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How to insulate floor trusses used in the ceiling

Edward Lord | Posted in Energy Efficiency and Durability on

We are building a 2 story lake front home in southeastern tn. The basement is 10 feet tall and walkout with about half the wall being poured and the other half being 2×6. The slab has 2 inch thick foam around the perimeter. The poured wall will have interior foam between the studs. The plane between the basement and first floor are 16 in flooring trusses.

The first floor ceiling is 16 inch thick flooring trusses and then floored with advantech in the attic to create a floor in the attic. The attic roof is stick framed and open with large windows for future use and is about 2500 sq feet . We would like to use the attic at some time in the future and thus the reason for the attic flooring.

For now we would like to isolate the first floor from the attic for hvac purposes .We would like to condition only the first floor /basement for now with the possibility of conditioning the attic later

Now for the questions
1) How do I insulate the ceiling of the first floor (which is actually 16 inch flooring trusses covered in advantech flooring and is the attic floor) . I cant used standard batts as the voids of the flooring trusses would not fill.I am thinking about spraying open cell foam at about r20-40 ( as this for now will be my house “attic insulation”) To spray foam at r40 is very expensive and was quoted at about 20k for 3500 sq feet ( quote for r20 was 10k)

2) I will later use the attic as it has been stick framed and engineered to be open. The attic will have a large cathedral ceiling created by the roof joists which are 2x10s. I plan to insulate the attic roof later with either foam or standard batts , I am not sure which yet. I am rather confused as to whether this should stictly be foam or a flash and batt of foam and fiberglass batt.

I am not an engineer/builder ( as is probably obvious) and my bulder has limited info /experience with foam.My building knowlege comes from this web site mostly.

Thanks in advance for ANY help / suggestions


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  1. John Klingel | | #1

    How are you going to ventilate (unless you spray foam it, which I guess does not need ventilation) and air seal this "roof"? Why fiberglass, when there are better insulation materials? I think you could insulate w/ foam sheets and Great Stuff the edges and gaps, then re-use the foam pieces in the real roof later (same measurement on-center, of course). Unless someone else says not to, ventilate over the foam sheets. Laborious, but less money than spraying????

  2. GBA Editor
    Martin Holladay | | #2

    If you are fairly certain that you will eventually be using the attic, then the most logical place to install your insulation is in the sloped roof assembly.

    Here's my advice:
    1. Insulate your sloped roof.

    2. Be sure that your insulation at least meets minimum code requirements for your climate. More is better.

    3. Don't use fiberglass batts.

    4. Read this article: Creating a Conditioned Attic

  3. Edward Lord | | #3

    Martin and John
    Thanks for the input.
    My thought is to try to create two "climate zones" (attic and below attic) for now . I am not sure how i will use the attic and thus for now I was trying to isolate it from the house below(like is tradionally done) and then have the potentional to use it later.The attic is about 2500 or so sq feet( a relatively simple gable roof) and peaks to about 20 feet.The volume of space seems huge to me and i worry about the cost to condition it , thus the idea to isolate it from the rest of the house (of course this generates cost to isolate it).If i put the thermal envelope up to the roof vs the attic floor, will this not substantionally increase conditioning cost.For instance if the volume of space in the attic is 50% of the volume of space in the floors below will the cost to condition it be 50% of the cost to condition the space below the attic.
    I have read your article Martin about conditioned atics(along with a lot of your other articles,which are all great by the way) and the million dollar question is which technique to use .The flash and batt technique of the roof deck using open cell foam and fiberglass batts below seems appealing to create the weather seal and insulation at a ? reasonable cost
    Another option , I was think about for the roof undersurface is to use techsheild as the roof decking , create a 2 inch air space below it for ventillation and then use some sort of blown in insulation at about 7 in thick and then to cap it off with 2 inch blue foam board strapped to the roof joists(2x 10).
    This sounds expensive though!

    So in a nut shell do i
    1) Isolate the below attic space from the attic space , and if so how? How do i insulate between the attic floor trusses.This may generate needless cost if the attic is later turned into conditioned space.Is this idea to isolate the attic space to save conditioning cost way off base or reasonable?

    2) How much and what type of insulation/sealing do i do to the attic roof .

