Best practice recommendations for this vaulted ceiling?

Hello energy pros,

Some relatives of mine recently purchased this 1917 house in Northern California and, as the family member who does "that energy stuff" in Southern California (I'm on the program side), my involvement was solicited.

I can't think of a better candidate for a home performance case study. It's got plenty of charm, good bones, knob and tube wiring, an ancient gas room heater, and zero insulation whatsoever. They are millennials, small business owners, enthusiastic about sustainable living, and the home is in a well-traveled urban location experiencing a rapid renewal. So the potential for it to raise the profile of comprehensive, design-based home upgrades is very good. Also, since major structural upgrades are required by the city before anything else can happen (they have to beef up the ceiling joists and rafters and support the extra weight with three columns from the ceiling to the foundation) and a lot of their time right now is devoted to a new baby, they don't anticipate doing envelope or mechanical upgrades earlier than this fall. I like this because it will give them a solid year of living in the house "as-is" before improvements, and me plenty of time to consider how to best approach their problems... and properly implement the solution. One difference that stood out to me immediately from the typical house: it will be occupied nearly round-the-clock, underlining the importance of comfort and operational affordability.

They plan to convert the basement to a commercial cheese kitchen (they've joked about paying me in truckloads of mozzarella, a proposition I've seriously considered) and the currently unfinished attic to office space under a vaulted ceiling. Right now, the rafters are 2x4s 32" o.c., which the city is requiring be upgraded to 2x8 16" o.c. The roof is supported by 1x4 skip sheathing. There is no eave venting.

My thought is to fit pieces of 1" foil-faced polyiso between the rafters flush against the skip sheathing from ridge to eaves. The width the polyiso is cut to will vary from one rafter bay to the next, as the old 2x4 rafters will remain in place among the new 2x8s. Then layer in batts, and close it off with another polyiso layer beneath the drywall to make a polyiso-batt "sandwich" and eliminate thermal bridging through the rafters. See the cross-sectional diagram below.

Question 1: Is FF polyiso a sufficiently weather-resistive barrier for this application, under the skip sheathing? What's the worst-case scenario that could arise from a moisture standpoint?

Question 2: We all know a radiant barrier needs a gap between it and a surface to function correctly. Is the 50% or so of the area where a 1" air space will exist between the FF and the shingles (the black chunks in the diagram, representing space between 1x4s) significant in terms of an overall radiant barrier effect?

Question 3: The blue line in the diagram represents Knauf EcoSeal applied around the edges of the polyiso to seal it to the rafters. I personally haven't used the stuff before but the contractors I've talked to who have are impressed. I will test this with a blower door before any drywall goes on. Are there any caveats I should be aware of using it in this application?

This is California Climate Zone 12, if that helps. Thanks in advance!

Griffin

Tags: attic, barrier, batt, cathedral, ceiling, ecoseal, foam, insulation, moisture, polyiso, More…radiant, rigid, sheathing, skip, vaulted

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Hi Griffin,

You definitely have a project there and as most undertakings like this go, it will cost twice whatever you estimate.  Understanding that you may have already covered these bases I will still list some thoughts for you.

Being almost 100 years old the house lacks everything we want to see in new construction.  From ground up there is no moisture treatment and from the picture there looks to be limited landscaping potential to deal with water from the outside.

Converting an old house to a food preparation facility will have a whole list of concerns all by itself.

Be sure to include an egress from both attic and basement.

Installing a 1" layer of foil faced poly between the rafters will not provide a wrb as the rafters will still be exposed and you will be unable to seal from above.  And polyisocyanurate does not like water.

To reduce the risk of getting half way into this and discovering all of the extra expenses, I would suggest getting an engineer in to design the final product from top to bottom.  Yes, an added up front cost, but the resulting design will get you through the permit process and construction with fewer complications.

I apologies for repeating steps you are already aware of, but I'm a long ways away so just guessing.

Good luck,

Bud

Hi Bud,

Yep, there's a little bit (or a lot) of everything going on with this one. I can't really offer the kind of hands-on involvement I would want to feel comfortable with significant responsibility for the outcome (they're an eight-hour drive north of me), but being that it's for family, and has opportunity for comprehensive improvement written all over it, I thought I would share with them a thing or two about what I think I know, and hopefully positively influence the end result.

I should add that they are currently working with an engineer and architect and, as far as I know, haven't finalized the design. This, along with the permitting and structural upgrade process, will probably take them the next several months. 

I'm not sure what their appetite for a re-roof is, but if I'm understanding correctly, it sounds like there's really no way to achieve a wrb without it. 

Thanks again for your input.

I can't really tell from the picture, but it would be a shame to go through all of this with a 30 year old roof up there.  Especially with the benefits that go along with changing it now.

As the old saying goes, you work from the foundation up.  If that can be made solid and dry, the rest is just money.  It is hard (actually impossible) to tell an architect or engineer what to do so advice will probably be limited to functionality as opposed to performance.

Sorry I can't help more.

Bud

Bud, I just talked to her, and the roof is about 15-20 years old now. So that's good news. I'm now heavily leaning towards recommending they replace the roof considering what's at stake. Like you said, it would be a shame to miss the opportunity.

Griffin,

That's a gorgeous attic! To my eye, the bungalow is a beautify style that can accommodate high performance discretely.  The only aspect of this building that looks less solid (visually) and a little inconsistent is the roof edge. 

