GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Can wood lap siding be done well without caulk?

J Chesnut | Posted in Green Building Techniques on

I am residing a house with beveled cedar lap siding. I would like to avoid using caulk but don’t know if that is prudent where the siding butts against window and corner casings; this being a vertical joint I’m not sure if flashing would be effective. If I flashed with role flashing centered behind this joint between the siding and vertical casings would that suffice? Would metal flashing here be overkill and an extra layer of felt suffice?

On the north face of the house there was some water damage the sheathing at these vertical joints where the original wood lap siding butted up to the vertical casings. The north face has a gable so the 9” overhang doesn’t do much to keep water off the siding. On the siding that I am taking off caulking had been done at some point in time but its hard to determine if the caulk as it degraded helped or exacerbated the problem by trapping water. The house was built in 1907 and has 1x sheathing and rosin paper over the sheathing. I will be removing the siding, adding felt paper and furring out for a drainspace before residing with the new wood siding. I just had the house insulated with dense pac cellulose (the house is in Minnesota.)

If you think I should use caulk at the vertical joints please describe what constitutes a high quality durable caulk. I’m a bit overwhelmed by the selection of caulks available.

Thanks,

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    J Chesnut,
    I'm a strong believer in caulk-free installations of wood lap siding. When you're talking about wood lap siding, caulk does more harm than good.

    Small squares of asphalt felt can be installed under vertical joints to improve watertightness. If necessary, gently lift the window casing with a flat bar to make it easier to insert the apshalt felt under the casing.

  2. Riversong | | #2

    If you're wrapping the entire structure in lapped felt, then that's all the flashing you'll need as long as it's integrated with the door and window flashings and flashings at any other penetrations. With a rain/drain screen, any water that does penetrate the siding will be able to drain or dry. Caulk will only limit the drying potential, require regular maintenance and no caulk lasts forever or has the same coefficient of expansion as wooden materials. Most people, including experience builders, don't realize that caulking is an art in itself and requires sufficient "keying", a specific section ratio, the appropriate modulus of elasticity, as well as compatibility with substrate in order to function as intended.

    Since you're furring everything out for the drainscreen, you must be extending door and window jambs as well and replacing trim. It's those details which are critical and require careful sealing, flashing and integration with the WRB. Make sure that the felt is wrapped around corners and not terminated at corners.

  3. J Chesnut | | #3

    Great, this is exactly what I wanted to hear.
    I'll be replacing trim w/ 6/4 thick stock to come out past the bevel siding. I have replacement windows in their old frames. I'm going to have to expose one to see in order to figure out the best flashing method.
    Drip cap flashing over the crowns of the window trim integrated with the felt paper will be done. I've looked at details that include another drip cap directly above the window and under the top window trim which I don't think I will include. Looks good in a window header section but is awkward in the way it releases onto the side window trim.

  4. Robert Farnham | | #4

    If you have all the siding off the house, isn't it a good time to be breaking the thermal bridge by installing exterior insulation? Any thoughts on this?

  5. J Chesnut | | #5

    Robert F,
    I have considered at length adding exterior insulation. In my work at an architecture firm we advocate for this all the time on remodels. However to make this project make financial sense for my family I'm doing the work myself (this is my own house) and the extra time, staging and detailing required is too much for me to handle as I try to get this done in a timely and affordable manner.

  6. Hunter Dendy | | #6

    I think you're right not to add exterior insulation on a house that old, especially in your climate. It would change how it's been performing all these years in regards to vapor transfer. Old houses are leaky and could be the worst place to create the barrier sandwich.

  7. David Meiland | | #7

    Another vote for no caulk. You don't mention what you're doing at the windows. Are you leaving existing double-hung units? If so, can you find a way to get the felt tucked all the way into the openings? In some cases I have simply removed the windows, installed my pan, wrapped the felt in, and reinstalled the windows. If you don't do this it'll be the weak point. It sounds like you are doing a nice restoration of an old home, a great thing to do.