    3) If the volume of space in the attic is 50% of the volume of space in the below attic area , is the cost to condition this attic space 50% of the cost to condition the space below( ie first floor and basement)

    This all seemed straight forward 1 year ago at the design stage and now I am at the framing stage and still have not decided on the interior insulation

    Thanks again!

  4. Jim Merrithew | | #4

    What is the surface area of the roof? You mentioned the attic floor space is 2500 SF. How much more insulation would you need to do the roof instead of the ceiling?

    You said you plan to install flooring. If you insulate the ceiling and add flooring to the attic, then, when you convert the attic into conditioned space, you will have to remove/lift the flooring in order to transfer the ceiling insulation to the roof. You would also have a vapour barrier in the ceiling and another one in the roof. This process looks duplication of effort.

    If you are already planning to invest time and money in ceiling insulation, why not go a little extra and do the whole roof instead? This could save a lot of work and money in the long run.

  5. James Morgan | | #5

    And on the other hand -
    if you are uncertain that the attic space will be called into play why not go ahead and blow cellulose into the truss space. This will be way less cost than insulating the roof deck and way more effective as insulation (the insulated ceiling area will be 60% or less of the roof deck area). If the attic ever gets used, the lost cost of the ceiling insulation will be minimal.

  6. Edward Lord | | #6

    The surface area of the roof is about 4800-5000 sq feet (as measured by my plans) and its peak is about 20 feet above the attic floor .The house is built with floor trusses for the attic floor / main floor ceiling and then advantech flooring was glued and nailed down for the attic decking .The house is just completing framing and thus the space between the attic and main floor is open from below and floored above thus any blown in insulation would have to stick to the underside of the attic decking prior to installing drywall (ie foam)or be blown in after the ceiling drywall is installed in the first floor(ie cellulose of fiberglass). Can i put pot lights , wiring etc in the space between the first floor /attic install drywall and then blow in insulation into the cavity created by the 16 in attic floor trusses?

    As i am thinking about it , it seems like i can insulate the 3500 sq ft first flooor ceiling or the 5000 sq ft roof decking.The disadvantage to extending the insulation envelope to the roof decking is that i now have to condition about an extra 38000 cubic feet of space

    James your idea of blowing in cellulose into the truss space sounds interesting and especially cheaper.How does one do this with the can lights wiring and maybe ducts sandwiched between the attic advantech decking and the frist floor drywall ceiling?I guess I would have to cut holes in either the advantech flooring or the drywall first floor ceiling. I recall seing a see thru membraine that cellulose is blown thru and holds the cellulose in place in a dense pack.Does this seem the way to go in that i would staple this membraine to the first floor ceiling prior to the drywall installation??


  7. John Brooks | | #7

    why not "think outside of the truss"?
    the glued down advantech is probably a fantastic air barrier
    You only need to "connect" the advantech deck to your wall air barrier
    add ventilation baffles
    blow cellulose galore on top of the attic floor deck
    no worries about wires, ducts and can lights

    It might be similar to my attic floor deck air barrier concept

    then if you expand into the attic later... the cellulose could be recycled

  8. James Morgan | | #8

    Yes, the cellulose can be blown behind a ceiling scrim after electrical and mechanicals have been installed, this is standard practice and quite straightforward. Just make sure the can lights are IC rated. You'll get a great insulation value and the attic floor will be free and clear for storage.

  9. John Brooks | | #9

    I have been thinking about site built ventilation baffles lately and how labor intensive they would be.
    Another thought would be to use off-the shelf baffles and a judicious amount of spray foam....
    to create a dam (container) for the cellulose.

  10. Edward Lord | | #10

    Interesting about the insulation on top of the advantech and then the insulation to be reused later.Dont think the wife would go for it as it would disrupt attic storage for now,but certainly needs some thought!

    Probably dumb question but what is scrim? I read on this web site where blown cellulose could NOT be used in a ?horizontal application(info section on blown cellulose- )

    Q. Is it possible to install damp-spray cellulose overhead, in an open ceiling?

    Hulstrunk: No, cellulose insulation cannot be sprayed overhead — except for specific cellulose-based fire-retardant products with a high glue content that are used as thermal barriers over steel or foam insulation.

