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Community and Q&A

Very Cold Climate — ERV

Thomas Dresser | Posted in Mechanicals on

I trying to decide whether to use an Energy Recovery Ventilator or Heat Recovery Ventilator to ventilate a tightly-insulated house in northern Maine, (Very Cold climate zone) to be used mainly during the winter.

Any insights as to which one is a better choice and why?

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Replies

  1. Thomas Dresser | | #1

    For clarification, the ski chalet will be in Zone 6.

  2. GBA Editor
    Martin Holladay | | #2

    Thomas,
    Either one would work, but I would choose the HRV.

    For more information, see HRV or ERV?

  3. Thomas Dresser | | #3

    Thanks, Martin.

    The article was very insightful, especially the large house, small house distinction. HRV seems like the right choice for me.

    I guess my next question is how do I size the HRV? The chalet has a volume of about 6000 cubic feet, with two bedrooms and a pull out couch for overflow. A supplier recommended that I plan for 1/3 ACH. Does this mean a 33.3 CFM HRV (6000 cu ft per hr / 60 min / hr * 1/3 ) will get the job done?

    Thanks.

    --Tom

  4. Thomas Dresser | | #4

    On a related topic, the current metal roof is vented with 4 inch soffit vents taht run along the full lenght of the eaves and gable vents under the peak the rakes. About six inches of foil-backed fibergalss insulation lies on the ceiling strapping, leaving a gap to the underside of the roof of several inches. I think this may be the main reason the blower door test ran about 40 ACH at 50 Pa.

    I would be interested in your opinion on the two solutions I am considering:

    One (expensive) solution I am looking into is 6 inches of closed cell foam sprayed to the underside of the roof, making sure to cover the soffit vents. The gable vents will be covered by exterior foil faced polyisio sheets. This approach is intended to upgrade the roof insulation and, most importantly address the air infiltration.

    The second (less expensive) solution would be just to seal the soffit and gable vents with foam and call it a day.

    Any direction from you would be most appreciated.

    Thanks.

    --Tom

  5. Thomas Dresser | | #5

    And finally, the uninsulated pine floor sits on pilings with minmal uninsulated skirting around the perimeter. My energy model indicates that this is the single loargest source of heat loss as the R value of an inch of pine is about 1. I am considering spray foam here as well to improve both insulation and infiltration.

    The question I have is with the water pipes They currently come up from the ground with minimal insulation on the incoming water and none on the waste water. I have never had any freezing problems.

    Do you think this is because of the current level of heat leakage, and if so, will my insulation strategy cause the pipes to start freezing? If so houw might I best address this issue? Should I build an insulated box around the pipes? Or should I make the entire area into a crawl space with 6 mil poly on the ground and insulated skirting?

    Any direction here would also be much appreciated.

    Thanks.

    --Tom

  6. GBA Editor
    Martin Holladay | | #6

    Thomas,
    Q. "How do I size the HRV?"

    A. ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. If your house has 4 occupants, your ventilation rate would be 30 cfm + 60 cfm = 90 cfm. More information here: Designing a Good Ventilation System.

    Q. "Should I install 6 inches of closed-cell spray foam or just close the vents?"

    A. Of the two options you presented, installing new spray foam insulation on the underside of your roof deck is the only reasonable option.

    Q. "Will my insulation strategy cause the pipes to start freezing?"

    A. Very possibly. There is no good way to solve this problem short of installing a new concrete foundation (a concrete-walled crawl space or basement). If you can't afford that, your best bet is to insulate the pipes very well with top-quality pipe insulation enclosed in an airtight insulated box. Include electric heat tape near the vulnerable pipes. Install an insulated airtight skirt around the exterior of your crawl space.

  7. Thomas Dresser | | #7

    Thanks so much, Martin.

  8. Thomas Dresser | | #8

    Martin--

    Two more questions, both related to the windows. I am applying 4 inches of polyiso over the existing sheathing and trying to use the "innie" window technique described in your May 6 posting. 1. Can you give me links to the correct flashing details?
    2. Can I substitute flexible flashing instead of copper for the sill flashing?
    Thanks.

