Rain and CLT

For anyone considering building a house out of Cross-Laminated Timber, please read this post carefully. CLT might be solid and strong but it is not rainproof. We’re finding out the hard way right now that however much you paste or tape over seams and cover cut ends and edges and knot holes, which we did after the last big rainstorm we had, the material itself will suck in water and deposit it inside the house.

We have had heavy rain over the last twenty-four hours. Right now we have water flowing down the insides of the CLT walls and dripping out the bottom, we have water being sucked along the CLT ceilings and dripping from saw kerfs, we have water getting in at the edges of windows. I emphasize that this is all despite extensive attempts to stop this from happening, with all the cut ends of the CLT taped up and the roof covered with layers of polythene (as a temporary measure) – and you can’t cover the whole thing in poly. The roof is okay as a result, but but the water just comes in through the walls. And comes out everywhere.

The worst thing about this is that a lot of the water won’t come out immediately. As the temperatures drop, a fair amount is likely to freeze in there and won’t come out until the spring. This means that we can’t really do a lot of the interior work we had planned this winter even if we get the roof and walls completed.

So, here’s what you should do:

  1. Don’t start building a house in autumn in a climate like Ontario’s, even if the experts tell you it will be up quickly and it will be fine. Lots of things will cause delays.
  2. Plan to wrap the entire CLT assembly in a water and air-tight but water-vapour permeable membrane immediately after it’s been put together. You could use something like eco-friendly like SIGA Majvest, or even Dupont Tyvek Homewrap, all sealed with the appropriate tapes at joins and around windows and doors. This will protect the assembly from rain and wind during construction, and it certainly won’t do any harm to the airtightness and water-proofing after the building is complete to have two vapour-permeable membranes.

We should have done this. We probably still will, even at this stage. With an new El Niño phase just starting, it’s likely to be a warmer, wetter winter than the last few.

Everything did seem to be going rather too well, didn’t it? Now, please excuse me while I go to empty a few more buckets and collects more wet towels to dry out…




6 thoughts on “Rain and CLT

  1. Carolyn

    So sorry to hear about the water leakage into your house and the subsequent delays – sending you all my best wishes for a drier rest of the winter and for your house building plans to continue as hoped x


  2. M&D

    A great shame about the rain
    Let’s hope it doesn’t get wetter
    and the weather gets a little better
    so that plans can commence again

    After Wordsworth we don’t think!
    Seriously though, we do hope things improve for you


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  4. ruxtonmutt

    Why did you end up going with Tyvek instead of the Majvest? I was thinking of using tyvek instead of building paper for a better air barrier but one of the posts about ceder siding and surfactants dissolving the tyvek someone installed a decade ago was pushing me away from tyvek.


    1. David Post author

      It’s what we could get locally at short notice. Given a bigger budget and more time, we would have gone with the Majvest. If this interior membrane were to degrade over time, it’s no big deal, it’s really mainly there to protect during construction.


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