    Yet I see it used in cathedral ceilings behind membraine(?insulweb).How would it be used in a ceiling under the advantech?Would the drywall be installed for the ceiling and then multiple access points/holes placed and then blown OR would a product like InsulWeb be used and then insulation blown on top off. The option of blowing cellulose on top of insulweb sounds good , but would the weight of the cellulose not cause the insulweb to pullaway from the ceiling floor truss. also would there be a bulge between trusses from the weight of the cellulose.

    Thanks everybody for the thoughts

  11. John Brooks | | #11

    funny that you mentioned wife would not go for it.
    I don't think Storage Galore is such a good idea

    Build it and She or He will Fill it

  12. Edward Lord | | #12

    Ha Ha !!! Love it, but she has her eye on it already , I need to watch my back if the storage dissapears!! Also found what scrim is . Sorry for not checking before asking

  13. John Brooks | | #13

    ok, build a temporary SMALL(notice I used caps) room above for dear wife's "stuff"
    at least it can be semi-conditioned

  14. James Morgan | | #14

    Yes the scrim will sag slightly until the ceiling drywall is installed, then it will be perfectly flat and secure.

    By the way, I doubt you need to enclose a space above to use it for general storage. It's done all the time without adverse consequences.

  15. Edward Lord | | #15

    So to make sure I got it right
    1) Ceiling of main floot is 16 " floor trusses,16" apart, that will have some duct work ,wiring in it.On top of this is nailed and glued advantech.Currently the underside is open as it will be the main floor ceiling and be drywalled later.
    2) my options are
    A) insulate the cathedral ceiling attic(10 inch lumbar) and pay to condition this attic space from the beginning OR
    B) insulate the floor trusses(attic floor-main floor ceiling) with either blown in foam OR blown in cellulose.The cellulose to be blown in behind scrim or insulweb while the horizontal first floor ceiling is exposed.Eventually when the attic is used for more than storage to insulate the roof deck .

    What about the referrence above from GBA which states that blown in cellulose cant be done in a horizontal ceiling?( could this just refer to an open ceiling?? and what is an open ceiling??)
    Great info guys/gals to help me think thru this!!!

  16. John Nooncaster | | #16

    Points of clarification--

    If the cellulose is installed in the truss space--

    Would the attic then need to be ventilated?
    Why would it be necessary to remove the cellulose from this truss space if the attic later became unvented?

  17. James Morgan | | #17

    Response to John N:

    1. If only the ceiling truss space is insulated the attic must be ventilated.

    2. If in the future the attic is insulated at the roof deck there is no particular reason to remove the ceiling insulation unless you just want to. It will provide some sound separation between the floors and will aid in zoning the upper and lower areas separately.

    3. This is true whether the future roof deck insulation is installed as a vented assembly or not.

  18. John Nooncaster | | #18

    So, Ted could insulate the truss space now with cellulose and spray the underside of the roof deck the required amount of foam to prevent condensation ( a few inches of either open cell of closed cell depending on the exact climate zone). Then there is no need to ventilate the roof deck. Money is saved as compared to foaming the entire roof deck to R40 with foam. When that space is occupied, the roof deck insulation can be upgraded with more foam or cellulose to R40. The attic stays unvented at all times.

    Anything wrong with these plans?

  19. James Morgan | | #19


  20. GBA Editor
    Martin Holladay | | #20

    Q. "What about the reference above from GBA which states that blown-in cellulose can't be done in a horizontal ceiling?"

    A. The Hulstrunk comment you read was in reference to damp-spray cellulose installed in an open cavity without InsulWeb.

    Horizontal ceilings are insulated with cellulose all the time. In most cases, the cellulose is installed from above after the ceiling has been drywalled.

  21. Edward Lord | | #21


    I am not sure what James meant by "pass"

    The suggestion of John Nooncaster to use spray cellulose and then a thin layer of foam on the roof deck sounds good.My original plan was to use techshield roof decking with vents and then insulate the attic floor/first floor ceiling with foam .The cost of the foam was a surprise and now I am looking for other options.The cost to use techsheild and vents is about 1500$ over the cost to use standard roof decking without vents and so if Johns option is reasonable this would reduce the foam cost by 1500$.I could spray 2 inches of foam against the roof decking ,not use techsheild and save on vents for a total cost of about 4-5000$.
    As the attic decking is already in place any insulation would have to be applied from below(ie the first floor) prior to the drywall? I dont see how this could be air sealed around the pot light cans.Any suggestions or articles about this?
    Any thoughts on the effectiveness of techsheild?


