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Whole home insulation ideas

In addition to my other post I would like to know about the best approach for certain areas of the house here are my concerns

ATTIC: 2" of open cell foam along bottom cord of trusses to seal drywall, also spray foam where top plate and drywall come together, also where insulation baffle meets top plate. 12 inches of blown cellulose on top of the foam
QUESTION: Is this a good plan to seperate the attic from the conditioned part of the house?

BONUS ROOM OVER GARAGE: 3" od open cell in ceiling of garage to keep floor warm
QUESTION: Floor area of bonus room is a concern. I have radiant floor heat in the garage floor concrete below and would like to allow that radiant heat to assist with keeping the above garage room warm. would 3" open cell in the ceiling of the garage under that floor be to much for thew radiant heat to work through? Is there a better idea?

BONUS ROOM KNEE WALLS AND ATTIC: Sealing off exterior end of the attic where it meets the soffit to create a thermal barrier, then 3" of open cell in the back of the knee walls against the drywall. 12" of blown cellulose on top of drywall
QUESTION: Where is the best place to create the thermal barrier. Part of the bonus room attic is above a porch and joins the main part of the house. My idea is to keep the thermal barrier as far away from the bonus room knee wall as possible to help with the conditioned space. Is open cell a good choice in this area?

BONUS ROOM CEILING: Open cell against ythe roof deck from the soffit to the ridge vent
QUESTION: Is open cell ok against the roof deck and how much air venting should I allow below that spray foam....every other rafter or every third rafter?

Asked by Anthony Hughes
Posted Aug 8, 2014 1:05 PM ET
Edited Aug 8, 2014 2:24 PM ET


9 Answers

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It doesn't take a full shot of 2" of open cell foam to seal the truss chords to the gypsum, but it should work. Separating the attic from the conditioned space is fine, if it's not being used for routing ducts, etc. Attic venting would still be useful in heating dominated climates, but in humid cooling dominated climate venting the attic will usually raise, not lower the moisture content of the wood & cellulose.

It's very difficult to adequately air seal behind kneewalls which also needs sealed air dams at very joist bay. It's usually easier to make the roof both the pressure & thermal boundary of the house in rooms with partial cathedralized ceilings.

If you're insulating the roof deck you DON'T ventilate that space (at all!). Whether you can get away with using open cell foam is climate dependent. In climate zones 1 & 2 it's pretty safe, in climate zone 3 it may depend on your roofing color & shading factors. In zones 4 & higher you would need some sort of interior side vapor retarder (but NOT a true vapor barrier like 6 mil polyethylene sheeting or foil.)

Answered by Dana Dorsett
Posted Aug 8, 2014 3:18 PM ET


Thanks. I was concerned about the open call against the roof deck but it is very common in this area. The bonus room does hav a ridge vent but not a lot of attic space...only about a 3ft wide 3 ft tall triangle area at the top of the roof peak. What would you suggest if not going the foam route?
Also, in the attic, all my duct work is in the attic and the attic is vented. I was thinking if I could keep my conditioned air in the envelop of the house then I would be working in the right direction? Do you not think this is a good idea?
I have another post about the wall stack up I would really like your input on as well! You have helped me in the past and I really could use it now! Trying to get started insulating next week!

Answered by Anthony Hughes
Posted Aug 8, 2014 3:41 PM ET


You haven't specified the location of "..in this area", which makes it hard to parse what methods are good/better/best here.

Ducts in a vented attic are a recipe for higher energy use. If you insulated at the roof deck you need to air seal the attic or the venting becomes a thermal & moisture bypass. When inside the pressure envelope (a sealed attic) duct leakage has fairly low consequences to infiltration drive & energy use, but in a vented attic those consequences can be quite large.

Answered by Dana Dorsett
Posted Aug 8, 2014 5:26 PM ET


I am in the 47460 area code. Climate zone 4 right on the line of 5.

Answered by Anthony Hughes
Posted Aug 8, 2014 10:08 PM ET


If you are considering an unvented conditioned attic, you might want to read this article: Creating a Conditioned Attic.

Answered by Martin Holladay
Posted Aug 9, 2014 4:23 AM ET


It seems like you are making this as complicated as possible? Is your garage fully conditioned? Just radiant heat? How warm are you planning on keeping your garage? What is the "bonus room" used for?

You're best to just THOROUGHLY air seal and insulate the bonus room from the garage and not try to count on any kind of "bonus heat" from the garage rising up and making the floor warm in what is conditioned part of your house.

Answered by Robert Kohaus
Posted Aug 9, 2014 11:05 PM ET


Not trying to make it complicated Robert, I am trying to confirm the things I am hearing from the local insulation contractors because I am building above the skill level and experience level of most of them. Hardly anyone does foam outside the sheathing around here and the "Energy Code" is brand spanking new. I appreciate your help and am merely trying to do what's right. It's pretty rough comaring and ensuring it's right.

Answered by Anthony Hughes
Posted Aug 10, 2014 7:29 AM ET


To answer your questions, I am not using the rigid foam on the garage so the plan is to airseal the studs in the garage walls and use a BATT or blown cellulose. I want to keep the bonus room above as warm as possible because we intend to use it as a multi purpose space for the time being. Art studio, play space, TV area. We have a big family and lots of usable space is important.
I did not put any HVAC inlets or returns into the Bonus room because I was figuring I would install a mini split in the future to use in that space. Its a large room 14x50 with 8 ft ceilings and knee walls, it also has a 12 ft wide functional shed style dormer with 2 windows in it.
I was hoping to benefit from the well insulated garage and the radiant heat and at least keep the floor warm. Airsealing the bonus room and isolating it from the garage will be challenging because of the knee walls but I can focus the foam on an envelop between the two spaces.

Answered by Anthony Hughes
Posted Aug 10, 2014 7:39 AM ET


I have researched open cell and closed cell foam for a few years now. 2 to 3 inches of open cell for air sealing is typically achieved using "closed cell" and then applying cellulose over that for added fire resistance. I know of a top spray foam expert who installed the insulation in his home this way. Closed cell will give you better fire performance and a higher R-value than open cell and chances are greater you will not have to deal with odors when humidity rises into the attic space from the living space. There are many topics about open cell odor in attics from off-ratio installations to rotting roof sheathing due to high humidity. One serious consideration you should think about for the safety of your family is adding a fire suppression system (sprinkler system) to the home when using cellular plastics as insulation in conjunction with engineered trusses and engineered lumber. These products combined when ignited do not give occupants much time to escape a fire.

Answered by Richard Beyer
Posted Aug 10, 2014 9:40 AM ET

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