Add supplemental heat to conditioned crawlspace
I built a 300 sq ft addition this summer on top of a fairly shallow (24″) ICF crawlspace. This is in the middle of Canada, and now that winter has set in im starting to see how the heating system functions.
I extended two separate branches from my forced air furnace into the addition, and opened up a small hole at the far end of each branch according to the instructions for air flow and air changes. There is an open passage way from the crawlspace into the existing basement for return air.
The addition itself stays close to the the temperature of my main house, perhaps a degree or two cooler (19 C), but the crawlspace is approximately 5-6 degrees Celcius cooler than the house (15 C). This obviously creates a cool, unpleasant floor and results in the addition feeling even cooler than it is.
I need to raise the temperature in the crawlspace without sucking much more air from the addition itself. My first test will be to add a few more holes into the ducts within the crawlspace, but i dont want to do so much that the addition gets colder.
If I choose to add a supplemental electric heater into the crawlspace, what type is my best option?
I was thinking that a simple baseboard heater, suspended close to the crawlspace floor might be my best bet? Im a little concerned though about whether or not two feet of head room above a baseboard is enough.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
A heater with a fan might be better - less localized heating above it. If your other heat source isn't electric resistance, then I'd try hard to use it for the crawlspace (for efficiency reasons). For example, add a booster fan to your branch ducts to move more CFM, making more available for the crawlspace.
You may want to consider adding a hyperheat mini split in the crawlspace. It would cost about 1/3 as much to run as an electric heater or having your HVAC guy set up the crawlspace. as its own zone with a thermostat.
Are you monitoring the humidity in the crawlspace? If so what is it?
Walta
If the crawlspace is that much cooler than the house even with some supplied conditioned air, there is a good chance that there are cold air leaks. Warm air in the house naturally rises, and it tends to pull cold air in through penetrations into the crawl space. Check carefully for air leaks, and seal all of them you find. This can make a big difference in crawl space temperatures.
DJ,
If you do end up opting for an electric baseboard heater, most manufacturers want a clearance of 12" above.
You want to consider a very small minisplit here. Electric resistance heat is usually one of the most expensive ways to heat a space. If you do want to go with electric resistance heat, a heater floor system with a bit of insulation underneath might be a more efficient option. Heating a crawlspace just to make the floor above warm seems pretty wasteful to me.
I’d also look to be sure that the crawl space is properly air sealed and insulated and fix that first. Any type of heat you put into the crawlspace is of limited use if you have big air leaks or uninsulated walls.
Bill
>"You want to consider a very small minisplit here."
No, you don't. That's beyond silly, edging into "ludicrous" territory here.
Even a half-ton Mitsubishi would be grotesquely oversized & inefficient for the design heating load of an attached 300 square foot addition's crawlspace, and wouldn't be efficient enough to come anywhere near "paying off" either the environmental or financial cost of the mini-split. You'd be better off spending the money on grid-attached solar.
>"If you do want to go with electric resistance heat, a heater floor system with a bit of insulation underneath might be a more efficient option. "
YES!!!
Electric mesh type radiant floor operated off a FLOOR thermostat (not a room-air thermostat) with R19 or so under it would raise the barefoot comfort considerably without a large impact on energy use, unless the floor temperature is raised to the point that it becomes the primary heat source for the room. A 15C floor makes a 19C room feel chilly, a 20C floor would be adding about 1000 BTU/hr (~320 watts) into that 19C, 300 square foot room. That yields a big shift in comfort, but still not that "cozy toes" radiant floor that some are looking for. It climbs another 1000 BTU/hr for every degree C higher than the average room temperature, so it's penny-wise to not take it too much higher and making it the primary heat source for the room.
>"... look to be sure that the crawl space is properly air sealed and insulated and fix that first. "
ABSOLUTELY YES!!
Even with the stemwalls well sealed and insulated the crawlspace the floor temp will run roughly ~18C or a bit less in a 19C room, and the crawlspace air temp will be roughly midway between 18C and the subsoil temperature, which in parts of the lower Canadian midwest can still be about 5C. A location decribed as "... middle of Canada..." could be even colder than that, depending on how that middle is measured. Centre of Canada Park (a bit east and slightly south of Winnipeg) is one thing, the true geographical "...middle of Canada..." would be much colder, near the border between Nunavut & Manitoba.