  8. J Chesnut | | #8

    Hunter,
    This house was built in 1907, has 1x6 sheathing backed by felt paper, empty stud cavity and the inside finish is plaster and lathe. No vapor barrier (although I can't be sure over the years what kind of paint has been used on the interior finish.) I think this would be a relatively safe existing wall condition to add a significant exterior insulation package. Keeping the stud cavity empty would allow drying to the inside. The 1x6 sheathing is less prone to moisture than plywood and OSB. As I understand from other post on this website 4"+ of exterior rigid insulation would keep the sheathing warm enough not to be a surface for condensation.
    The challenge is in all the details, the cost, and maintaining a respectable exterior appearance but I think a 'deep energy retrofit' on an older home like this could be done (if executed well considering air tightness and added ventilation) with low risk.

  9. J Chesnut | | #9

    David,
    The home has replacement insert double hung windows. The replacement units (Pella) fit into the old 1x frames and are caulked around the top and sides with a backer rod behind the caulk. From what I have seen so far the old window frames and sheathing around them have held up well even without drip cap flashing over the window nor a proper WRB (their is rosin paper over the sheathing but it has virtually disintegrated). The fact that everything has held up so well is a testament to solid wood and lots of thermal transfer out the walls.

    A window on the north side has a rotten sill but most the original sills have held up and I'm hoping just to prime and repaint to keep things moving ahead.

    My thought about the sides and tops of the windows is to remove the existing caulk and backer rod and try to tuck the felt paper around the old 1x frame, then re-caulk. And again I'll put drip cap flashing over the window head trim but not directly over the window unit. Replacing the trim will allow me to cover the caulk joint between the insert window and old frame. Currently the caulk joint is visible.
    This leads me back to a question I have about caulks. What do experience builders consider the best caulks to use over a backer rod and 1/4" gap?

  10. David Meiland | | #10

    lots of thermal transfer out the walls

    It's not just heat, but your previously-uninsulated walls have dried very nicely due to the air circulating thru the empty cavities. By insulating you have completely changed that, so flashing becomes a lot more important.

    Above it sounds like you are planning to wrap the existing, old jamb? I would probably not do that, I would try to tuck the felt between the jamb and the framing. As you have seen the most critical area is the sill, but the sides matter too. This part of your project is a fussy one.

  11. J Chesnut | | #11

    David,
    Now I think above the windows I won't tuck the felt paper at all but instead extend it as far down as I can to act as flashing allowing water to pass over the caulk joint and release onto the window frame.

  12. Hunter Dendy | | #12

    I would think that 100+ years of repainting (probably a few layers of oil based) could constitute a fairly impermeable interior layer. You're right though that with open cavities and some likely leaks at the top and/or bottom it probably isn't too much of an issue. It sounds like you're putting a lot of effort into restoring a historic house and I commend you on that, I just finished one myself.

  13. Riversong | | #13

    I don't know what David is getting at with tucking the felt. The felt WRB should remain on the exterior plane of the structural envelope (sheathing) and be properly shingle-lapped to direct water downward.

    I suspect that the widely-recommended sill pans may not be the panacea they're made out to be, since they can pool water under the sill or prevent the framing from drying should it get wet. With cellulose in the walls, there will be lots of hygric redistribution to deal with any small leakage.

    You're certainly right to cover the caulk joint, as no caulk is immune from UV degradation and weathering. Urethane caulks are among the most durable, most adhesive, and with sufficient elasticity to maintain their seal during cyclical expansion. But backer rod and tooling is essential in order to create the proper "hourglass" cross-section of the caulk joint to allow one-dimensional expansion.

  14. J Chesnut | | #14

    Thanks Robert for the information on caulking. I did not know about tooling the caulk to an hourglass section for proper functionality.

    Hunter, my house is old but I wouldn't say 'historic' and I'm more learning from my house than restoring it.

