a doorway to the main 2-story 1952 house that is almost always kept open. That area has its own furnace and AC and separate duct system.

I have trouble believing that even with extensive air sealing, considering the original drafty windows are still there, that there would be any issues with the furnace not getting enough air.

I think I will see how everything is with the sealed crawl, and then add a supply down there if needed, and a dehumidifier if that's needed.

It was about 35 last night and I went into the crawl with a thermal camera and I couldn't find any spots anywhere that were lower than 51 degrees, so I think that's good.

 

I didn't mean to backtrack the discussion, but what is the main reason to add a supply in that space? Are there water pipes in the crawl space? Does it get really cold in your area? If not, as user_12345a said, the radiant heat from that long and big supply trunk may be enough, with support from a dehumidifier. Sealing and insulating the crawl space is part of "conditioning." With a sub-floor, underlayment and solid wood flooring, there is probably no risk of heat loss from the living area above. May be worth observing over a full cycle of seasons before permanent changes are made. There is solid science behind HVAC, but in existing dwellings, making small changes and observing the effect may work better.

 

I wanted to follow up saying that I don't see any requirement in 2018 IRC (R408.3Unvented crawl space.) indicating any fire damper requirement, just that:

"1. exposed earth must be covered..." and there is none, just a concrete floor.

"2. One of the following is provided..." "2.2.Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code."

So it sounds like an open transfer grille might be okay on its own, in addition to a 6" or 4" supply with damper, but that I should additionally insulate the perimeter walls.

Unless there is a different fire damper code requirement not referenced there, which I am unaware of?

 

Just wanted to provide an update about what I did.

Drilled a big hole in the trunk:

Traced around the 6" takeoff:

Cut out 6" hole:

Bent back every 4th tab, screwed in with 1/2" self-tapping sheet metal screws:

Screwed on a straight/offset register boot, used foil tape at that connection and at the trunk:

Register with built-in damper (thanks for the suggestion):

Screwed on the register because the end of the boot was warped:

8"x10" register between the first floor and crawlspace at the opposite end of the crawlspace from the supply, utilizing the access hatch in the floor of the closet next to the furnace:

I will report back to let everyone know how this works out during these last couple cold months and also in the humid summer. Not a perfect professional job but I can always undo it if needed.