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Solutions for exterior insulation on home with poly warm-in-winter side?

user-6924093 | Posted in Energy Efficiency and Durability on

I love this site!

Home is in 4C or 5 (Clackamas County Oregon – DoE shows 5 on map, but lists marine 4 on county by county) and of mid 90s construction.

I am an interior decorator working for an HO with severe PTSD who will not allow any other tradesman near her home (I met her at a volunteer event for abused children), who has been having me do many different jobs on her home, which is in need of care. On some projects I am over my head, but she knows and is willing to compensate where I make mistakes. At least today it is not like in the days when you had to find an old salt in person willing to share – today we can all get access to expertise from those willing to share!

So a deck removal revealed the deck was thru attached to the sill plate, sandwiching original LP siding. Not a good find. After the siding is off, and the lower plywood, I advised we put the time into a rainscreen solution. She has requested Ledgestone but I am trying to keep her with siding – it appears by all measures to be less problematic (particularly as this home is over the Juan de Fuca fault line and her HO insurance earth movement policy precludes masonary veneers) and she has a limited budget – I am only trying to give her the most ethical solutions.

Adjacent to the gypsum there is dreaded poly sheeting. I have already decorated the interior walls with murals and plasterwork, and cant address it that way anymore.

On the exterior, there is 1/2″ CDX with approximately 1 thousand staples per sq meter. Its a nightmare to remove.

I need to insulate the rimjoist, as the crawlspace was previously encapsulated and is now part of the conditioned building envelope; furthermore the finished flooring is 2″ sleepercar T&G subflooring (found as well between the treated sillplate and the standard bottomplate) with no vapor barrier or other materials, and will create a lot of transmission to the exterior.

Q: Can I foam or even prime the edges of the sleepercar flooring to both seal it against endgrain moisture intrusion and prevent airflow?

Q: if I leave the poly sheeting next to the drywall, will I be doing her a disservice (ie, must I remove the plywood – its a 2 story tall building, hillside – cut out the sheeting and put new ply back up (or clip staple ends and use the same if its in good condition))?

Q: if no, and the sheeting is left in place, drying to exterior, it appears no rigid foam is suitable. Is this correct? The articles here suggested Roxul Comfortpanels as a solution but I cannot find them in small quantities (1100 sq ft of sheathing) reasonably priced.

Q: Any other advice you can provide about this situation that was not covered in these articles?

Thank you very kindly. I am doing my best to help this terrified lady to keep the house she so depends upon in good shape, and trying to get good advice where I have little experience.

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  1. user-6924093 | | #1

    Update, as I did not mention:

    2x6 walls gypsum->poly sheeting->2x6/fiberglass batt->plywood->currently nothing

    Crawlspace is radon vented (despite no radon) for moisture management I assume, under scrim reinforced thick white poly, with EPS under that judging by the sound/movement/feel - against the soil along all the footwalls. The white poly is powder attached to stripping to separate poly running up the concrete walls with a butyl strip, and polyiso over all of that wherever there was concrete. Exterior crawl vents are all closed and polyiso'd. The HVAC system vents into the crawl. The rimjoists, few that there are (its post and 4x10 beam with beam set direct into foundation wall notches) are completely sealed away behind polyiso with canned foam at the edges. Concrete on top of the wall under the rimjoist is bare - insect inspections? Its very interesting and first time Ive ever seen something like it. Brightest cleanest crawl space Ive ever seen, could eat down there.

    I mention it because I know it affects the stack affect and air must move right through the T&G sleepers/finished flooring to the living space, or right out the edges of the T&G to the outside...

    Mr Holliday answered a question here:

    "Although it would be better if the poly weren’t there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven’t been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits aren’t as risky as some people fear, and he believes that these homes likely will be fine."

    Q: When polyiso is added to the exterior of the sheathing, what prevents rain splash from wicking back up the foam and reducing its performance? Can this be flashed with off with SV-5 coravent or some other material?

  2. GBA Editor
    Martin Holladay | | #2

    First, can you tell us your name?

    This post is confusing. At first, I thought that you needed advice on attaching a ledger board for a deck. Although you didn't really tell us, I eventually figured out that you have stripped all of the siding off the house, and you are asking advice on whether it is safe to install rigid foam on the exterior side of the wall sheathing, as part of a siding replacement job.

    In general, as noted in the Fine Homebuilding article you quote -- and as explained in my article, The Exterior Rigid Foam is Too Thin! -- you can add exterior foam to this house, even though there is interior poly, as long as you follow a few common-sense steps (as explained in my article).