    With my re-siding job I'm trying to demonstrate that wood siding is a durable option that I hope my clients will consider as an alternative to cement board siding. I was also trying to demonstrate the use of FSC certified siding which I have been able to source but at great difficulty. I unfortunately discovered FSC wood lap siding is currently not a viable option in my local market mostly due to the fact lumber mills are too large of operations here to mill orders less than 10K board feet at a reasonable cost.

  15. Riversong | | #15

    Most caulks should be tooled into an opening against a backer rod such that the depth of the bead is half the width, with a broad surface for adhesion on each side. This allows the caulk joint to stretch and compress with expansion and contraction of the substrate(s).

    Thus, if caulking is planned, gaps should be left between abutting materials. Smearing a bead of caulk on the surface or into a tight 90° intersection will not create a durable seal.

    In this graphic, the bottom cross-section is correct:
    http://www.buildingdiagnostics.com/articles/sealant1.jpg

    Here is another image of a proper caulk joint:
    http://www.conservationtechnology.com/images/Building/SealantsGoodCaulkJoint.gif

  16. David Meiland | | #16

    I don't know what David is getting at with tucking the felt. The felt WRB should remain on the exterior plane of the structural envelope (sheathing) and be properly shingle-lapped to direct water downward.

    Are you saying that you don't wrap the WRB into the window and door openings? That's what I do and I think it's better than not, by a small margin. I am generally remodeling or building new so it's much easier than the OP's case. He has existing double hung jambs in place, it may be difficult to do it effectively.

    I suspect that the widely-recommended sill pans may not be the panacea they're made out to be, since they can pool water under the sill or prevent the framing from drying should it get wet. With cellulose in the walls, there will be lots of hygric redistribution to deal with any small leakage.

    Again, easy to do with a new opening. I generally set a piece of 1/2 x 6 bevel siding on the rough sill and use a flexible membrane to make the pan. I have seen new, expensive windows leak right away and would never set a window without a pan that drains out over the WRB.

    You're certainly right to cover the caulk joint, as no caulk is immune from UV degradation and weathering.

    I'm mystified by this part. It sounded to me like he had finless windows installed into existing double hung jambs, with stops and sealant. How can he cover the sealant with the felt? Is he adding more trim over the stops, so this will all be hidden?

  17. Riversong | | #17

    "Are you saying that you don't wrap the WRB into the window and door openings?"

    Just enough (about 1") to staple in place if housewrap, and not at all if felt. My double-frame walls have CDX window boxes that are Tremco'd in place to be part of the air barrier. I want to foam the window jambs to the CDX to become part of the air barrier assembly. The last thing I would want is indoor air to leak past the window jambs and get behind the WRB that is turned into the RO so that it would cause condensation on the backside of the water-impermeable housewrap.

    With an X-cut in the housewrap and triangles of it stapled into the RO, any water that might leak near the top of a window into the RO could also run behind the WRB pennants and get trapped. If the WRB is to create a weather-proof membrane, integrated with flashings and/or sealants at the penetrations, then I want to keep that membrane in the outer plane of the house and not direct it in to any openings.

    "[Sill pans are] easy to do with a new opening. I generally set a piece of 1/2 x 6 bevel siding on the rough sill and use a flexible membrane to make the pan. I have seen new, expensive windows leak right away and would never set a window without a pan that drains out over the WRB."

    We quickly forget that we've been setting windows without sill pans for hundreds of years, and for decades since we began to insulate our houses well - and problems are still rare and generally caused by poor installation or poor maintenance.

    A flanged window, installed to AAMA standards, should not leak. If there is minor leakage, those standards require a water-open bottom flange for drainage. Wood, don't forget, can tolerate minor wetting as long as it can dry. Once we cover it with self-adhering membranes, it can no longer dry.

    "I'm mystified by this part [covering the caulk joints]. It sounded to me like he had finless windows installed into existing double hung jambs, with stops and sealant. How can he cover the sealant with the felt? Is he adding more trim over the stops, so this will all be hidden?"

    He said, "Replacing the trim will allow me to cover the caulk joint between the insert window and old frame."

