I'll be building a slab-on-grade house that's simply a walk-out-basement with a roof over the basement, so three sides are earth-covered to the roof and the south side has all the doors and windows. It's a nice passive solar design for my cold Ontario climate.

Passive radon venting is recommended by the EPA (http://www.epa.gov/radon/pdfs/buildradonout.pdf), where they suggest a simple loop of drainage pipe under the slab. One end drains water downwards to an exit pipe (in my case on the side the hill the house is on), and the other end brings gases upwards through a vertical stack on an inside wall of the house. The warmth of the house causes a chimney effect and presto, free venting of any gases collecting under the slab.

But doesn't that constant pulling of cold air from the hillside (passing under the foundation and then through the interior wall) steal heat from the house, raising my heating costs? If I insulated the stack, that would remove the chimney effect (free drafting). Should I instead have an outside stack out in the backyard, with a small solar-powered inline fan? Any other suggestions?

Thanks,
...Terry

First, I’ll say that I didn’t think you Canooks paid any attention to the US EPA. If I were you I probably wouldn’t. :-)

Joking aside, personally, I think the amount of energy lost to the cold pipe going up through the conditioned space would be negligible as there will be no air leakage from the conditioned space into the pipe. Further, the amount of air moving through the pipe will not be a huge volume. That is just an unsupported opinion though.

If you are in an area of suspected radon (rocky soil) I wouldn’t hesitate to do it. After the fact radon mitigation systems are expensive to have installed, they have operational costs, and are just another thing to break down. I had one installed on a house I built and it was around $1800 and ugly on the side of the house to boot. A passive system would be far preferable. The unfortunate thing about radon is you never know if you are going to have it. You can only find out after the fact, although obviously certain conditions and geographies predispose a home to it.

If you are that worried about the wasted energy you could build the house with it but install a non-glued pipe cap at the top of the stack. Then after being in the house for a little while get one of those send-away radon testing kits and test it. If you need the radon mitigation, just remove the cap.

Or you could go with the solar fan powered stack out back, or even insulate the pipe going up through the house with the solar fan somewhere around the top of the stack. Further, the solar powered fan probably wouldn’t be very hard to retrofit later if it became necessary. The solar powered fan would have some finite lifetime though – whatever that may be and would need to be replaced at some point. I’d be more likely to just have an outlet up in the attic near the stack for a regular powered fan install later if necessary. BTW – do you have a link for one of these solar fans? I’d like to see it.

One thing about Radon is that if the house under slab vapor barrier is detailed properly and is all inclusive, and you have a really good waterproofing job on your walls the chance of radon infiltration is greatly reduced.

One word of prudent caution though. Assuming you do go with a 3” or 4” stack, you may want to make sure there is enough room for the pipe going up through the house so that the drywall will not be in contact with the cool pipe as it could be a condensing surface. In other words, run it up through a 2x6 wall.

It's not pulling air from the house, but it is pulling heat from the house. It's a pretty low load though- it doesn't take a lot of volume, just a enough to de-pressurize the slab so that the bulk of the soil gases are vented via the stack and won't permeate the slab. If a passive system works it'll be much lower energy usage than if active ventilation system needs to be used.

Don't cap it- let it run. If costs you even $10/heating season in fuel costs I'd be shocked, since the air volumes are so low. The flow is unlikely to be high enough for condensation to occur on the pipe, but if it does, insulating the stack with R2 (bubble-pack or self-adhesive R2 duct insulation) would still allow enough heat transfer to drive the stack effect but keep the drips at bay.

On my neighbor's radon system (a higher-impedance retrofit), we unplugged the fan and put a venturi-type vent-cap on the top of the stack, which was sufficient to get the levels down near the remediation levels (but not as low as with 60watts of 24/365 active ventilation). Should you somehow fail a radon test with the passive system, this approach would be preferable to apply as a first step before going with active systems (solar or grid).

Terry,

google Certainteed Form-A-Drain, it has some interesting info. And try google new construction Radon mitigation - there is a lot of possibilities there.

I would install 4 inch perforated septic drain system piping in a loop 2 ft away from the footing around the perimeter of the foundation buried in 6 inches of washed stone (also installed thruout the floor area); Tee or ell to about 1 ft above finished floor elevation and cap it temporarily; then 6 ml plastic; then 4 inches (minimum) EPS or XPS then PEX for radiant (you will have the perfect set-up for radiant even if you are not planning heating with radiant at this time); then the concrete. Follow all recommendations for sealing the edges of the slab to the walls with caulk; seal all cracks around all openings in the floor with caulk; then you can extend the 4 inch piping up thru a wall leaving access near the roof for addition of a fan if necessary (make sure you have power within 6 ft of that location).

This system should operate passively just as you, Matt and Dana have written.

On the exterior of the footings install 4 inch perforated pipe and washed gravel with a layer of material to keep out fine sand and silt and run all of that system to grade. Certainteed has a nice product for the foundation walls for water proofing (obviously install at least 4 inches of foam on the exterior of the foundation) I have always backfilled foundations with clean gravel if available or washed stone if gravel is not available (gravel is usually WAY less expensive than washed stone). Since the mid 70's I have never had a problem with water or dampness in a basement and have done many retro-fits using the same exterior system. And that includes a passive solar underground home in upstate NY (cold) in 1979!

Please let us know what you decide on!

Best of building to you.
richm

The "chimney effect" doesn't work unless you allow ROOM AIR out your vent. This is actually counter productive for radon and heat loss.

An in-pipe ventilation fan works best in my experience. You need POWER to draw the radon out from underneath a sealed slab.

Thanks, Matt, for the energy-loss reassurance. I've got high drainage soil (sand/gravel), but I'm installing a passive radon stack because there's a history of it near some uranium mines 60 miles south, even though no one in this area has found any.

If you google 'solar vent fan', you'll see several such gadgets, including one at amazon.com.

Thanks for the emphasis on vapour barriers, waterproofing, and not touching drywall with cold pipes too.
...Terry
PS. Hey, I only looked at the EPA stuff because it was referenced in our Canuck CMHC site!

Lots of great advice there, Richm. I did consider Certainteed Form-A-Drain, but I'm trying to buy local wherever possible.

No, I'm not planning radiant heating but you're not the first person to suggest I put in the PEX anyways just in case. I'll give that more thought.

I like your system. I'll be using 6" of XPS under and on the outside of the slab (including Hi-40 under the footing portion).

I think my final decision on the radon will be to go with the EPA's recommendation -- do the standard 4" drainage on the outside perimeter for water but not do an inside-perimeter loop for the radon. Instead, just install a T-pipe into the 4" or more of gravel under the slab, leading up a 4" pipe through an interior 6" wall and out the roof. Keep it simple, especially since I have high-perc soil.

Thanks again,
...Terry

Terry,
I believe it is worthwhile installing the 4" perf. pipe inside perimeter loop. I bet it will be about $100 in material. Cheap labor if you do!
richm

Not True Robbie.  The radon system is sealed from the room.  It is open on both ends - top where the air is escaping from, and at the underslab level where it is drawign from.  You of course have to have a vent at that low point to allow air to circulate through.

From the net:

The US EPA has put it plainly, stating, "Any radon exposure has some risk of causing lung cancer. The lower the radon level in your home, the lower your family's risk of lung cancer."