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Community and Q&A

What’s wrong with this wall?

bdrfab | Posted in Plans Review on

From inside out…
1/2″ Drywall
2×4 framing
Diagonal T brace or let in 2x bracing
2″ XPS caulked and taped (doubles as WRB)
Horizontal Furring strips
Extira Siding

2×4 framing w/ 2″XPS makes window bucks easy. Extira (exterior grade MDF) is the probable siding choice as the desired look is approx. 2’x4′ panels, and smooth texture is preferred. Running the furring strips horizontally allows the panels to be fastened in a location the makes sense for the panels, as well as allowing some customization of panel size. What am I missing/overlooking

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  1. wjrobinson | | #1

    Walls are just one part of a home. For me I would at this point aim for a much higher R wall, but your wall is fine IMO.

  2. Foamer | | #2

    I would offset the furring from the foam to allow back-venting of the MDF panels.

  3. bdrfab | | #3

    AJ, if it was up to me, I'd like higher R as well, budget is what budget is. Hopefully I can at least talk them into 2x6 walls.

    Torsten, I'm a little confused, what do you bean by offsetting the furring strips? The furring strips (probably would kerf the back) would act as a rain screen and allow ventilation. That should be sufficient, correct?

  4. Billy | | #4

    The horizontal furring strips are the problem if you you don't provide for (1) sufficient drainage for the rain screen (this will be blocked by the horizontal strips), and (2) sufficient vertical airflow behind the siding (this also will be blocked by the horizontal furring strips.

    I doubt you can cut enough kerfs to provide for adequate drainage AND airflow.

    I have three alternate suggestions, although there may be others.

    1. Can you run your furring strips vertically? This would be best.

    2. If you must run the strips horizontally can you space them about 3/8 inch out from the XPS foam?

    3. Use Cor-A-Vent SV-3 strips horizontally --> see

    Whatever you do make sure you detail the rain screen properly at the bottom of the wall to allow drainage and airflow and at the top of the wall to allow airflow, and use bug screen.


  5. GBA Editor
    Martin Holladay | | #5

    1. I think your horizontal furring strips will work fine. Of course you won't get vertical ventilation channels, but your siding will still dry out after a rainstorm -- it just might take a few hours longer to dry. Not a big deal.

    2. I prefer polyiso to XPS, because it is more environmentally friendly.

    3. When you use rigid foam for your WRB, flashing details for the windows, doors, and penetrations have to be carefully thought out and executed.

  6. Billy | | #6


    How will bulk water drain with horizontal furring strips interrupting the drainage plane?

    Without vertical ventilation channels how will the siding dry out? Does the water vapor just pass through the cladding material as with a non-rainscreen wall? If so, this design doesn't take full advantage of rain screen design.



  7. GBA Editor
    Martin Holladay | | #7

    You're right -- if you install horizontal furring strips, you aren't taking full advantage of rainscreen design. I know that.

    I also know that for decades, siding was installed tight to the sheathing, and most such walls performed adequately. Installing an air space will be a huge improvement.

    The amount of bulk water that gets past siding is small. It dries out by evaporation (diffusion). This drying is accelerated by sunlight and by daily changes in temperature. These temperature changes have the effect of a pumping action that aids air exchange in the air space behind the siding.

    The siding on my own house was installed on horizontal furring strips 30 years ago. Every time I have had an occasion to open up a wall, everything is doing just fine.