  18. GBA Editor
    Martin Holladay | | #18

    Robert,
    You wrote, "A flanged window, installed to AAMA standards, should not leak." That's simply untrue.

    The simplest way to disprove it is with a garden hose. Do a little experiment next time, after the WRB and windows are installed, but before you have installed insulation or drywall. After you've seen what happens with your own eyes, you won't write sentences like that any more.

  19. David Meiland | | #19

    I'm sure I'm not the only one who has had a brand-new, high-end window leak during the first rain, and I'm talking about a leak thru the unit itself, not between the unit and any of the flashing or other installation materials. Just about all of the wood/clad units are mitered at the frame corners, with sealant in the miter, and I hate to say it, but there's no way that's going to be perfect every time. You might stand a better chance with a vinyl window, but if the "welded" corners ever separate the same type of leak will probably occur. A pan will keep the water off of the framing.

  20. Riversong | | #20

    Martin,

    You're being ridiculous.

    I'll state it again, since it IS a true statement: A flanged window, installed to AAMA standards, should not leak.

    "Should not leak".

    It's tautologically true that a defective window MAY leak, and it's also obviously true that it's the installer's responsibility to check such things as seals at mitered cladding before installation.

    What DOES leak, however, in the kind of pressure-washer test that Martin describes are the taped diagonal slits in the WRB that are required for AAMA methods A1 or B1 with WRB applied (as is now typical) before window installation. Tape simple isn't as secure or reliable as a simple shingle lap (such as occurs with felt applied after the windows). But the gasket and caulk installation method makes a virtually leak-free installation, with a drainage path at the sill flange. This should be all that is necessary for weather-resistant and durable installation.

    Of course, the use of moisture-vulnerable materials - like OSB - and impermeable, non-hytgroscopic insulations - like spray foam - decrease the durability of the installation because even small amounts of "normal" water intrusion become concentrated in the framing and sheathing and can reach high enough levels of saturation to become problematic.

    I've never had a new window leak, or any window in a renovation project leak. Careful inspection and installation makes that unlikely.

  21. Michael Chandler | | #21

    Robert, the test Martin described was a simple garden hose test, not a "pressure-washer test", and the fact is that many new, correctly installed, and non-defective windows, when tested from the outside with a low-pressure garden hose while closed and locked, will leak even though as you correctly state they "should not". After a couple of seasons a very high proportion of flanged, clad windows will leak occasionally during wind driven rain events when closed and locked. This is why we all use sill pans and both tape and foam the openings to prevent air pressure differentials from pulling moisture into the space between the window and the framing as best we can while providing a drying potential. I suspect that in practice you and Martin are very much on the same page despite minor differences in the details.

  22. Riversong | | #22

    I would demand replacement by the manufacturer for any window that leaked through the frame or weatherseals.

    Of course, any assembly can leak under sufficient momentum and wind pressure. But that's a good reason to avoid any type of sliding window. I normally spec casements, awnings and fixed units. Tilt-in double-hung windows are a foolish trade-off of weather tightness for the convenience of easy cleaning.

    we all use sill pans and both tape and foam the openings

    Is this the royal "we" or do you speak for all builders and window installers?

    There are many ways to flash and seal around a window, and the choice needs to be appropriate to the materials and methods used in the entire structure.

    It goes without saying (or should) that the more moisture-vulnerable materials used (like OSB), the more extreme measures must be taken to prevent moisture accumulation and retention. But too often those extreme measures, particularly the overuse of self-adhering bituminous and butyl tapes, are as likely to trap moisture as to keep it out. There are many cases of OSB rot where such tapes at the head flange are applied directly to the sheathing.

    I still contend that, with quality windows, careful pre-installation inspection, appropriate installation methods, and moisture-tolerant and moisture-buffering materials, sill pans are not the necessity that many would believe. If sill pans must be used, then non-adhesive molded units would be preferable to adhesive materials that don't breathe. And a sill pan is worthless without an outward bevel and side dams.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |