Trombe wall (non-vented) with exterior moveable mode of insulation?
I am in the process of completing my energy efficient house design. Approximately 80% of the Building Envelope will be a Double Stud Wall Construction allowing for roughly 11″ of Cellulose Insulation in addition to a 1″ Rigid Insulation outboard of the Sheathing and inboard of the Hardie Cement Fiberboard Skin. The Remaining 20% of the Envelope will be Double Glazed Fiberglas Windows with emphasis placed on the South Elevation (Perhaps 60% on South, and 20% each on East and West). However, on the South Elevation I also have two sections of Trombe Wall – each 8′ in width and 12′ in height. The Trombe Wall will have a triple glazed exterior window treatment with a 2″ air gap between the Glazing and the Black Face Brick. Obviously, I want the advantage of the Solar Gain and the Thermal Mass associated with the Trombe Wall. The disadvantage of course will be the Heat Loss during nights and days without Sun. For this I have designed an Exterior Operable Roll Up Shutter that would hopefully provide sufficient Insulating capabilities to minimize Heat Loss when the Trombe is not contributing to the warmth of the house. Recently, I have begun to consider substituting an overhead Bi-Fold Type closure in lieu of the Roll Up Shutter as I believe this would have better insulating capabilities and most likely have fewer maintenance issues, I have looked at Schweiss Doors on their website, but I am concerned that it may be somewhat impractical to use the Bi-Folds since it may require a significant clearance outside of the Trombe Glazing to accommodate the operating mechanisms (either Hydraulic or Winch Driven with Straps or Cables). Any comments/suggestions? Anyone have a similar condition? TIA
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Replies
Thomas: where will your house be built? Trombe walls may not make sense in most US climates, because you end up with a big, poorly insulated wall. Why triple glazing in front of the Trombe walls, but double elsewhere?
Thomas,
Energy-efficient homes don't have Trombe walls. To minimize your energy bills, stick with well-insulated (up to R-40, depending on your climate) opaque walls, punctuated by just enough windows to provide you natural light and views -- no more.
Double-stud walls should not include any exterior rigid foam. Double-stud walls need to be able to dry quickly to the exterior, because the exterior sheathing tends to get damp in February. Adding exterior rigid foam to your double-stud wall will make the wall more susceptible to dampness, mold, and rot.
Stephen,
The house will be located in New York's Catskill Mountains at 2600' Elevation. And Yes, the Winter conditions can be quite extreme - Typical maximum cold reaches 20 below zero and there is a strong wind factor as well.. I recognize the inherent inefficiencies of Trombe Walls - yet without actually performing the needed calculations, I thought that I could offset the Negatives by applying the concept of the Moveable Exterior Insulation. Based upon yours and Martin's responses - it may we wise to abandon the Trombe's - though to be perfectly honest - I hate to do it as I truly wanted the benefits including the Thermal Mass which I frankly saw as a stabilizing interior "moderator". The Double Glazing at the Vision Windows is intended to maximize thermal efficiency without the loss of acuity. The Triple Glazing at the Trombe was intended strictly for the additional R Factor.
Martin,
As I mentioned to Stephen, I must now consider abandoning the Trombe Walls. I have read many of your posts in the past and I am well aware of your observations re: Moisture within Exterior Wall Systems. Before fully committing to my Exterior Wall Design, may I pose a few additional questions? I AM committed to the Double Stud Construction with the Cellulose Insulation. Given that, what specifics would you suggest between the Exterior Studs and the Hardie Fiber Cement Board Cladding in order to minimize the possibility of condensation and permit acceptable "Breathability"? It was my original intention to adhere the Exterior Rigid Insulation to the Sheathing with 3/4" Unistrut channels which would also serve as the fastening mechanism for the Cladding, thus creating an airspace between the Rigid Insulation and the Hardie Board. I have printed out your article regarding where to place the Housewrap such as Tyvek or other depending on whether the windows are "Innies" or "Outies" and will seek to finalize this matter shortly. It seemed from earlier readings that you are not 100% opposed to the application of Exterior Rigid - but rather that it should be within the context of a well developed system - and not simply an "Add-On". Thanks
If you want thermal mass, you don't need a trombe wall to do it. No reason why you can't have thick interior plaster, structural masonry with outside insulation, exposed concrete or tile floors, etc. But New York is hardly the right climate for passive solar stuff. It gets too cold and there's not enough sun. The only place in the USA where this stuff can really work well enough to be worth it is in the high desert southwest, where I live, and people have made it work here. But most still don't do it, because even if you've gotten religion, there are far easier, more cost-effective, and less user-intervention-required ways to gain many of the benefits of passive solar design principles in a cheaper, more conventional design: with proper orientation, proper window sizing, glazing, and shading, interior thermal mass, and outside insulation.