I'm more in the "wait 'til you have the money to do it right" camp than the "do what we can afford to do now" camp.  Buildings are long-term assets, and in the big picture, "ownership" is more like a brief hand-off from one owner to the next.  Decisions implemented by current owners will affect people on down the line.  It seems like this is a building worth care.  But that said, it is also important that the current owners get the most enjoyment that they can from the building. 

Is the architect clever?  Is the engineer creative?  Will the contractor be ambitious and conscientious?

It looks to me like that front dormer might be over-framed. And you say there is some structural work going on.  Would it be possible to put in a structural ridge?   (The engineer might have to be creative to make that work without messing up windows.)  That might make it possible to do a SIP roof.   I'd hate to see them loose head room in the attic.  What about over-framing or framing the 2x8's from the outside.  The existing framing could be a service cavity (or left open).  Or the bottom of the new rafters might just be flush with the existing rafters and everything covered on the inside.  A conscientious and ambitious builder should be able to carefully remove the dormer, brackets and rafter tails.  A clever architect should be able to figure out how to remount these in a way that looks natural and hides the thicker structure.

Lots of ways the roof could be done.  This bungalow looks like it has simple enough roof planes that it could be vented (except around the dormer).  Unvented would give a thinner roof.  Don't do foil-faced polyiso on both sides.  

The potential here is exciting!  It's also nice if they can live in it for a while before making major changes.

Good luck!

Ken

Ken, I couldn't agree more; bungalows, Craftsmen, and American Foursquares are some of my favorite homes to work on (I'm a little jealous as I couldn't dream of finding a house like this in San Diego at the price they paid). 

I'm with you in the wait-until-you-can-go-comprehensive camp, with the exception being that carefully staged, logically ordered work that doesn't interfere with the enjoyment of the home or preclude better performance down the line can work if done right. In their case, I think the structural and electrical upgrades, plus any foundation work (I'm including sealing and insulating the crawlspace in this) make sense to do now, and then work upwards, per Bud's advice. 

I don't have previous experience working with architects and engineers, but I'm hoping theirs at least have open minds.

What do you mean by the dormer being over-framed? Are you referring to the rafters that extend past the opening? I think I understand the idea of a structural ridge, at least as a necessity for a SIP roof, but how would that interfere with windows?

Sorry for the barrage of questions, this is just outside my arena of experience. 

I'm not sure what I'm missing, perhaps it's that I work exclusively in the multifamily sector, but couldn't you just apply a two-part high density foam insulation and be done with it? The high R-value of the spray foam will more than make up for any benefit lost by eliminating the radiant barrier.  And the added expense of using spray foam is offset by the time and labor it cost to cut and fit the polyiso in and around the structure. 

Jaime,

Thanks for your reply. That certainly seems to be the simplest, quickest, and nominally highest R-value solution, and one they did ask me about. However, I'm hesitant about spraying foam on skip sheathing for a couple reasons: it seems like there's high potential for performance-compromising imperfections due to the more complicated geometry, and it seemingly leaves the door open for bulk water issues in an enclosed assembly if the roof becomes damaged. I'm not sure how polyurethane handles water in those instances, but I don't want them to find out the hard way.  

Maybe if they had a standard plywood roof deck with tar paper it would be feasible, but as Bud pointed out, in this case the only option to get a true weather-resistive barrier to protect the rafters is a reroof. Fortunately, I just asked them about its age and they told me 15-20 years, so this option might be more palatable for them.

  That's some nice looking existiing wood, and not a lot of headroom;  maybe they should consider tearing off the old roof, adding some stained plywood or other, then re-rafter above the old roof with 2 x ?.  You could then dense pack/batt the resulting cavity, deck it with an inch or so of rigid (R-5 in SF Bay area), and put a new roof on over that.

Voila!   A really well insulated, thermal bridge free assembly that is condensation safe!

Charles Bennett

Mr Insulation

San Rafael, CA

Hi Charles. Yes, I think I'm going to recommend they tear off the old roof and talk with the engineer and architect about building up/out. They have so much to gain (more headroom and thermal bridge elimination, as you pointed out, as well as peace of mind about moisture problems), and since the roof is 15-20 years old anyway it's easier to make the case for it.

What do you mean by stained plywood? And when you say R-5 of rigid, are you referring to XPS board?

Thanks!

Griffin

Griffin,

  It looks like tar paper above the skip sheeting, so when you tear off the old roof you'll have

nothing over top of the skip sheeting.  If indeed you want to have the existing rafters/skip sheeting now be the "finish" ceiling (if your clients would like that sort of look) then whatever you deck the roof

with will show on the inside and will be part of the "finish" ceiling. So they could find some kind of

finish ply or other they like, then stain it or other as they see fit. Could even be something like drywall, if that first layer over the existing roof didn't need to be structural and you could figure a way of finishing it.  Then complete the new roof assembly over that.

  In our climate zone (Bay Area/Berkeley?), the minimum amount of closed cell material needed

to prevent condensation within an unvented ceiling assembly is generally considered to R-5

(check with your local architect or building department).  You could add more than R-5 for peace of mind, and depending on the total R-value you are trying to achieve.  I don't believe XPS is a closed cell material, although it probably gets used in place of such.  Polyisocyanurate rigid foam board is closed cell and is commonly used for these type of assemblies. It is also commonly available.

  There are several Passive House architects and engineers in the SF Bay Area that are experienced

with designing these assemblies. 

  So, did that make any sense at all?

Charles Bennett

Mr Insulation

San Rafael, Ca

Yes, that makes perfect sense now. Thanks!

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