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Question DetailsAsked on 2/14/2018

Question for LCD about gas smell....

After smelling gas from a hole in wall, we had several leaks repaired in our house line and now it pressure tests well. The neighbors downstairs had leaks as well. They were repaired. We are still getting residual gas smell from the wall where we first found gas. Wondering if it's old drywall/insulation loaded up with mercaptin, or if there's still a leak plaguing us. The gas company comes and finds 30-40 ppm of what might be gas in the wall, but they say this is nothing to worry about. What should we do? I've painted the wall w/kilz (oilbased, two coats). That definitely reduced the smell, but when I open the fusebox on that wall, theres's still a strong smell. Plus we can smell hints of it right when we walk in from the fresh air. (We immediately stop noticing it though) The story is longer and more complex than this which is why I'd love to chat if you're available b/c you gave the most thorough/thoughtful answer re: drywall gas odor.

Thanks for your time.

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3 Answers

Voted Best Answer

Well, there are different odorants used (including several members of the mercaptin family like ethyl and methyl and enahnced butyl mercaptins among others - there are about 73 commonly used chemical odorants used for safety purposes like this and at least 12 I found in a list for sale for use in fuels alone), so it can depend - some of them are much more readily detected by people, some linger longer, etc - though all I have encountered will stink up an insulated wall and adhere to and outgas from contaminated pipes for months if not thoroughly ventilated or chemically/soap and water washed off surfaces.

The odorants, by law, have to be detectable (based on subject testing) to 50% of people at 20% of the lower explosive limit (LEL) of the gas, so should result in odor detection at or before the point where the natural gas or propane concentration in the air is 1/5 or less of the way up to the concentration where it will explode if ignited. For ethyl mercaptin for instance (the odorant I am most used to) that 50% detectability limit is a bit less than 2 parts per BILLION - a very, very low concentration - about 1/1,000 to 1/10,000 the nose detection limit for most materials. Commonly about 2-5 times that amount is actually put in the gas system to ensure it will be detectable well below the legally required level and to guard against variations in concentration in the system. The level in household gas is commonly a bit higher that that from concentrating of the chemical in the house hold pipes over the years from the gas just sitting there most of the time, thoroughly saturating the piping.

Now - your gas company was detecting 30-40 ppm of (presumably) methane - 20% of the LEL for natural gas is about 1% concentration of the gas in air, so they were detecting about 0.4% of that LEL concentration - FARRR below a hazard level, as they indicated - so at least as far as where they tested there is not enough gas concentration to have any concern about, in my opinion. Especially if the pipe repairs were done in the past month or few, so there would still be residual gas in the unventilated walls which could be detected.

Because this is an apartment/condo building I am a bit unsure why YOU are pursuing this issue at your cost (once the leaks in your unit were fixed, assuming utilities in the walls were your responsibility in the first place - like with an "owned" condo or apartment or zero-net lease agreement). The neighboring apartment owners or condo association would normally be responsible for any gas smell coming from further out in the building.

Simple logic - since both you and the neighbors had gas leaks several places, it would be rational to assume there are leaks in other units as well (sounds like aging pipe problem or they used the wrong or some of the early pipe teflon tape which eventually dissolves in natural gas so leaks at joints over time), which should be tracked down and repaired, unless the smell can be proven to NOT be coming from leaking pipes. You did not say WHERE the leaks in the pipes were - if all at joints, then one would logically presume that all the gasline joints in the building are equally likely to leak at this age - due to poor assembly procedure, poor brazing/soldering if copper, improper/defective joint tape if taped rather than using pipe dope to seal threaded connections. Or if the leaks were pinholes in the pipe, than the pipe is either defective or if corroding out so should be replaced building-wide regardless.

Several approaches here:

1) to purely stop the smell, assuming this is residual smell only, in addition to the Kilz on the drywall (which would not be expected to be highly effective, as drywall by itself with any paint at all on it is not highly gas permeable), would be to also seal the base of the wall(s) affected with siliconized latex (to be paintable) or pure silicone caulk (unpaintable) - at the interface of the drywall and the bottom wall plate, or if drywall goes clear to the subfloor at the drywall/subfloor interface. This would require pulling the baseboards/base trim to get at the interface - at leat on that wall. A quicker and dirtier (and not so nice looking) solution would be just caulking at the top and bottom trim/baseboard interface with wall and flooring.

