Condensation from Porch Concrete?
ZZ54e3arn5
| Posted in General Questions on
A few of our new houses have zero step entries with the porch concrete coming up to the top of the band board. Only felt paper separates the concrete from the wood framing at the porch. The interior of the bands are insulated with 2” rigid foam and sealed with Dow Gaps and Cracks. We’ve discovered moisture in the basement along these band boards. I think we should have installed rigid foam against the house before the concrete was poured. Any suggestions on how to fix the current issue? Thanks.
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Replies
You need to remove the concrete from contact with the rim joist and create a condition where the rim (a) can dry to the exterior and (b) stay dry. Putting foam on the exterior and then placing concrete against it would probably rot it out eventually. You need an air gap and a way for any water that falls in to drain away. Hopefully the whole thing is covered by a roof.
Jason,
You don't explain whether you have a roof over this concrete-floored porch. But whether the porch is roofed or unroofed, the concrete can easily be damp; even a change in temperature can allow the concrete to absorb moisture from the outdoor air.
The fix will not be easy. The best solution is to get a concrete saw and cut back the concrete, creating a gap between the concrete and the building. Ideally the gap will be wide enough to allow you to protect the band joist with a peel-and-stick rubber membrane.
If the inside of the band joists are covered with foam board, where are you seeing the condensation?
Yes, it was a mistake to use only felt to separate the highly hygroscopic concrete from the wood framing. Felt is a good rain barrier but not a capillary break, since it will absorb water and pass it though to the other side.
Unless you can pull the concrete away from the house and install a vapor barrier material, probably the best you can do is to seal the joint with a good urethane caulk like PL or Sikaflex.
I would remove the interior foam board, however, until the wood can dry out. If you cannot seal the exterior joint, then it would be best to allow the wood to breathe to the inside.
2. The porches are covered by roofs.
3. Water was dripping from behind the foam.
Jason,
Are these porches, or grade level patio"s?
Do they extend the full width of the structure, or do they terminate before the corner'(s) of the building?
And finally, does the water infiltration show up evenly along the full width or just at certain locations?
5. These are porches, not grade level patios. One style has a porch that extends the full width of the front wall and the other porches extend from one exterior wall to the outside corner another exterior wall. It seems that the infiltration is along the full width of the porch.
Jason,
I think you need to determine whether what you're seeing is leakage from the outside or condensation from the inside because of the cold conductive concrete against the wooden band joists.
If the moisture is uniform and not localized, I suspect condensation. You might want to check the relative humidity in the basements. If the water is leaking out from behind the foam, then it can't be very well sealed and humidity can easily pass through to the band joist.
If the porches are covered, my guess is it is condensation and not bulk water. The foam insulation may actually be hurting you in this case - keeping the heat from reaching your band joist keeping it below the dewpoint. Sounds like a combination of high humidity and cold band joist. Reduce the humidity and raise the temperature of the band joist and see if that makes things better.
Hard to make recommendations without knowing if the basement/crawl is vented, your location and other site specific information. Not a great long term solution but maybe while troubleshooting - install a dehumidifier to lower the humidity (we typically try to avoid these long term since they are such energy hogs) and maybe remove half the foam to see if the condition improves in one or both places.
Jason,
I think Robert and Danny have given you a good place to start. Let us know if removing a test area of the foam dries up the situation.
Thanks for your answers
Removing the foam did allow rim joists to dry. It's interesting that one of our houses without the zero step entry and same rim joist insulation did not have any moisture. Maybe the thicker gravel base against the house is a factor? I'll update after more troubleshooting and monitoring. Thank you for all of your suggestions.
With the insulation removed we have not seen any more moisture. As for now we are leaving out the insulation and continuing to monitor. A laser thermometer showed the uninsulated rim joist at the porch to be the same temperature as the rim joist insulation on a different section of the house. It may be best to leave the rim joists at porches uninsulated.
Jason,
The temperature readings were the same because an IR thermometer measures surface temperature. The rim joist should have been insulated on the outside, to isolate it from the conductive concrete, to keep it dry and to allow drying to the interior.
Given what you've got, leaving it open to dry to the interior and remain closer to interior temperature is the sensible solution.