    --Tom

  9. Thomas Dresser | | #9

    The interior 2 by 4 walls all ready have a mix of kraft faced and foil faced fiberglass installed. I am using 3/8 " beadboard as my interior wall. Should I replace the insulation? If so with faced or unfaced fiberglass batts? The insulation was installed in 1967.

    Thanks.

  10. GBA Editor
    Martin Holladay | | #10

    Thomas,
    Q. "I am applying 4 inches of polyiso over the existing sheathing and trying to use the "innie" window technique described in your May 6 posting. Can you give me links to the correct flashing details?"

    A. Yes. Here are two detail drawings:
    https://www.greenbuildingadvisor.com/cad/detail/exterior-insulation-retrofit-window-sill-1-12-rigid-foam

    https://www.greenbuildingadvisor.com/cad/detail/exterior-insulation-retrofit-head-furring-strips-padded-opening-1-12-rigid-foam

    Q. "Can I substitute flexible flashing instead of copper for the sill flashing?"

    A. Flexible flashing may be used to flash the rough sill of a window opening (to create a site-built sill pan). It cannot be used to flash a sill exposed to the weather.

    Q. "The interior 2x4 walls already have a mix of kraft-faced and foil-faced fiberglass installed. I am using 3/8 " beadboard as my interior wall. Should I replace the insulation? If so with faced or unfaced fiberglass batts?"

    A. I don't understand why you would want to remove fiberglass batts and then replace them with fiberglass batts. Either leave them in place or remove them -- your choice -- but if you are going to the trouble of removing them, you should certainly insulate the stud bays with something better than fiberglass batts, which are the worst available type of insulation.

    One important point: if your interior finish material is tongue-and-groove boards, be sure that your walls have a decent air barrier. Boards are not an air barrier.

  11. Thomas Dresser | | #11

    Martin,

    Thanks for the answers to my questions. Very insightful.

    Regarding the third question, my understanding is that the new layer of housewrap to be installed over the sheathing and under the new polyiso foam sheets will serve as an air barrier. My reason for using 3-ply beadboard plywood on the interior is that it will withstand extreme temperature variations from room temperature 70 degrees down to minus 30 when the heat is truned off midweek, as we use it mainly on week ends. Another reason for using the plywood is that I expect it will help the wall to dry to the interior in the event that mosture condenses on the inside surface of the foam.

    I would like to make sure that the insulation achieves a good balance of insulation value and drying potential. It would seem to me that the insulation in the wall cavity should be made uniform so that all areas of the wall have the same insulating value and drying potential. I was also thinking that the 1967 vintage insulation might have lost some of its R value by now.

    What do you see as my options in this situation that take advantage of what is currently lhere? Also if you had the option of picking any cavity insulation material, what would that choice be, and why?

  12. GBA Editor
    Martin Holladay | | #12

    Thomas,
    1. Housewrap isn't the best air-barrier material, because of the many fastener penetrations and the possibility of rips. You are probably better of using the two layers of polyiso as your air barrier material. Pay attention to caulking and seam-sealing (with a compatible tape) when installing the polyiso.

    2. I thought you were using boards for your interior finish material. If if is plywood paneling, that will be easier to air seal. You can install caulk under the perimeter of each piece of plywood to improve air tightness.

    3. Dense-packed cellulose is far preferable to fiberglass batts as a wall insulation.

  13. Thomas Dresser | | #13

    Refering back to post 10, first answer:in which you supplied two details for me:

    Q. "I am applying 4 inches of polyiso over the existing sheathing and trying to use the "innie" window technique described in your May 6 posting. Can you give me links to the correct flashing details?"

    A. Yes. Here are two detail drawings:
    https://www.greenbuildingadvisor.com/cad/detail/exterior-insulation-retro...

    https://www.greenbuildingadvisor.com/cad/detail/exterior-insulation-retro...

    My builder and I started to discuss the head detail this week end and we encountered an apparent problem with the head exension. The 2 by 2 block between the foam and the extension will act as a thermal bridge and degrade the insulating value of the window area significantly. What altermatives can you recommend?

