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Q&A Spotlight

Attic Ventilation for a Revamped Timberframe

Will these plans for a vented rainscreen include enough ventilation for the attic?

This timber frame home in New Hampshire is getting new siding over a vented rainscreen. The homeowner wonders whether his plans include an adequate amount of ventilation for the attic. Photo courtesy Jonathan Sinclair.

Jonathan Sinclair’s home, a 225-year-old timber frame in New Hampshire, is getting a facelift. Sinclair is residing the building with fiber-cement lap siding, and he plans to install the siding over a vented rainscreen—a thoroughly modern detail.

“My question comes from building an adequate soffit vent that will flow air,” Sinclair writes in this recent Q&A post.

As he explains, the original builder included 2×6 soffits with a 2 1/2-inch-wide gap between the building and the edge of the soffit. That gap was then covered with molding. Sinclair plans to incorporate this gap into the top of his rainscreen so that air will have a continuous path from the bottom of the house to the attic. Ridgevent 20 (manufactured by Air Vent Inc.) at the top and bottom of the rainscreen will keep out the bugs.

“Does anyone have an opinion if this will allow adequate air flow to act as a soffit vent or will I need to drill holes up through the soffit beyond the crown molding through the 2×6 to give adequate soffit venting,” Sinclair asks. The house has no ridge vents and but it does have gable-end vents at the peak.

Venting the rainscreen into the attic is OK

GBA editor Brian Pontolilo replies that it’s generally fine to vent a rainscreen into an attic, although Sinclair’s building inspector may have other ideas. If the inspector is okay with it, there is still the question of whether it provides an adequate amount of intake area for effective roof venting.

The International Residential Code (IRC) does include some guidance on how to calculate what’s called the “net-free ventilating area” for a roof, which you can read more about here. Pontolilo also suggests an earlier GBA article on the topic (see the first entry in the “Related Content” sidebar below as well as this article from Fine Homebuilding magazine).

Sinclair replies that his concerns stem from the age of the house, and the probability that warm, moist air from inside the house will leak into the attic despite his best efforts to air-seal and insulate.

“If it is your belief that the amount of moisture will be very minimal that is added to the attic through the rain screen venting into the soffit, than I will just not worry about the additional small amount of moisture,” he writes.

The 3/4-inch-wide gap at the top of the rainscreen will add up to a net-free area of 2 1/2 square feet over the 40-foot length of the house, well shy of the 4 square feet he should have for an attic of that size. With a like amount from the front of the house, he’d get 5 square feet of vent area at the soffit. He also says he would consider the option of using a gable-vent fan or opening windows in the gable end to increase air flow.

A soffit vent may be needed

Malcolm Taylor notes that as far as he knows, only British Columbia does not permit a rainscreen to be vented directly into the attic.

“The argument in favour of doing so is to increase the air-movement in the wall cavity,” Taylor says, “but especially with two story walls like your house has, the amount of air being supplied will be well below that necessary to vent the roof space.

“Given that you are ideally trying to situate at least 50% (or better still 60%) of the required roof ventilation at the eaves rather than the ridge,” Taylor continues, “I don’t see any good way around supplying some form of soffit vent.”

At the top of the wall, there’s a gap between the 2×6 soffit and the wall.

Roof ventilation isn’t as much of an issue in new houses that are well air-sealed, but in older homes roof ventilation provides an important layer of protection. Taylor says his hunch is that the benefits of increased air flow from connecting wall cavities with the attic would be outweighed by the potential for adding more moisture.

Plus, Taylor says, using an exhaust fan in the attic can depressurize the attic, which would pull more moist air from inside the house.

The reason that the air channel in the rainscreen won’t provide a lot of ventilation air is because of friction losses, Taylor adds.

“The reason Joe Lstiburek cites for connected rainscreens not being a problem for flame-spread is that the friction losses in such a small cavity mean there is very little air-movement,” Taylor says, “and that friction increases with the length of the wall cavity—especially as many are not going to be continuous from foundation to roof because of windows and lower roofs. I don’t think you can rely on the total open area of the wall cavities supplying the same amount of air as you would get with vents the same size at the soffits.”

Rainscreens are a capillary break

Akos suggests that the real benefit of a rainscreen is as a capillary break between the siding and the water-resistive barrier.

“Rainscreens are about as effective with only the bottom vented and top sealed up” as they are when vented both at the bottom and the top, Akos says. “This is a very common detail.”