  8. bdrfab | | #8

    Thank you for your thoughts. With exposed fasteners holding the panels to the wall, horizontal flashing really is better, and cor-a-vent wouldn't provide enough holding power. Thank you for sharing your expierence Martin, that encourages me. I wasn't sure if the siding being in panels would allow enough airflow for adequate drying. I assume your comment about flashing details was a subtle hint to get off my butt and pony up for a Pro Membership? ;-)

  9. Billy | | #9


    Despite what Martin says I think you'll be a whole lot better off if you can go vertical OR space the horizontal furring strips off the foam sheathing by even 1/4 inch. I don't know what type of siding Martin has but I suspect it is much more vapor permeable than Extira which is a resin molded MDF -- there's no way you're going to get much if any vapor diffusion through that material. Plus, comparing siding installed flush to wood sheathing (presumably separated by tarpaper or not) is a LOT different from the situation where the sheathing is XPS or polyiso foam. The vapor profiles of these wall systems are entirely different! I don't think it's an apples to apples comparison, especially if you are creating a sandwich between two vapor barriers (XPS or polyiso foam and the Exira panels) without good airflow in between them.

    I have a bigger question about your approach which is the use of Extira as siding on this house. After looking at information on the Extira website the manufacturer recommends it for signs and trimwork, but nowhere do they recommend it as siding. From the website:

    "Extira panels are specifically designed and manufactured for exterior use, unlike MDF. Extira panels also offer high performance in high-moisture, interior environments. Use instead of wood for non-structural, paint-grade applications such as:
    • Column Wraps • Shutters • Planters • Brick Mould
    • Exterior Signage • Sub-Floor
    • Marine Applications • Carriage Style Garage Doors • Door and Window Par ts • Decorative Mouldings • Brackets"

    I also note that they provide only a 5 year warranty and they have all kinds of warnings about painting and not letting your siding come in contact with standing water.

    I suggest you speak with the manufacturer directly about your application. It might be fine but I'm not sure it's a good idea.

    Wouldn't you be better off with Hardi cement board panels? It is more rot resistant and it allows vapor diffusion.

    Also, how do you plan to finish the seams in the Extira panels? Will the seams be exposed? Will you put battens over the seams? Manufacturers typically recommend Z-flashing on large panels used as exterior siding. If you have horizontal trim boards covering the seams make sure the top edges of the trim boards are sloped to allow rain to run off.

    Let us know what the Extira manufacturer says after you speak with them.


  10. wjrobinson | | #10

    William, I agree with Martin, horizontal works and may even be better in some ways. The gap is mainly a capillary break and pressure equalization plane more than a vertical-large-amounts-of-actual-running-water space. Also vertical spaces open top and bottom could be worse in a fire and they if large could be reducing the R value of the wall assembly.

  11. GBA Editor
    Martin Holladay | | #11

    Your points about the vapor permeability of Extira and its suitability as siding are well taken, and I appreciate the fact that you looked into Extira (which I didn't). Thanks for sharing the information.

    Aaron -- I think that you need to research the points raised by William and should perhaps consider a more conventional siding material.

  12. bdrfab | | #12

    I'll give them a shout, but I guess I fail to see the difference between using Extira in a signage application and a siding application. There won't be battens, its more of a reverse board and batten. Any joints would be rabbited/overlapped, with a bead of caulk on horizontal joints. Our problem with fiber cement is that unless you have better suppliers than us, smooth panels aren't smooth. That's the appeal of extira. We've used it as soffit material before, using a CNC to cut the shapes and then pre-painting before installing it. For horizontal grooves, I was thinking it would be , cut with a dish carving router bit, which has a radius on it so water can't sit on it. To be perfectly honest, I'm not crazy in love with this system either, but am struggling with a more cost effective way of getting the desired look. I'll try to find a picture to post for alternate suggestions.

  13. bdrfab | | #13

    Hopefully this is a little clearer. Proportions would be closer to 2'x4' panels

  14. wjrobinson | | #14

    Aaron, I think your build will be fine. The roof stops the rain number one. The water stop for your wall will work and is behind your siding. The siding will also stop water but mostly it protects the layers behind it from UV as it should in your case.

    I say build it, blog it, and post pics.

  15. doug_horgan | | #15

    Similar MDF trim products sold ten or fifteen years ago are falling apart, on projects where we used them back then.
    I would be very wary of this type of material unless it sees very little water.

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