Thomas,
Q. "What specifics would you suggest between the exterior studs and the Hardie fiber cement board cladding in order to minimize the possibility of condensation and permit acceptable breathability?"
A. The answers can be found in these two articles:
How Risky Is Cold OSB Wall Sheathing?
Monitoring Moisture Levels in Double-Stud Walls
Briefly, the recommendations are: (a) Choose a sheathing other than OSB (board sheathing, fiberboard, DensGlass Gold, or plywood -- definitely not rigid foam); and (b) Include a ventilated rainscreen gap between the back side of the siding and the WRB.
Thomas,
Q. "It seemed from earlier readings that you are not 100% opposed to the application of exterior rigid foam."
A. There is nothing wrong with installing exterior rigid foam, as long as the R-value of the exterior foam is properly balanced with the R-value of the fluffy insulation installed between the studs.
Exterior rigid foam is never installed on double-stud walls, because the rigid foam would have to be absurdly thick to properly balance the R-value of the fluffy insulation. Rigid foam is usually used on 2x4 walls or 2x6 walls -- not on 10-inch thick walls or 12-inch thick walls.
For more information on the principles behind the use of exterior rigid foam, see this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
Thomas,
You have expressed an interest in thermal mass. Nathaniel gave you good advice: if you want to include thermal mass in your building, there are much better ways to do it than with a Trombe wall.
The main problem with a Trombe wall is that the thermal mass has little or no insulation on the exterior side. For more information on this topic, see All About Thermal Mass.
Thank you for your valuable input Gentlemen. I have parted with some of my earlier Idealism and as a result of your comments - taken a more pragmatic approach. The Trombe Walls are out and I have revised my intended Exterior Wall construction as follows (from Interior to Exterior).
1/2" Drywall
Intello Plus Barrier
Double Stud Wall with Cellulose Insulation
Denseglas Gold Sheathing
Tyvek (or Similar Housewrap)
Furring (with 3/4" air space utilizing) via Unistrut Channels
Hardie Cement Fiberboard Cladding
Now, as regards my additional questions...my intent is to sheet the topside of the 2nd Floor Ceiling Joists upon which I expect to have 24" of Cellulose Insulation in the Vented Attic Space. Would you suggest the similar combination of Denseglas Gold and Intello Plus as on the Exterior Wall? And further ....in the space between the 2nd Floor Ceiling Joists and the Underside of the Attic Sheeting, I have assumed 4 layers of 2" Rigid Insulation directly above the Double Stud Wall Cellulose Insulation. Do you see anything "Inadvisable" regarding this - particularly with respect to potential condensation issues? TIA
Thomas,
If you are creating a vented unconditioned attic, you don't have to put any sheathing on top of the attic insulation -- so no DensGlas Gold above the insulation on your attic floor.
You also don't need a stack of rigid foam above your perimeter walls. As long as you have at least 26 or 27 inches of room between the top plates of your exterior wall and the underside of your roof sheathing, you'll be able to install 24 inches of cellulose on your attic floor. Install an insulation dam at the perimeter, and leave a ventilation gap at the top of the cellulose.