    Because we are using casement windows, not double-hung as illustrated, we are having difficultty finding a way to attach the pine extension box. There is no room to toe screw them in, as sugggested in the "Innie Outie" article.. The article also suggests fastening them from the inside with wire clips. Do you have any imore information on these clips including the name of a vendor that would shed some light on this approach.

    Finally, we want to install these boxes without trim so they would attach to the cladding (pine siding). If we were to take this approach, the extension box would pass through the rain screen and I would think it would need some flashing.

    With all the "Innnie" windws that have been installed, I would think there might be some details available on how to do these. Any leads?

    Thanks, again, in advance.

  14. GBA Editor
    Martin Holladay | | #14

    Thomas,
    Q. "My builder and I started to discuss the head detail this week end and we encountered an apparent problem with the head exension. The 2 by 2 block between the foam and the extension will act as a thermal bridge and degrade the insulating value of the window area significantly. What altermatives can you recommend?"

    A. If you want, you can nail window flanges through foam instead of a wood block. For more information, see Nailing Window Flanges Through Foam -- although you should note that Joe Lstiburek doesn't recommend this method for foam more than 1.5 inch thick.

    Q. "Because we are using casement windows, not double-hung as illustrated, we are having difficulty finding a way to attach the pine extension box. There is no room to toe screw them in, as suggested in the "Innie Outie" article.. The article also suggests fastening them from the inside with wire clips. Do you have any more information on these clips including the name of a vendor that would shed some light on this approach?"

    A. Side straps or masonry clips should be available from the window manufacturer. Just contact the manufacturer and explain what you are doing. These clips are used all the time when windows are installed in brick buildings.

    Q. "Finally, we want to install these boxes without trim so they would attach to the cladding (pine siding). If we were to take this approach, the extension box would pass through the rain screen and I would think it would need some flashing. With all the "Innnie" windows that have been installed, I would think there might be some details available on how to do these. Any leads?"

    A. For more information on flashing "innie" windows, see this JLC article by Thorsten Chlupp. There is also further discussion of Chlupp's details in this Q&A thread on GBA.

  15. Thomas Dresser | | #15

    Martin--

    Thanks for all the leads. I think I am finally set.

    Last weekend I also saw black stains on much of the fiberglass insulation that had been removed for electircal work. I am speculating that this was caused by air infiltration through the very loose board and batten siding. I got the analysis from this link.

    http://www.homeenergy.org/show/article/filter/1998/id/1360

    What is your opinion, and what should I do about it?

    Thanks,

    --Tom

  16. GBA Editor
    Martin Holladay | | #16

    Thomas,
    If you have board-and-batten siding over a stud wall insulated with fiberglass batts, it's no surprise to find black mold on the batts. You probably have a lot of air leakage and a lot of migration of interior moisture into your cold stud cavities.

    The solution is to warm up the stud cavities and to include a tight air barrier in your wall. There are lots of ways to do this:
    - The air barrier can be established with new plywood or OSB sheathing with taped seams.
    - The air barrier can be established with new interior drywall installed according to the Airtight Drywall Approach.

    In either case, penetrations -- electrical boxes and electrical penetrations through the bottom plates and top plates -- have to be addressed.

    The stud cavity can be warmed up by installing a layer of exterior rigid foam insulation. This will greatly reduce the chance of condensation or mold in your stud cavities.

    The fiberglass batts can be replaced with an air-impermeable insulation like spray polyurethane foam.

    There are many ways to build a wall, but your home now has walls that are poorly designed.

  17. Thomas Dresser | | #17

    Thanks for the warning. I brought in a mold inspector and fortunately is was only air-wash. Better safe than sorry.

    I am using the GBA approach for window installs with Tyvek and exterior foam. I reviewed the product specs for W R Grace Vycor for the flashing and it indicates that it is designed for use over uneven substrates such as OSB. There is no mention of adhesion to housewrap. It appears that I may have the wrong procduct.

    What products should I be using for flashing over housewrap?

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