Are holes in exterior plywood sheathing necessary?
Dreamhouse67
| Posted in Building Code Questions on
We are building a new home on the west coast of BC. Our framer has placed large diameter holes in the exterior plywood sheathing saying this is necessary to meet code and allow the exterior to breathe. Our concern is that when other protective layers placed over the sheathing break down over time (plastic and tape and glue only last so long) we have a welcome entry point for water, rodents, carpenter ants ect. How does this practice fit into an ‘airtight’ home??
We are not builders but can’t find anywhere noted that these holes are necessary. Thoughts, advice or comments appreciated.
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Replies
Frances,
I'll answer this as a mea-culpa to Martin, who some time ago patiently explained why my insistence on it being a good idea was incorrect.
The holes in exterior sheathing came as a reaction to the building failures of the BC Condo Crisis, where widespread moisture problems in walls occurred. The idea was it would give the wall an path to dry to the outside, but walls dry by diffusion through the sheathing, and the area of the holes were far to small to affect the rate of drying, while compromising the air sealing qualities of the wall assembly. The practice has been discontinued, I haven't seen anyone drilling holes in their sheathing in BC for years and there is nothing in the BC building code that requires it.
Thanks for the response!!
I should add that the holes are fist sized in places and on the weekend my husband was so frustrated that he filled them. Everyone is up in arms saying they will need to punch them out to pass sheathing inspection and we could run into major moisture problems without the holes.
Maybe they should build it right (with a rainscreen under the cladding) instead of deliberately building it wrong…
Francis,
Your local building inspector may also be working off old assumptions and insisting on the holes, however, as I said, there is nothing in the building code requiring them.
Even fist sized holes simply don't make any appreciable difference in the drying that will occur. Unlike air barriers, holes in vapour barriers (or vapour retarders like sheathing) work as a percentage of the area of the wall that is open. So even the large holes your builder has drilled probably only consist of at best one percent of the wall surface. So one percent of your wall is drying at the increased rate of the building paper covering it and the rest is drying at the rate the sheathing allows.
In the rainy coastal BC climate it's fairly important your builder comes to understand the mechanisms that can cause moisture problems in walls. I hope he uses the discussion you are having about your house as an opportunity to do that.
Frances,
Drilling holes in your wall sheathing encourages air flow through the wall assembly, and certainly undermines the thermal performance of your wall. Building scientists agree that the best way to limit heat flow through your building assemblies is to reduce air leakage. The more airtight your sheathing layer, the better your wall will perform. So: no holes.
If you want to encourage rapid drying of your wall sheathing -- and it's a good idea to do that -- you should include a ventilated rainscreen gap between the back of your siding and your WRB. That approach is science-based, and makes much more sense than drilling holes in your wall.
Here is a link to the previous Q&A thread on this topic -- the one that Malcolm Taylor was referring to: Am thinking of the following wall and roof assembly in BC as a cost-effective way to achieve a high R-value and tight envelope.
Martin,
No that's not the one - i still didn't get it until a year or so later then it came up in another discussion and you laid it out again. Unfortunately i can't find that thread.
Malcolm,
Maybe you're right -- but this dialog (the one that I linked to) seems relevant. It's a blast from the past -- from the early days of GBA.
Martin: "The practice you suggest -- drilling holes in exterior sheathing -- is fairly common in British Columbia. It started as a fad among builders who were distressed at the massive failures that became known as the Leaky Condo Crisis. However, this method has no basis in building science. It was simply a panicked reaction by builders who were tearing out their hair because of the wet-wall disasters they were seeing."
Malcolm: "The practice may have originated with builders, but it was backed by research into wall cavity drying by the CMHC, and although not specifically addressed in the code, was recommended by both building inspection officials and government publications such as the BC Building Envelope Guide for Houses. ... This is recognized in the wall details for Pacific Northwest construction put out by BuildingScience too. You would do well to re-think your advice on this one."
Martin: "You are wrong about the advice given by Building Science Corporation for wall assembly design in the Pacific Northwest. Here is a link to the relevant paper: High-R Walls for the Pacific Northwest."
Malcolm: "Their practice isn't to randomly drill holes in your sheathing, but to increase the ability of the wall cavity to dry to the exterior in wall assemblies designed to dry that way. This is done by leaving a small gap between sheets when applied horizontally and drilling holes in the stud spaces that are fully enclosed, such as under windows."
Martin: "I can't find any evidence in the paper you linked to that Joe Lstiburek or John Straube has ever recommended that builders drill holes in their wall sheathing."
Malcolm: "Got to go now and drill some holes in my sheathing."
John Straube: "I am with Martin: drilling holes in your sheathing is nutty."
Yeah well - it's not much fun remembering i got called nutty by both you and John Straub in one discussion :)
Malcolm,
The GBA community is grateful that you gave us a second chance and stuck around. Our conversations would be more impoverished without you. So we're glad you stayed.
Thanks to all of you. I did some research last night as well....is there anything out there that supports this process that the inspector is going to pull out of his hat or am I going to need to hire a building envelope specialist to deal with this??? Are you aware of any research that shows what happened as a result of the VPD reactionary craze? This house has been designed for us to live in into old age and I want it to last as long as the one we tore down!!!
Frances,
The Building Science Corp. report I linked to (High-R Walls for the Pacific Northwest) is a start.
You might also refer to the old thread on GBA -- especially since it includes a comment by John Straube, one of Canada's most respected building scientists.
You might even refer to this thread, since it includes statements pointing out that "The practice has been discontinued, I haven't seen anyone drilling holes in their sheathing in BC for years and there is nothing in the BC building code that requires it."
Frances,
The 2012 BC Building Code (which came into effect in 2014) has extensive requirements for both the air tightness and use of vapour permeable materials at 9.25.3 and 9.25.5 along with commentary on the reasoning for them in the appendix. No where does it recommend or mandate any gaps in the exterior sheathing except those left for expansion of the sheets.
What happened as a "result of the VPD reactionary craze" was a lot of research into stopping the problems reoccurring, and the results being incorporated into the new code. If your builder follows the requirements of the 2012 code you will end up with a well-built, resilient house you don't need to worry about.
Malcolm ... Is there anywhere in the code that says the holes are not an acceptable practice or is it just not stated that it must be done ? I couldn't find anything specific to vpd's but I have not read it completely.
Also, the mjority of the holes have been sealed by my other half with patches and a tar like compound so not sure if that will cause an issue??
Finally, would you consult on the project, lol???
Frances,
No there is no interdiction against the holes. The code allows situations where there is no sheathing used at all.
Given where you are now, with a bunch of already cut holes in your walls, and that you are probably including poly on the interior, so the air sealing of the sheathing isn't as important as it might be, I wouldn't be too worried by the holes. A well-sealed sheet WRB like Tyvek will help. Hopefully your cladding will keep out pests, so while not ideal, I don't see whether your husband's repairs get to stay or go as a huge problem.
I think they still require it in the District of North Vancouver. Last year, I saw a large condo unit with holes drilled every where.
I think that you should seal the holes in an airtight manner. Otherwise you will have excessive air flow through your wall assembly, increasing your energy bills.