When and where to caulk wood siding
We live in a 1920 balloon framed house, Climate Zone 4. The house has wood clapboards nailed directly to the wood studs, no sheathing. To make things worse, blown in cellulose was installed in the wall cavity years ago. And since we have had spray foam installed a few areas on the second level during a renovation – it too is in direct contact with the siding. I know this is not the best building practice, but we have begun to remove the vinyl and paint the old wood. The wood is in surprisingly good shape, but I’m guessing the vinyl has protected it. And after removing the vinyl I’m glad we did because we realized some things were installed incorrectly and caused damage to some window sills and adjacent siding.
The contractor working on the house insists we caulk the underside of the siding. His reasoning is since there is no sheathing, the caulk will create a weather barrier to keep moisture out (and bugs too). From what I understand, the horizontal lap joints should never be caulked so any moisture in the wall can escape, particularly vapor/humidity driven into the wall the winter.
About a year ago, we did try caulking an area and with 6 months a few paint bubbles appeared…under the siding lap. I’m thinking it’s moisture, the contractor is saying the siding was not prepped properly.
So my question: is there ever a reason to caulk the lap joints, like in our situation with cellulose and no sheathing? My thought was to caulk a few wide gaps to keep bugs and wind driven rain/snow out, but leave everything else uncaulked.
Or…are we just crazy for even attempting to paint the siding as is? We just really don’t want to go back with vinyl and can’t really afford to replace it all and add sheathing.
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I just saw Dr. Joe Lstiburek of Building Science Corp talk about this last month, which is at least the third time I've heard him say the same thing but it's good to have reminders. The result of his graduate school thesis project was that not just leaving that joint uncaulked, but adding small shims to allow moisture to drain, was very effective for keeping paint on the siding--and interior mold at bay--when working on older homes. The gaps can be tiny, just enough for water vapor to escape, which it often can't do through painted siding.
I tried finding a reference paper on their website but it's not showing up. But at minimum, I would not caulk that joint, and preferably I would add small shims at each nail.
Huh, interesting - thanks for sharing, Michael. I've often wondered about this as homes get repainted over the years - it seems like it'd be pretty difficult to repaint a house with wood clapboard siding and NOT have the paint seal up the horizontal overlap seam.
I agree, but with differential wood movement, that gap often opens up again. Or latex paint will span the gap but a single layer or two is still vapor-permeable.
Michael,
It's interesting how all that changes when all the cladding and trim are mounted on rain-screen strapping. When things can dry into the cavity, sealing at the exterior plane begins to make more sense.
Malcolm, I totally agree. I have long thought that the single biggest advancement in building science in the last 20+ years was the introduction of rain screens, as they solve so many problems so simply.
I agree with you on the rain screens. They are a similar concept to vented roofs: provide an easy ventillation path and mositure becomes much less of a problem.
I'm a firm believer in trying to use simple solutions wherever possible, if for no other reason than it's less likely people will mess up simple things :-)
Bill
Malcom,
I'm leaning away from caulking - no caulk but no shims either - I'm a little concerned about trapping moisture in the wall from humidity/vapor in the winter. But are you saying it might make sense to seal up the exterior in this situation if they can dry to the interior? The interior walls are plaster and lathe (mostly - a few areas have renovated areas have gyp. board), not sure what paint is on the old plaster though.
arkitkt,
No. Rain-screen cavities provide an alternate drying path to the exterior, so with that gap sealing up the cladding makes sense. If you don't have a gap you need to maximize the cladding's ability to dry to the outside, so I wouldn't seal it.
The vapour-drive in heat dominated climates is outwards, so you won't get much drying to the inside.
Thanks Michael, I've read some of Lstiburek's posts but not on this subject...if you can find them that would be great!
I'll keep looking for it. I'm pretty sure I've seen articles he's written on the subject. Or possibly it's only something he talks about in presentations, though I doubt it.
Paul, we've done a little painting and gaps under the siding are still there...so I believe moisture can escape.
Michael, one clarification: Do you think adding shims and opening the gap more is a good idea with no sheathing? Will that allow more wind blown rain/snow in and saturate the cellulose?
That's a tough one, but think of it this way: in order of importance, according again to Lstiburek and other gurus, the building envelope control layers, in order of importance, are:
1. Water control layer. If you can't keep the weather out, you're in trouble.
2. Air control layer. You don't want the walls to "breathe," they should be as airtight as possible, with mechanical ventilation added if the building has low air leakage.
3. Vapor control layer. Less important than the two above, as damage due to vapor alone is relatively rare, though it adds up over time so it's still important.
4. Thermal control layer. Once everything else is addressed, do your best to keep the interior at a comfortable temperature with low energy expenditures.
Following that logic, I would say that you are asking a heck of a lot from a single layer of clapboards and coats of paint, but that keeping rain and air out is more important than vapor control, which is basically what shims provide.
Michael, so are you thinking no caulk but no shims...leaving the small gap at the lap siding for a little air movement but not sealing it up? Or caulking so it creates a little bit better water and air control layer - hoping moisture/vapor in the wall is not a problem?
I fully understand that what we are doing is not best practice...but it's our goal to keep the house as original as possible...and the renovation work affordable at the same time.
Thanks again for your comments.
Yes, that's what I talked myself into after going through the layer priorities. I'm gearing up to paint the walls on my 1830 house, which has sheathing but with large gaps, so I understand the compromises.
You might want to catch our next BS+Beer Show; we're talking with Justin Fink and Patrick McComb, who have both researched and written about exterior coatings. Justin's a big fan of traditional linseed-oil paint, as he wrote about here: https://www.greenbuildingadvisor.com/article/resurrecting-linseed-oil-paint
So Michael just to clarify...you've talked yourself into no caulk/no shims? Is this what you will be doing on your house? And just curious - with your house, what primer/paint do you plan to use?
I've talked myself into it for you, but I haven't made a decision about my house yet; I have a few more projects to tackle before that one becomes the priority.
I think caulking the bottom of the siding is asking for trouble, I'm pretty sure the paint will peel in no time. I also can't see it helping much in keeping water out. The water typically moves up through capillary action between the boards where they overlap, this is why the wedge suggestion helps.
Overall, you need to fix your wall. Insulated wall without a WRB of some kind will fail. There is no bandaid to get it to last. Actually there is, the vinyl siding that you took off is what you need.
If you want wood and insulation, remove the siding, fix the lack of sheathing, WRB and window flashing and re-install.
The section with spray foam (assuming closed cell) is fine as it isn't effected by any water leaks. The siding finish might still suffer over time.
Unfortunately this is one of those cases where the right solution is very costly and all other ideas will simply mean more work but still fail.
Akos,
I agree this is definitely not ideal. The vinyl needed to come off and I'm glad it did because there were a few issues behind it (moldy foam core instead of a true WRB for example). But I'm having a hard time going back with vinyl. But maybe that is what we need to do :(.
When you say failure, are you saying paint failure or structural failure from moisture in the wall?