Also, a significant amount of your air leakage comes through electrical openings in walls and ceilings - switch plates and outlets and light fixtures - so removing the cover plates and any wall-mounted light fixtures and putting in foam weatherstipping (premade, like from FrostKing) will essentially eliminate that source. Ditto to caulking/sealing around the ducts at any forced air vents (remove vent, seal around duct at the drywall with duct tape or foam-in-a-can or or caulk/culking foam rod, replace vent cover). Ditto to sealing around any pipe penetrations through walls or floors (especially commonly needed with any steam or hydronic heating system pipes).

2) To remove the odorant smell from the insulation, if the odor is a real irritation or for best safety so you do not have that smell around just in case a real leak develops and gets ignored because of that, one can put a ventilation system in the wall - same type of partial vacuum system one would install for radon removal under a slab but installed to draw air from the wall and vent it outdoors. Of course, fan unit would be MUCH smaller than for a radon system - just enough to put a slight vacuum in the wall so the odorant does not escape into the living spaces - something more on the size of a large computer ventilation fan than a bathroom fan, to avoid excessive wastage of conditioned air.

3) One could go around the building, checking each appliance and at various places in the walls for gas concentrations - homing in on the highest concentration location. This needs to be done with all doors and windows in the building closed and HVAC fans off. I the highest concentration they find is in the low ppm's like in your wall, then unlikely to be a significant gas leak anywhere - though could still be pinhole leaks. Bear in mind the natural gas system in the building normally runs in the fraction of a psi pressure - so there is VERY little internal pressure in the pipes to force the gas out into the building, so a small leak can go undetected for ages just because there is enough ventilation and diffusion in the air to not be noticeable. That is why the odorant is chosen to be detectable at a very small concentration - to allow leak detection before it gets strong.

4) both of the above would be if the odor does not spread significantly with time in a vacant apartment (with doors closed) - if the smell become significantly stronger and more pervasive over time in multiple rooms, I would be suspecting a current gas leak diffusing through floors/ceiling or walls. A determined search throughout the building for gas leaks, with a sniffer checking all around all wall and floor and ceiling openings, removing representative electrical outlet cover plates and sniffing there (with a powered sniffer like gas companies use - which has a little pump in it which pulls air in to be sampled, not just a cheap pocket "probe" type which has to be stuck into the gas pocket to detect it), checking the entire building for natural gas, would seem to be in order at a minimum.

5) One thing needing checking - a simple nose check should probably work for your area but other units and public areas should be checked too - sniffing around for a gas burner or pilot putting out odor because the gas is not cleanly burning, or because the odorant has been changed to one which persists even after burning. The draft hoods at all appliances like furnaces, boilers, and water heaters should be checked when they fire up - because there is commonly a short burst of combustion gases which comes out of the draft hood at the top of the appliance when it initially kicks on. Usually detectable odorant smell only for maybe 3-6 feet around the appliance and only for a few seconds until the draft hood starts pulling ambient air into the flue (this for gravity flue devices, not direct vent), but a partly blocked flue, a firebox with rollout or inadequate combustion air, and leaking flue can also put the odorant smell into the surrounding building. Each appliance should be checked for this, and flues cleaned (if needed) and inspected with camera for any combustion gas leaks.

6) Safest thing would be for a three-part check building-wide: item 5) above, then a handheld gas sniffer inspection per 4) and 5) above at ALL appliances in the building (dryers, ranges, furnaces/boilers, water heaters, gas fired heating units on heat pumps, any other natural gas consuming appliances. Then, shutting off the shutoff valve to all of them (because they cannot take the test pressure), doing a building-wide long-term pressure test (usually 12 or 24 hour depending on state law) pressure test on the pipes, using usually nitrogen, to detect any other gas pipe leaks. Of course, not highly feasible in cold conditions because the gas heating appliances would be shut off for 12-24 hours, and requires coordination with all residents because heat (and generally central A/C as well) and all gas-fired hot water and gas cooking/drying will be out of service during that time period. This sort of test would have to be coordinated by building management, and in almost all cases would also be their responsibility to pay for.


I have heard of cases in large cities where building management refused to diligently track down and repair small gas leaks like this (because it can run a thousand to few $ in an apartment building, PLUS any repairs), so the residents ended up calling the fire department daily about the gas smell accumulating, until the fire department got tired of the calls and contacted the city housing enforcement bureau to have the building cited for non-compliance. Or in some cases the fire department will, if a leak like that is not fixed in short order (with a few days or less) may just condemn the building, evcuating it until the leaks are fixed and zero gas can be detected in an inspection. Course, that means the residents have to be put up in temporary housing duyring the interim, which in most areas has to be paid by the building owner.

Answered 2 years ago by LCD


Adding more info: it's a large apt buliding, 12 years old. The plumbers have come and pressure checked every line above and to the sides and below us and everything is good after repairing a few small leaks. The super informed us that the previous owner of the apt noticed a gas smell for years but the gas co never found anything. However when we had them do a pressure check, it was obvious that there were many leaks on our line. Trying to guage if the smell we are experiencing is old leftover gas smell, or if there is still a small leak in the bldg below us somewhere that is giving us odor.

Answered 2 years ago by mattrohrer1


Thanks LCD. I really appreciate your thoughtful response, and your time. A few responses/clarifying questions:

The leaks were found in the joints. I agree with your assessment that most likely many joints in the building are leaking b/c they were all put together at the same pt. However, the building is taking it's sweet time getting to the diff apts to check, and the plumbing company seems to think it's not that urgent as well.

am a bit unsure why YOU are pursuing this issue at your cost -

I am not. Our building management company is coordinating with the plumbing company, per our request, and the building is paying for it, however, they are dragging their feet (it's taken over a month to check 6 apartments (out of 32 in the whole building).

to purely stop the smell, assuming this is residual smell only, in addition to the Kilz on the drywall (which would not be expected to be highly effective, as drywall by itself with any paint at all on it is not highly gas permeable),

I'm a little confused here. Are you saying that the kilz on the drywall will be effective, b/c it renders the drywall not gas permeable, or the other way around?.

Our apartment is a large loft like space (one big room w/kitchen, living room, entryway, and then a separate bedroom) and the smell is always strongest right by the entry door, although it does spread to the whole room (which is one open space, about 600 sq feet) albeit quite faint. So it's hard to say if it's an active leak or not... The gas company and plumbers say they think it's not. But they also didn't believe there was a gas smell until we had called 5x and they finally did a pressure leak and detected 6 leaks, so it's difficult to trust their judgment.

Unfortunately the wall ventilation system would be difficult b/c our entry wall ( the one that stinks) faces the internal hallway in the building and is opposite the window (about 35 feet from any outdoor space ( a vent pipe woudl have to be put in through the entire apt, which would be expensive and ugly). Hoping that the Kilz, and then sealing the light switch and fusebox with some of the sealants you mentioned will help, (although won't air still come through the switch itself regardless if the area around it is sealed?)

Here's what I'm thinking is our plan, based on your feedback: We do another layer of Kilz. Seal the base of the wall and the fusebox/switches as best we can. Encourage the building mgmt and plumbing company to continue to check the lines below us, and if smell continues, pressure the fire dept to continue to check until issue is resolved in a more forceful way. Although, I'm not sure if this would happen, b/c the level detected is so low (IE 30/40 ppm in walls)

Thanks again for your time. Any more thoughts feedbck would be much appreciated.

Kind wishes.

Answered 2 years ago by mattrohrer1


Paper surfaced drywall is not highly gas permeable, and there is negligable pressure differential across the drywall in any case and may actually be from living space into the wall, so not only is there little "driving force" for the gas to go through the drywall, but it also would not tend to transmit much anyway. Certainly the Kilz you have done should be more than enough for the through-the-drywall factor - the oil-based Kilz does a real good job of stopping odors like cigarette smoke and meth lab chemicals and such from outgassing from contaminated drywall.

The outlet/switch sealers I am talking about look like following link - come in packs of about 6 or so and also bundle packs of both switch and plug ones - just sheets of foam which you punch out the appropriate pre-perforated holes for the outlet(s) and switch as applicable, put in under the cover plate, and replace cover plate. As long as you are careful not to jam a finger or screwdriver into an electrical box or outlet when doing this or putting the screws back on (always work one-handed and not touching the cover plate for safety), easy DIY job. Of course, turning the breaker off first is a safer idea, especially if not comfortabel bare-handing around exposed outlets.

