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Thoughts on shell, air barrier and insulation

Last Post 27 Nov 2010 11:08 AM by Viking House. 5 Replies.

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cgal

New Member

Posts:15

11 Nov 2010 07:39 PM
I need help. After hours (and hours) searching the web and this and other "green" building sites, I'm still struggling for the best answer. We're building a new 6300 sq ft home in Lakeland, FL 33803. I have an experienced reputable builder chosen because of a longstanding personal relationship. Although he does not have the energy efficient experience I would prefer, I am not willing to dishonor that relationship by choosing another builder. I recognize that is a limitation. This home has been designed (by my wife and I) to meet the needs of our family, elderly parents, etc. We are on a tight budget because of the amount of square footage we need. The floorplan/layout is final. The layout is based on life function, not maximal energy efficiency. The structure will be concrete block on slab 1st story, 2X6 second story and 4:12 engineered truss roof. Roof will be engineered for tile but we'll use shingles for now. I have supplied a great deal of data to by builder (e.g. building america best practices for hot humid climates, Building Science Builders Guide, etc). He is very open to the info, just hasn't done that stuff before. I'm figuring we'll do something similar to the Maitland Enclosures that Work from the Building Sciences site. I intially thought I'd be going with spray foam but the quotes came in so high I started looking for other options. I'm now thinking cellulose may be the way to go because of cost, insect resistance, mold resitance, and sound proofing. I guess my question is, do you think we can be successful at air sealing the house with housewrap and using spray or dense packed cellulose in the upstairs walls and under the roof deck (with netting)? I'm really just looking for the most cost effective most effective solution that doesn't require a complete change in technology (like SIPS, ICFS). I appreciate any thoughts/comments all of you have. Thanks. Clark

Bob I

Veteran Member

Posts:1435

12 Nov 2010 05:47 PM
You don't need to use expensive technologies; celuilose will work as well as anything else in most applications. Here are a few things to keep in mind: 1. Find a way to do a blower door test BEFORE you insulate so you can find the leaks and seal them befoe they are buried. 2. Be suire you have a truss that allows for sufficient insulation directly over the walls. In some cases it may be a "raised heel" truss, but it'll vary with the design 3. BSC specializes in details. Using the general idea without following the caulking details for instance will not give you the right results. 4. The "dirty little secret" is that none of this is really that hard; you can certainly achieve your goals if you pay attention to details. PS: I'm a little concerned about the avalanch of details you've read about or passed on; most are based on the same science and may be different ways of saying the same thing, but may leave him feeling overwhelmed. I think if I was in his shoes, I'd prefer to follow ONE concise guide - Building Science would be my choice - and leave the others for the occasional odd detail.

Bob Irving<br>RH Irving Homebuilders<br>Certified Passive House Consultant

cgal

New Member

Posts:15

17 Nov 2010 07:39 PM
Bob, Thanks for your feedback. I think your advice was on the money. I am an analytic and I agree that I was "avalanching" my builder. I guess my question was not particularly interesting since it generated hardly any response. I really do appreciate you taking the time to respond. Clark

Dana1

Senior Member

Posts:6991

18 Nov 2010 02:54 PM
You may have to dense-pack it to at least 3.5lbs to avoid long-term sag issues in the walls at the humidity levels of FL, but cellulose works fine. Specify "borate only" or "sulfate free" fire retardents, which won't corrode metals when wet the way aluminum sulfate does. It'll be a better insect & rodent agent too, with higher borate levels. Under the roof deck you can go with a lower density, but put in enough to allow for ~10-15% reduction in R over the next quarter-century. eg: If you want it to still be over R30 in 25-50 years, give it a full 10", which will be ~ R35 on day-1 at 2.5lbs density, but may sag to R32 before it fully stabilizes. Using wet-spray "stabilized" material it'll keep day-1 performance for a long time, but just how long depends on the absolute humidity cycling. (all wet-spray stuff is borate-only.) If you're insulating the CMU first floor with an interior studwall, put at least a half-inch of XPS between the studwall and the CMU as a vapor retarder (1" would be better) to limit the rate of moisture flow into the cellulose. An inch of XPS + 2x4 celluose studwall adds up to about the same R as 2x6 stick-framed w/cellulose. With taped or foamed seams the XPS makes a near-perfect air-barrier too. Tile has a much lower net solar gain than composite shingles, but it's pricey. To lower the cooling load go for a higher pitch, and use a cool-roof shingle type that's been tested by the CRRC: http://www.coolroofs.org/products/search.php You're looking for something with a high aged solar reflectance, as well as a high emittance value. Choosing the right shingle can make as big or bigger difference than adding a radiant barrier on the interior. Doing a full pass of air sealing with blower doors & smoke machines, foam guns at the ready as soon as the shell is up and the windows & doors installed is the first 90% of getting the air-sealing right. A lot can be missed even when you THINK you're being careful. It's much easier to seal the exterior well BEFORE the insulation & siding are up than after.

matteo

New Member

Posts:56

23 Nov 2010 07:49 AM
Hi, I'm a newbie in this forum site and thankful for the information that you shared with us here about the proper way of building a house..

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Viking House User is Offline

New Member

Posts:37

27 Nov 2010 11:08 AM
Hi We developed this externally insulated and airtightened timber frame method to cost effectively build Passive Houses. It may be of interest to you if you are constrained by budgets. I still think that the house you are building is way too big though, you'll know all about it when the oil is gone!

Can you afford not to build a <A href="http://www.viking-house.ie">Passive House</A>? <a href="http://www.viking-house.co.uk">www.viking-house.co.uk</a>