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!

1 Answer

Are there HRVs that can be installed in unconditioned space?


To combat elevated humidity levels in my home, I am planning to connect an HRV to draw air from the HVAC return duct and to then to dump fresh air into the basement. Due to the limited space in the mechanicals/HVAC room, I am planning to install the HRV in the garage, but I see HRV manufacturers want it installed in an operating environment that's at least 50F, so my unheated garage would not be a proper location.

In GBA Pro help | Asked By Venkat Y | Aug 31 14
6 Answers

Practical insulation and air-sealing techniques needed for a (sort-of) enclosed crawl space

(If this should be posted elsewhere- feel free to move it, or tell me where to repost it.)
I have a fairly typical Chicago-style, narrow (like 22'), 2-story, balloon-framed house that was built in the 1880s or 1890s, and converted to a 2-flat later.
At some point (maybe in the 2-flat conversion?), the 2-story back porch was enclosed, and the new siding was continued down to grade level- by adding some framing between the wood columns (on piers) that support the porch.

In GBA Pro help | Asked By Ben Rush | Aug 30 14
2 Answers

2 x 6 cathedral ceiling insulation plan

I have a 1 1/2 story cabin that I am insulating. The second story was constructed using engineered attic trusses. The top chords are 2 x 6 and the knee walls are 2 x 4. Metal roof with soffit vents and ridge vents. My plan is to fur the sloped top chord with a 2 x 4 on edge. This would provide me with 9" of space. I will put allow 2" of ventilation space and install 1.5 " XPS rigid foam board as my baffles, followed by 5.5" of roxul comfort batts, 6 mil poly vapour barrier and drywall. The 2 x 4 kneewall I will build out also with 2 x 4, giving me 7 inches of space to fill.

In General questions | Asked By Dawn Roberts | Aug 30 14
1 Answer

Insect screen for an exterior rigid foam wall

I live in the San Francisco bay area and am going to use 1-1/2" rigid foam board and a rain screen on the exterior of my house. I have read numerous articles in regards to using insect screen at the tops and bottoms of the rainscreen wall, however no specific details about the type of insect screen is ever mentioned.

There are numerous types of insect screens (Aluminum, Fiberglass Bronze, galvanized, etc.) available.

In Energy efficiency and durability | Asked By Randy Mason | Aug 30 14
6 Answers

Insulating walls in an old house

My 1927 house has balloon framing, real plaster walls on rock lath, and no vapor barrier. It has cedar shingles over tar paper.

If I blow insulation into the walls, will I create a problem with vapor from the house condensing inside the walls? Even without insulation (current condition), can I close the top of the walls in the attic or will that also allow water vapor to condense in the walls?

In General questions | Asked By RUTH HENDRICKSON | Aug 29 14
5 Answers

Is it cheaper to run a dehumidifier than an ERV in winter?

I am in zone 5A and I have a problem of excess humidity in my ICF home with no abnormal sources of moisture (5 occupants of a 2150 sft home with a full basement). This results in condensation on the wall of windows facing a pond on the north.

Since a dehumidifier will also dump heat into the home helping my all-electric furnace heat the home, I am wondering if it would cheaper to run the dehumidifier than to use an ERV to lessen the humidity in the home. I realize in the latter case, there's some loss of sensible heat that the electric furnace would then have to compensate for.


In Energy efficiency and durability | Asked By Venkat Y | Aug 29 14
1 Answer

EPS over polyiso on reroof?

So based on everything thing I am reading should we be using R3 R4 or R5 for each inch of Polyiso since it decreases in performance as it gets colder. I seem to be seeing some variation in Answers. I am in zone 5 and was planning on putting 5 inches of Polyiso.(thinking that was R 30 Roughly but if is is only R 15 when coldest that is a problem) Now it seems like I would be better served with three inches of polyiso with EPS on top. I have access to 1.5 or 2.75 inch EPS. Would I still be OK with 3 inches of polyiso and 2.75 inch of EPS on top?

In Energy efficiency and durability | Asked By terry grube | Aug 29 14
0 Answers

Looking for an HVAC contractor for a project in Sudbury, MA

Hi Folks,

I am advising on a home renovation in Sudbury, MA. It has become obvious that the structure -- about 20 years old -- was built without thought to airsealing. The resulting home performance problems are likely predictable to GBA readers.

Now, the owners are turning a 3-season sun room into a 4-season room and need help figuring out how to best heat and cool the new space. This project also presents an opportunity to address other related issues.

In Energy efficiency and durability | Asked By c talwalkar | Aug 30 14
3 Answers

Air sealing penetrations

My new house is designed to limit penetrations through the building envelope to as few as possible. Nevertheless, sillcocks, outside electrical receptacles, outside lights, HRV intake/ exhaust, etc. need to be sealed.
Are there methods or products that are more effective than others? How about ease of installation? I've seen photos of tape covering holes and that seems like a pretty clunky solution. I'm probably going to either do much of the airsealing myself, or at least supervise it.
Thanks for any advice.

In Energy efficiency and durability | Asked By stephen sheehy | Aug 29 14
1 Answer

Flash foam and Roxul?

I understand from prior posts there are concerns with the flash foam and fiberglass insulation in terms of moisture vapor.

Would the use of proper barriers and Roxul instead of fiberglass help to abate that concern?

In Energy efficiency and durability | Asked By George Levicki | Aug 29 14
Register for a free account and join the conversation

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