    When you install exterior rigid foam, the foam will cover the exposed ends of the 2-inch-thick tongue-and-groove flooring, sealing the air leaks (mostly). If you want to caulk the ends of the exposed flooring before you install the rigid foam, you can. More important: pay attention to airtightness when installing the rigid foam. It can't hurt to install a bead of caulk around the perimeter if each piece of rigid foam, especially toward the bottom of the wall. (The caulk would go between the rigid foam and the sheathing.)

    Plywood sheathing is not a problem. Leave it in place.

    To deal with splashback, the bottom of your wall will have metal flashing protecting the bottom of the rigid foam. To learn more about this flashing, see this video: How to Install Rigid Foam Insulation Outside a House.

  3. user-6924093 | | #3

    My account name should not be USER-digits, but L. David Loomis. I am not sure why it ended up without my own name attached, which I feel is important on any internet board. It promotes ethical, responsible, and respectful behaviour. I do sincerely apologize for the confusion. My speech therapist suggests I mention that I am diagnosed Autistic, and I think somewhat differently than neurotypicals. I dont know what you know, nor how much you need to know, so give as much information as is reasonable to ensure fluid communication. I think engineers are more used to the way we think and get us eventually!; the difficulty is generally with reactionary thinkers or the impatient. My company is a sole proprietorship, Artistic Concepts. I generally do murals, plaster bas-relief or other handcraft work such as copper, or even acid staining or inset stone patterns. Despite this handicap I do care a great deal and my work is never beneath above standard! I'm just lucky to have found a niche, only 19% of Autistic (ASD) people work in any capacity; generally part time or tech.

    Yes, I was concerned about double vapor barrier, but found your fine homebuilding article after posting.

    Thank you for the video link Martin.

    Would you happen to have any direction on heatloss through her flooring?

    Ext. Wall Diagram:
    Stud 2x6
    Bottomplate 2x6
    Sleepercar 2x6 T&G subfloor under entire structure & perimeter
    Treated Sillplate 2x6

    The Sleepercar T&G has been utilized (post and beam construction) as subfloor and finished floor both. There is no vapor barrier, nor insulation beneath, because the crawl is completely encapsulated with white plastic, very similar to your article. The crawl has ventilated ducting I assume for moisture control and the heat rises directly through the large gapping of the T&G (very much appears to be an old barn floor).

    My concern is the T&G flooring is not tight. Anywhere. At the perimeter it is in locations only halfway onto the sillplate where it laps over perpendicular. There is quite minor moisture or red ant rot on some of the ends. I treated those locations for insect, allowed them to dry out (post sheathing removal) and will fill with bondo as I cannot screw lift and repair such a design! I could replace the bottomplate but there seems to be no great way to address the lower sleeper car sandwich.

    It is installed square and not diagonal so only two sides expose endgrain, but those same gaps permit air exchange and loss right at the sill, which is going to worsen the moisture problem I am seeing here. It must be a large amount of air gap if aggregated. The only barrier against infiltration in winter as the stack pulls cold directly in off the sillplate, is the plywood sheathing, not terribly tight, which I am given to understand I cannot seal at the bottom? But I am guessing I *can* seal off the sleepers. But I do hate guessing.

    I have no idea how to help this situation other than to "can-foam" all of the sleeper ends.

    PS- Do you also have details of windows flashed innie and outie, for subscribers? I have seen the other free pages but as I am a visual thinker drawings are most helpful. I am happy to subscribe especially if there are pictures.

    Thank you very kindly and most respectfully,

  4. GBA Editor
    Martin Holladay | | #4

    L. David,
    First of all, I'm sorry that the GBA site registration process makes it difficult to attach your name. Here is a link to an article that explains the problem and the solution: How the GBA Site Displays Readers’ Names.

    If you have removed enough of the plywood sheathing to expose the subfloor, you should finish addressing any rot. When you reinstall the plywood sheathing, you should install the plywood in an airtight manner, and trust the plywood to limit air infiltration. The bottom of each sheet of plywood should be caulked -- usually to the mudsill. I assume that this bottom piece of plywood sheathing will cover the exposed ends of the 2x6 subfloor boards. The seams between the plywood sheets should be taped with a high-quality tape. In this way, the plywood sheathing becomes your air barrier.

    For links to GBA details for both innie windows and outie windows, visit this page: Building Details for a Deep-Energy Retrofit.

    For more information and illustrations of window flashing issues, see these three links:

    ‘Innie’ Windows or ‘Outie’ Windows?

    Installing Windows In a Foam-Sheathed Wall

    Video series: Flashing Windows

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