# Q&A: A Forum for Green Building Experts and Beginners

## Q & A Instructions

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### Calculating furnace efficiency

I know (or at least I think I know) that to figure steady state efficiency, divide the BTU output by the input. But many furnace spec plates don't have a BTU input listed. Instead they'll have Gallons (or Litres) Per Hour. Now, if 1 gallon of residential fuel oil has about 138,500 BTUs, can you do a straight calculation using these numbers?

The example I've got in front of me right now is a Kerr Gemini furnace (K5RE-115) with a Riello Burner. 96,000 BTU/hr output, 0.85 USGPH input. Nozzle size is 0.75. Can somebody help me figure the efficiency of this model with this info?

In Mechanicals | Asked By Peter Rogers | May 19 16

### Use of polyiso below a slab

In CZ-4, I will be building a Net Zero slab-on-grade ranch, on a flat lot, on top of a ridge. All water drains away from the site. The slab itself will be raised about 2 feet above the current grade level. Under the slab will be a sheet of poly, 2" of sub-slab insulation, and about about 18" of gravel, all above the current grade level. There will also be 3½' Overhangs. Foundation drains will drain to air.

In Energy efficiency and durability | Asked By Ted Cummings | May 16 16

### Hot water closed loop at 135 degrees F

Hi , I have set up an anaerobic digester at my horse property , I have made a solar heater ( black hosepipe behind some upvc ) to heat it ,for best results I need to get the digester up to max temp 135 deg F , just had my first pump seize up , what of off the shelf pump can cope with that kind of temp , its running water around a closed loop of hosepipes

any and all suggestions welcome

In Mechanicals | Asked By David Lowe | May 19 16

### Pretty Good Historic Double Hung....

We're doing a DER on a house in a historic district and tying to balance performance with aesthetic appeal that will be tolerated by the local historic commission.

This is a house in the Boston area and so largely a heating zone.

Is there an 'acceptable' window that gets the obvious thumbs up from our community?
Marvin's next generation double hung looks good and is available with a narrow 5/8th muntin.
I can't find much information on its performance BUT as I begin to (quickly) research are there other specific products within other lines that should be considered.

In Green products and materials | Asked By Edward Cambridge | May 17 16

### Wall design

I am adding a small addition to my house and have been struggling with choosing a wall design. I'm trying to find a balance between efficiency, ease of construction, and cost. I was a fan of double stud walls until I read about potential issues with damp sheathing. It occurred to me at some point that if we ventilate ceiling assemblies, why can't we ventilate a wall? Specifically, I'm thinking about ventilation on the inside of the sheathing in addition to a rain screen on the exterior of the sheathing. Has this already been done?

In Green building techniques | Asked By Eric Colbeck | May 19 16

### DensShield backerboard - does it need waterproofing membrane?

From what I read in the posts on backerboards in GBA I see several comments on how well RedGard membrane is performing for the shower and tub areas. I also read that DensShield backerboard has a waterproof layer over gypsum core. We chose DensShield for its better environmental attributes. Do we need to apply RedGard over it or would it be an overkill? If we don't use RedGard, what should be done to the seams and corners...?
Thanks,
Lucy

In General questions | Asked By Lucyna de Barbaro | May 18 16

### Wood stove chimney flashing

Installing wood stove chimney to go through cathedral ceiling with 12/12 pitch. This is new construction. Roofing material will be galvalume R panel. On the roof, what is the recommended layering system to prevent moisture from entering at the chimney penetration? I have seen metal roof with the flexible high temp boots on top sealed with silicone, and I have seen 12/12 metal roof flashing adhered by sealant directly to the roof sheathing with the roof panels cut around the flashing and sealed , and a combination of many other ways.

In Green building techniques | Asked By Scott Smith | May 16 16

### How to construct a window buck for continuous exterior insulation with a rough opening that is too small for the plywood?

My window rough openings are 1/4" wider than my windows per the manufacturers instructions. Problem is, I didn't leave room to install a buck. I found this article on GBA that talks about nailing the windows directly through foam insulation: http://www.greenbuildingadvisor.com/blogs/dept/musings/nailing-window-fl....

In General questions | Asked By Jeremy D | May 15 16

### Need alternative to crawlspace for unique design

GBA folks,

I'm looking into building a house with the Blue Sky steel framing system with pier-and-beam construction. I've been reading through various GBA articles on crawlspaces, and it seems like there aren't very good reason to go with one. The only thing it would provide is access to drain pipes (all other plumbing and electrical are in service conduits and walls).

In General questions | Asked By David Gadbois | May 17 16

### Insulating floor of mudroom before closing it off

I'm building 5' by 8' mudroom addition where there previously had been a dilapidated porch-like structure. I've poured the foundation and secured the sill. The concrete foundation is about 5 inches above soil, then 6 x 6 sill. I will be framing the floor with 2x 6. I'm stuck with these dimensions because of grade etc. This "crawl space" is NOT connected the the main house's crawl space and will be 100% inaccessible once the subfloor goes down. Also, foundation is non vented.
Anyway- the plan is to-

In General questions | Asked By Joseph Moore | Apr 29 16