Insulation of Floor above damp basement
I have a cement floored basement with some humidity in summer which I control with a dehumidifier. I am having the sills and bays air sealed with foam and am planning to add insulation to the ceiling (of the cellar” with R-19 fiberglass. THe contractor is offering 3 choices, but no advice. 1. Place batts up against the floor with vapor barrier against the floor; 2. place unfaced batts up against the floor with poly ethylene barrier stapled to the bottom of the joists; 3. insulate the bays with “DryRight” fiber glass, with a vapor barrier that regulates moisture diffusion in response to rel. humidity changes. I do want to improve energy efficiency without adding a moisture problem. What method would you suggest?
Thank You.
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Replies
Jack,
I would not recommend insulating the floor joists because you can potentially cause more problems than solving as well as increase your energy usage. If you insulate the floor joists, you are then going to make the basement colder, in turn will make the basement even wetter. Not to mention the fact that if your furnace and ducts are in the basement, plus you have the potential of frozen pipes. It is always better to bring the basement into the thermal boundary by insulating the walls.
The first thing you want to do however is to address the moisture. Check your downspouts, grade against the home, etc. In the process of locating a good article to recommend to you from Building Science Corporation, I came across this PowerPoint presentation that was recently published and discusses what I have mentioned, as well as shows a couple of strategies to fix moisture related problems. http://www.buildingscienceconsulting.com/services/documents/file/20100615_Foundation_Insulation_Options_Neuhauser.pdf
Jack,
You'd be better off investing in rigid foam insulation for your basement walls.
If you do decide to insulate between your joists, don't include any polyethylene.
Sorry should read:
Not to mention the fact that if your furnace and ducts are in the basement, your system will have to compensate for being in a colder space, plus you have the potential of frozen pipes.
Jack,
The other responders are correct: first locate and fix the moisture problem or it will remain a problem no matter what you do. Once that problem is fixed, you'll be far better off with the basement within the conditioned envelope. You've already insulated the band joists, now continue that insulation down the walls with rigid foam.
As to the options offered:
1. Place batts up against the floor with vapor barrier against the floor;
If the basement remains outside of the conditioned space, then the vapor retarder should be adjacent to the subfloor, isolating the insulation from the heated space. But I would use only kraft-facing and not polyethylene, which can trap liquid water that spills on the floor above.
2. place unfaced batts up against the floor with poly ethylene barrier stapled to the bottom of the joists;
This puts the vapor barrier on the "wrong" side of the insulation and creates a trap for liquid water, but would seem to make sense only because the primary moisture source is below rather than above - a condition that requires correcting.
3. insulate the bays with "DryRight" fiber glass, with a vapor barrier that regulates moisture diffusion in response to rel. humidity changes.
This would completely undermine the value of the variable-perm membrane, since it will allow basement moisture into the insulation rather than facilitating drying of the insulation.
From the DryRight literature:
DryRight is not suitable in buildings with exceptionally high, constant indoor humidity levels...
Use of DryRight is not recommended where residential humidification systems are set at relative humidities greater than 50%.
DryRight with MemBrain facing is not intended to be used as an exterior vapor retarder.
DryRight must be installed behind and in substantial contact with the unexposed surface of the ceiling, floor or wall finish.
Joshua, Martin, and Robert,
Many thanks for your comments on my cellar Question.
I would like to add to my description the fact that the moisture problem only exists in the summer and that there is R-11 fiberglass installed by the previous owner flush with the bottom of the joists, vapor barrier down. I was planning to have it removed and have thicker insulation properly installed. It gets down to 45~ 50 degrees at the lowest in winter. Never a problem with frozen pipes, and our ducts are air sealed and insulated.
Downspouts are directed away from the house, but we are on a down hill lot. We have foundation drains that work and have added curtain drains on the uphill side.
We really don't want to heat the cellar, as we store food and vegetables there and also want to have warmer floors.
Does this change the picture for you?
Thanks,
Jack
Jack,
Just keep in mind: the warmer the floor the colder the basement.
I should add that, if the bulk water sources are well-managed (as you say) and the humidity problem occurs only in the summer, it's because warm/humid air is entering the basement and cooling from the lower earth temperatures. As air cools it becomes more humid and eventually reaches the dew point.
If you want to keep the basement dry in summer and not have to run a dehumidifier, then keep it closed up, perhaps allowing just enough air exchange by controlled ventilation to maintain your root cellar conditions.
This is the same problem commonly experienced with vented crawlspaces. In the summer, they get wet because of the entry of outside air. Seal them up and at least semi-condition the space and the moisture problem disappears.
Robert,
Thanks, I think your last comment is really helpful. I think air sealing will help with the intrusion of moisture, and removing the exisiting, mildewed insulation will help the cellar atmosphere and maybe bring the dew point up in the Summer. Our next step later will then be to insulate the walls.
Thanks again.
Jack