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

Q & A Instructions

[Click map to enlarge]

The GBAGreenBuildingAdvisor.com web site has a wealth of articles on a wide variety of construction topics. Before posting your question, you may want to check out the articles on this page: How To Do Everything. You just might discover an article there that provides the information you seek.

Please register for a free account or sign in to ask and answer green building questions.

If you want to post a question, the usual rules of courtesy apply:
1. Be nice.
2. If you can't be nice, be polite.
3. If you can't be nice or polite — well, please be brief.

To attach a photo or illustration: Under the box labeled “More explanation,” look for the words “File attachments.” Click that, and you should be able to attach a photo.

Thanks for joining the conversation!

5 Answers

Exterior Detail Between Poured Porch/Cold Cellar and Main Floor?

CZ6A, Ottawa ON

Having trouble figuring out how to handle exterior wall details at the front entrance where we have a poured slab over a cold cellar meeting up with the house at the front door. The slab will only be an inch or two below the front door (no distinct step, as flush as possible), but the main floor joists will need to sit on the foundation ledge 8" or so below the slab level.

Asked By Lance Peters | Mar 16 18
18 Answers

New Home Construction, Slow Bids, Confused Builders

I'm at the southern edge of Climate Zone 5A and want to build a better than code house, what some might call a "pretty good" house, one that includes the most cost effective of the concepts described in Martin Holladay's "Musings of an Energy Nerd" book. One builder I spoke with discouraged having a continuous layer of insulation on the outside of the walls, or on the basement floor stating that such improvements weren't needed and were too expensive to be worthwhile.

Asked By Illinois Prairie | Mar 14 18
1 Answer

Fixing Unvented Cathedral Ceiling Insulation


Asked By RobJ13 | Mar 17 18
3 Answers

Tiling around shower tips


I built a small guest house and will be tiling around a one piece shower soon. I want to make sure I take the proper steps to installing the tile correctly and making sure it is waterproof so no moisture can get behind the walls.

I used wedi board as my backer board and will be using thinset over that to place my tiles.

A few questions I have
1) Is there a waterproof grout product out there?
2) If not, do I need to seal over the tile/grout? If so, what product do you reccomend?

Asked By MichaelBa | Mar 17 18
0 Answers

What should I use to replace moldy fiberglass batts in exterior bathroom wall?

I am in the process of replacing shower tile in a main floor bathroom. After removing the tile wall assembly (installed in 1977), I discovered some moldy fiberglass batts covered with a plastic vapor barrier in the stud bays of the exterior bathroom wall. I'm not planning to do any more wall demo at this point, so I'm wondering what the best solution for replacing these batts would be. I would add rigid foam insulation to the interior side of the studs, but then I won't be able to transition to the rest of the wall.

Asked By Michael Schonlau | Mar 17 18
13 Answers

Under insulated roof

Hi all,

Asked By Randy Williams | Mar 15 18
7 Answers

Greatest (new) thing in air sealing?

The importance of air sealing is emphasized at every point by GBA and other organizations that use building science for designing (!) and retrofitting residential housing. It is a cornerstone for energy efficiency and for controlling air quality, and also for comfort in buildings. Good air sealing is a prerequisite for using increased amounts of insulation that may reduce energy flow and drying potential in walls and ceilings.

Asked By andrew c | Mar 16 18
6 Answers

No need to tape outdoor sheathing if walls are all spray foamed?

Hi. I asked my GC about if he was planning to tape the gaps between the outdoor plywood panels for air sealing. He said there was no need to do that because he'll be spray foaming (open cell) all the walls on the inside.

Note that he's using Tyvek Drainwrap on the outside, and then a 2 ply layer of asphalt felt paper on top of that, with cedar shingles as the exterior cladding.

What do you think? Taping the sheathing doesn't sound like that much work, but if it doesn't buy you anything no point in doing that.


Asked By mike myers | Mar 16 18
11 Answers

Should I vent a cathedral truss with a metal roof and foam insulation?

Using 2 layers of 2 “ Poly foam under the roof strapping then 2” air space and 1 more 2” Poly going across the top chord of truss to eliminate thermal bridging.

Asked By User-7034059 | Mar 10 18
1 Answer

Thermax over XPS rim joist

I believe the answer is no, but no problem layering foil faced polyiso over XPS in the rim joist, correct?

The XPS was can spray-foamed in, no issue if there is a slight air gap between the layers, correct?

Lastly, should I airseal the thermax perimeter with additional spray foam?

I have leftover 3" thermax and only 2" of XPS in that area so I wager it's a worthwhile application.

Thank you.

Asked By Justin Brown | Mar 16 18
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!