Exterior wall drying behind tub
CavalryCanuck
| Posted in General Questions on
Good Day,
Redoing my upstairs bathroom. The shower/tub is on the exterior wall, and I intend on doing a tile surround. I’m concerned about vapour drive from exterior to interior during the summer deadheading against the tile & causing moisture issues. I’m in Climate Zone 3a, so the wall is supposed to dry to the exterior (Using Intello Plus throughout the rest of the house).
What can I do to make sure that section of wall is able to adequately dry during the summer months? This renovation was prompted by already-existing moisture issues, and I want to eliminate the risk of further problems.
Thank you for your time!
Matthew B
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Replies
Hi Matthew,
Can you elaborate on "already existing moisture issues"? A little more information on the existing problems might change how we address your question.
Randy
Hi Randy.
The rim joist and a pair of joists in the cavity above the sill plate* are rotten, subfloor was rotten in the same area, and mold radiating out from the rot area.
The most likely primary cause of these issues would be due to water getting in behind/under the tub and surround over time. The prior construction for the tub surround was tile over standard 1/2” drywall. It doesn’t appear to have been sealed properly, as the entirety of the drywall was severely degraded due to moisture.
However, the secondary issue is that the area where the rim joist and joists were rotting had no opportunity to dry. The cavity above the sill plate* was stuffed with pink batt insulation, & blocked off with 2x6 boards before the joists spanned over the basement area. Additionally, 6 mil poly went all the way up the interior of the basement wall from the floor to these blocks between the joists.
*the house doesn’t actually have a sill plate. Joists and rim-joist were layed directly on top of the 8” wide concrete foundation wall.
Matthew,
The water damage you experienced sounds like the issue was with the original tile assembly causing a bulk water leak. If the area behind the shower gets wet regularly due to using the shower, wetting exceeds the areas capacity to dry and things decay. Drying caused by a bulk water leak, whether the leak is exterior or interior is usually slow.
Wetting due to vapor diffusion is a different situation. The wetting process is slow and in a hot and humid climate will be dependent on exterior to interior relative humidity and the temperature of the interior wall finish (air leak from the exterior also plays into this). In the case of a shower, it's more important that the shower not leak water into the wall. The wall needs to be "waterproof" which means it will likely have a slow vapor diffusion from inside the wall to inside the home. I don't know of any products with a high permeance that is also waterproof that can be used inside a tiled shower enclosure.
One option might be to not tile the space between the ceiling and top of the shower. If I remember correctly, tile does not need to be above 6-foot 8-inches from the shower floor. You could leave this space as a painted drywall to promote inward drying. Another option might be to not tile and instead use a tub/shower surround. This will leave a space between the shower enclosure and exterior wall where vapor can dry into.
Hope this answers your question.
Randy