Given that the humidity in both the house and crawlspace should be the same, as they are supplied by the same forced air (maybe that's not a good assumption?), is there any danger that the much cooler crawlspace will accumulate moisture?
>"Given that the humidity in both the house and crawlspace should be the same, as they are supplied by the same forced air (maybe that's not a good assumption?), is there any danger that the much cooler crawlspace will accumulate moisture?"
Not really, unless there is substantial, active humidification of the conditioned space air going on. In most midwestern Canadian homes the wintertime indoor air's dew point would be no higher than ~5C, the dew point of 21C/35% RH air. Only in summer would the conditioned space air's dew point be substantially above the temperature of the crawlspace floor, but the crawlspace is being delivered air-conditioned air (not merely ventilation air) it will have a similarly low dew point.
The deep subsoil temps in Winnipeg are still north of 5C, the southern border of Nunavut, not so much. So it really matters which "...middle of Canada..." location we're discussing. In permafrost territory it's probably worth insulating the crawlspace floor.
I am in fact in Winnipeg, so not exactly the middle of Canada, but definitely cold enough. Its been -20 to -30 C with the windchill this past week and my crawlspace is hanging out around 14 C, with the room above about 19 C. Humidity in the house is about 35 % while in the crawlspace its about 45%. I think thats too be expected as most of the lumber down there will still be drying somewhat.
The crawlspace itself it perfectly sealed, I was meticulous with a 10 mil vapor barrier, on the ICF walls up to the 4" of ridgid foam in the joist bays, all gaps foamed or caulked. The sump pit is sealed as well as I could, but there may be a small leak around the hole for the discharge pipe.
Im tempted to open one of the HVAC ducts in the crawlspace considerably more to get more heat into the area. If the small room above (mudroom) that Im stealing hot air from suffers much temperature wise, I can add a small forced air electric, in wall space heater in there. I think that getting the crawlspace up to 18 to 20 degrees will go a long ways to have the room above it stay more comfortable.
I think Id rather have a little electric heat in an occupied area where it will be seen rather than tucked away in the crawlspace anyways.
>"...my crawlspace is hanging out around 14 C, with the room above about 19 C. Humidity in the house is about 35 % while in the crawlspace its about 45%. I think thats too be expected ..."
The dew point (a meausure of absolute humidity) of 19C / 35% RH air is 3.2C. Cooling off that same body of air to 14C would yield an RH of 48 %. So the air in the crawlspace is at about the same humidity level as in the room above.
>"The crawlspace itself it perfectly sealed, I was meticulous with a 10 mil vapor barrier, on the ICF walls up to the 4" of ridgid foam in the joist bays, all gaps foamed or caulked."
Do you have 4" foam cut'n'cobbled under the full floor, or are you talking just at the band joists, right at the top of the ICF stemwalls?
>"Im tempted to open one of the HVAC ducts in the crawlspace considerably more to get more heat into the area. "
That will do it. Be sure there is an adequate return path commensurate with the supply too, even if you have to cut in a floor grille somewhere. Simply pressurizing the crawl space with just a supply duct will depressurize the rest of the house relative to the outdoors.
I'm going to assume that the existing furnace is using a reasonably cost-effective heating method, and is adequately sized to heat the house, the addition and the crawl space. If that's the case, you don't need another heat source. Full stop.
You need more heat in the crawl space, which means more air into the crawl space. You don't talk about zoning. If the existing system is unzoned, or the addition is part of a large zone, that means balancing the flow. Is there a path for return air out of the crawlspace? Air can't go in without air coming out. Beyond that, you need bigger openings into the crawlspace so it gets more air. That won't necessarily mean that the rest of the house will be cold. The thermostat will hold temperature and it will mean that the furnace will run longer -- but the point of heat is comfort, remember. Once you've added ducts and a return path in the crawlspace and gotten it to the temperature you want, then you can deal with balancing if part of the house is dramatically different in temperature. Balancing can be achieved using dampers, booster fans or zoning. But don't assume it's necessary, it would only be necessary if part of the ductwork is dramatically mis-sized.