On the fuse box thing - depending on how comfortable you are around electrical - you could also loosen the mounting screws on the cover plate (usually 4, in the corners) just enough to slip in self-stick foam weatherstripping just under the edge fothe cover plate. Put the sticky side against the cover plate so it does not stick it to the wall, because it has to be able to come off for maintenance. And do not use a metal tool to slide it in there. Of course, if electrically comfortable removing the cover plate entirely 9exposing the wires inside) or the building has a maintenance guy who will do this, then just take it off, put a bead of self-stick foam around the outer edge of the inside face, and put back on. here is the procude I am taling about - the smallest size should work fine -

The outlet and switch themselves are largely or totally sealed on the back for fire safety if that age, so once you put in the foam sealer to stop air from coming through or around the electrical box, should be pretty much airtight - unlike the old days where you could feel cold air gushing out of the outlet slots. You could do that sealing first (minimal effort or cost) and see if that cuts off the smell acceptably, then think about the baseboards as a possible third step ony if needed.

On the gas smell at the door thing - use a burning match or such or even a bit of sifted flour dropping to see which way the air moves at your door - if moving outwards under the door into the hall, then gas smell is coming from your wall, etc. If the air is coming in, then smell is coming from outside (maybe hall) and whatever you do in your unit is unlikely to have any effect. Ditto at wall (though sometimes varies through the day) to see if air is coming out of wall at outlets and baseboard or into the wall.

Like you say, at those low gas levels, hard to get people too excited and personally, if in your posiiton, I would not go ballistic about it as long as they are progressing on solving it. But if you want to make waves, talk to the Fire Marshall (larger cities have one, and each state has an overall one) or the local Fire Inspector (sometimes in Building Safety, sometimes in Fire Department) - ANY detectable gas level (the 30-40 ppm) other than immediately at the moment of ignition when a slight pluff of gas escapes (like a stove burner or furnace of such igniting, detectable only a foot or few away) is out of code compliance. But with the low detected gas level, with 6 units inspected (and hopefully fixed) in a month, sounds like they will be done in a few months, so unless you are afraid, I would (unless the smell gets stronger) maybe just let them keep going and hopefully between airing out time and fixing gas pipe leaks, it will be solved by summer.

BTW - on the mercaptin and natural gas - studies have found no adverse health effects to the low levels you are exposed to if the mercaptin smell is just noticeable, not highly offensive. Get up to 10ppb, where it causes running/teary eyes and nose, through irritation, respiratory distress (similar to tear gas effects) it can be a health hazard with several hours of exposure daily, but you would never let it get that high and that high a level would be indicative of a distinct leak, not residual smell. Note those type of guidelines and test do NOT apply to people with severe medical issues, respiratory distress, or babies, if applicable - ask your doctor if one of those conditions applies - and on doctors recommendation (though I doubt it at these levels) might recommend evacuating the unit till the problem is fixed (which would mean puotting a hurry-up on the management), and in many locales they would have to pay for temporary housing (because the unit you are paying for is uninhabitable for you/your family) till the condition became safe, which also would put the fire under them to get it fixed.

On the safety issue - assuming you have an independent HVAC system for your unit only - sometime when you are home AND awake for an extended period of time (so you can detect the buildup of any gas) like on a weekend day, try turning the HVAC system totally off - fan too - and let it sit for say 4-6 hours. If there is an active leak causing the smell in your unit, the odor should build up MUCH stronger and you will know there is a leak. If stays much the same or just gets a bit stronger but still in "detection" rather than "strongly objectionable" level then likely residual smell.

NOW a major safety caveat on that test - your senses will eventually "tune out" smells and sounds, as you noted before that you don't notice it after you have been home for some time - so during that test you need to go outside for 10-15 minutes every hour or two to flush out your nose and deacclimate it, because just sit inside the whole time and the gas could build up (if there is an active leak) and you would not recognize it because of the "tuning out" effect, until the mercaptin concentration got so strong that it started causing eye and nose burning and coughing - at which concentration the gas level could also be getting too close to the lower explosive limit for safety.

Good Luck

Answered 2 years ago by LCD

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