He would cap the siding slightly below the soffit and cover the gap with a piece of perforated flashing to serve as the eaves vent. If the gable vents are the right size, Sinclair might not need soffit vents, Akos adds, but he should check before “going crazy on this detail.”

Taylor endorses this approach. Connecting the rainscreen gap directly with the attic might work, he says, adding, “but rather than chance it, I would do as Akos suggested and block the top of the cavity close to the soffit, allowing the ventilation air for the attic to be introduced from behind a new frieze board instead.”

New plan of attack

While GBA readers are offering ideas, Sinclair has gone back to work.

“I decided to pull down the old 2×6 solid soffit board,” he writes. “I then ran a level line with the bottom of the existing fascia boards on the house itself, attached a ledger on that established line out of a 2×4 (provides the hard stop for any air flowing up the rain screen), and then attached 2×4 blocking from the ledger to the outer fascia.

“This allows me to now install 2-inch-wide louvered soffit vent for the whole length of the house,” Sinclair continues, “which will get me more than enough of required soffit vent and terminate my rain screen into a vent below the frieze board.”

Ridge vent material at the frieze board will keep insects and rodents out. Sinclair plans to use 5/4 material for the frieze board, with a rabbet at the bottom to drop over the top of the siding.

Consider adding exterior insulation

Bob Irving, a fellow New Hampshire resident, tells Sinclair that it’s distressing to see the value of homes like his decrease because they are uncomfortable and hard to heat.

The solution? A layer of exterior foam insulation, which is possibly only when redoing the siding and roofing. It could make a huge difference in how comfortable the house will be.

If Sinclair’s house has rafters and horizontal purlins, which is common in older colonial era homes, it will be impossible to vent from the inside of the roof, Irving says.

“Assuming it’s a continuous rafter roof, I’d suggest a separate soffit vent for the roof, which could be brought in through a screened gap between two of soffit boards which will be less visible,” he writes. “Provide a 1-1/2-inch to 2-inch interior space for the vent, and air seal the attic side of the channel.

“If you have a rafter and purlin roof,” Irving continues, “the only good way to vent it is to add a vent channel above the roof (again air sealed to the inside), at which point it makes sense to add exterior foam under the vent channel. (and close off the existing vent space). Both wall and roof foam can be pretty well ‘hidden’ so they aren’t very noticeable from the exterior, especially if you can adjust the roof trim at the same time.”

Our expert weighs in

Peter Yost added these thoughts:

This is a great exchange here driven by Jonathan Sinclair’s original post. I am concerned that there may be too much focus on venting and its contribution to moisture management in place of air sealing. You can’t vent your way out of many if not most air leakage-driven moisture problems.

I have two Climate Zone 6 timber frame projects with moisture problems related to wintertime condensation driven by air leakage. One is a new home blower-door tested at 1.2ACH50, the other built in early 1980s, blower door tested at 5.5ACH50.

In both cases, it is not just the overall air leakage that is the main issue; it’s where the majority of air leakage is taking place. And that is in the cathedral roof assembly. In the winter, the interior warm and often moist air is leaking through well-defined pathways through the roof assembly, condensing, and then running back down into the building interior.

The roof on the new home is vented soffit-to-ridge; the older home has an unvented roof. In both cases I am way more concerned about the roof assembly air leakage than whether or not the roof system is vented.

So, I was surprised to see the excellent and extensive discussion lack any mention of blower door testing. Particularly in older buildings (where the pathways are particularly complex and difficult to identify) and particularly if you have not done much air sealing of older buildings, you are going to need the relentless guidance that blower door testing provides. And when you test, spend as much time and worry on the location and extent of air leaks as the final number.

You may know that I am pretty skeptical of how much we really know about what drives soffit-to-ridge air flow (for more, see Wingnut Testing of Soffit-to-Ridge Venting). If you can, test the air flow on your own building. I have found that roof pitch seems to be a big driver of roof vent flow, as well as how much solar energy your roof sees.

Having said that, I have always thought that connecting wall and roof venting should work just fine, but now you have me thinking: break out the theatrical fogger and let’s test it!

And in terms of a rainscreen open at the bottom and not the top: I have heard more than once that closing off the top means that (more buoyant) warm moist air collects at the top of the wall assembly and results in paint peeling in the top couple of courses of horizontal siding, but I have not actually seen this myself. And yet another testing configuration to consider…

I do agree that the free drainage resulting from creating a space between any cladding and the rest of the wall is the primary benefit and how much air flow we get in that same space is a terrific secondary gain for moisture management.


Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. Peter Yost, our expert, is GBA’s technical director and founder of a consulting company in Brattleboro, Vt. called Building-Wright.

Editor’s note: GBA members’ quotes may be lightly edited for spelling, style, grammar, and/or clarity as we strive to make these articles as useful as possible. For their unedited comments, see the original post: “Soffit venting antique timber frame house.”

20 Comments

  1. Wooba Goobaa | | #1

    Interesting. A moisture problem in a new home testing at 1.2ACH50. My noob expectation would be 1.2 ACH50 ... we're good to go, no worries here.

    1. Malcolm Taylor | | #2

      Yes, me too. I guess it's good reminder that tight houses can still have large enough localized air-leakage to cause problems.

  2. Jonathan Sinclair | | #3

    I would be happy to provide Scott Gibson with additional pictures of the rainscreen details and soffit system that I constructed if he could contact me. I have attempted to find his email or contact info through google searches and reaching out to Fine Home Building but I haven’t gotten a response.

  3. User avater GBA Editor
    Scott Gibson | | #4

    Here's my email.
    Jonathan—You can reach me at scottgibson51 (at) gmail (dot) com.
    Scott

  4. User avater
    Jon R | | #5

    > closing off the top means that (more buoyant) warm moist air collects at the top

    Don't block off the top where reservoir cladding and solar drive pushes moisture into the cavity. And keep paint perms > 10.

    https://www.buildingscience.com/documents/building-science-insights/bsi-061-inward-drive-outward-drying

    1. Jonathan Sinclair | | #6

      I appreciate the response. From what I am understanding about the perm rating, you feel as though the perm rating of tyvek at 50-60 is to high for this application?

      1. Malcolm Taylor | | #11

        Jonathan,

        The problems with inward solar vapour drive depend on the amount of water that can be held by the cladding. Painted siding would not be considered by most people to be a reservoir cladding with worrying about.

        1. Jonathan Sinclair | | #14

          Thank you for the insight!

        2. User avater
          Jon R | | #15

          Joe L says "So what is a reservoir cladding? ... Fiber cement? Yes, unless you paint it or coat it on all six sides…and even then. "

          I'd like to see numbers, but in any case, I'd go with the expert and use lower perms (like Typar) where there are high perms all the way to a cool surface (which may not apply in this case). Fully adhered moderate perm WRB if needed to stop airflow.

          1. Jonathan Sinclair | | #16

            The fiber cement siding is primed in all 6 sides. So atleast it should abosorb very little in my mind.

          2. Trevor Lambert | | #17

            The Hardie Panel I installed (and trim and battens) was only painted on five sides (not the back).

          3. Jonathan Sinclair | | #18

            Trevor, interesting point you bring up. I always was under the impression from hardi that the back was also primed since the color is more or less the same but it is rough in texture so I thought that was the reason for the color inconsistencies. Turns out that there is some primer on the back from the rollers at the factory but it is not straight coated like the other 5 sides. I did some more research and hardi claims you don’t need to prime the back or top cost it because of their primer which is mixed in with the mix? Interesting subject.

  5. Jonathan Sinclair | | #7

    I have almost completed the project at this point. The only things I have left are a second coat of Sherwin Williams Emerald and the top facia shadow board which will go on after the pump jack brackets come off. Below are pictures of the rain screen strapping, pre assembled pvc window trim and ledger board with solid blocking out to existing facia boards.

  6. Jonathan Sinclair | | #8

    Additional pictures of the soffit framing, bug screen to keep insects out of the attic and the top nailer for the frieze board.

  7. Jonathan Sinclair | | #9

    Top solid nailer, Air Vent brand ridge vent. It into 2 inch strips and nailed in between strapping for vent/bug screen, frieze board notched to drop over the top of the top of the siding and allow an air path.

  8. Jonathan Sinclair | | #10

    Frieze board across the top nailer and notched for air vent, solid nailer attached for crown molding backer.

    1. Malcolm Taylor | | #13

      Nice job!

      1. Jonathan Sinclair | | #19

        Thank you sir.

  9. Jonathan Sinclair | | #12

    Crown molding up( I would have gone with 4 1/2 inch crown for more air flow but 5 1/2 matched the original crown), first coat of paint.

  10. Jonathan Sinclair | | #20

    The project is complete with two coats of Sherwin Williams Emerald. The last piece is the top shadow board for the facia that needs to be scribed and cut to slide up into place.

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