  22. John Brooks | | #22

    Ted, since the house is currently under are under the gun to make a decision that may vary in cost by $thousands$ ... and if you make the wrong decision...potential not-so-good consequences.
    Martin is probably the only one who "slept in a Holladay Inn last night"

    Hulstrunk:"The intent of the code is to prevent moist air from having contact with the sheathing"

    How many years before you expand into the "bonus space"?
    why not post a few photos
    does your first floor include a "code-size" stair to the attic/bonus space?
    how many square feet of attic storage space does your wife "need"?

  23. John Brooks | | #23

    Ted :"The disadvantage to extending the insulation envelope to the roof decking is that i now have to condition about an extra 38000 cubic feet of space"
    actually... the volume is not-so-relevant
    air-tightness, exterior surface area, R-value and WINDOWS are more important factors.

  24. James Morgan | | #24

    More to Ted: your floor deck can be the air seal so you don't have to worry about the can lights. Just check it from below and seal any gaps with a can of Great Stuff before the scrim and cellulose go in. By the way the cellulose is blown in dry, not sprayed - the latter term usually refers to wet application as sometimes used in walls.

    John B: yes, the additional area is critical but the volume can be a factor too especially in the heating season if it allows hot air stratification resulting in a higher Delta-T through the insulation. Admittedly this is more of a problem in a vaulted ceiling than in Ted's case where an intervening floor structure will mitigate this effect.

  25. James Morgan | | #25

    By "pass" I meant a truly informed response to John's scenario is above my pay grade. Having said that: I'd be concerned about unintended consequences of a hybrid approach; if there's 16" of cellulose in the ceiling there's going to be precious little leaking of heating/cooling into the attic to mitigate potential moisture problems in a sealed but under-insulated space; we all know how to reliably make vented attics work; it sounds like an unnecessary expense to avoid a simple venting of the roof. Plus, for macro-environmental reasons I have a gut aversion to whimsical use of spray foam.

    But that's just my opinion.

  26. Edward Lord | | #26

    james : I thought maybe it was not a simple increase volume of conditioned space and one increases conditioning cost by an equal proportion and so that is why i am considering just extending the envelope to the roof deck.Any thoughts on the percentage increase conditioning cost to simply extend the envelope to the roof deck,which would actually increase the conditioned space by about 50%

    The advantech is glued/ nailed down already , does this not provide an adequate air seal already? Where would I use the great stuff ?along every connnection between the advantech and floor trusses ,any deck penetrations of wire etc,or around any can lights ,hvac outlets, electrical outlets.

    The attic access will be by formal stairs and be part of the stairway from basement to attic.There will be a exterior grade/sealed door at the top of the stairs to access the attic.There will be 12 windows in the attic and they are all high performance low e glass windows.
    I am attaching some pics to hopefully give an impression.


  27. James Morgan | | #27

    How much more it would cost to condition the attic? The most important factors in this calculus will be the differential R-value of the insulation at the enclosure and the additional area of the enclosure. You have already calculated the second of these. The differential R-value will depend on how you insulate the roof deck, the worst case scenario being getting talked into a code-deficient but expensive R-20 spray foam treatment compared to an easy R-50 with 16" of cellulose at the ceiling.

    The other side of the question is though, how important is it to have conditioned storage space? If the answer is not very, then any additional construction cost and energy cost makes little sense.

  28. John Brooks | | #28

    Ted, your house/attic looks similar to what I imagined.
    Except even more exterior surface area...don't forget the side walls.

    If it will be several years before you expand into the attic...
    I still think the best use of $$ and resources would be to add ventilation baffles and blow cellulose on top of your existing almost perfect air barrier...
    I don't agree that unconditioned attics are a good place to keep stuff....
    but you could build a storage platform/deck above the Cellulose or even better a small insulated semi-conditioned storage room.

    The Thermal Barrier should be in Full contact with the Air barrier... so if you choose to blow between the trusses ...then be sure to FULLY fill the 16" cavity and keep your fingers crossed.

    Have you read Lstiburek's "Don't be Dense" article?
    Lstiburek:"I’ve got news for you. Even with trying to get a perfect
    air seal, dense packing a cathedral ceiling or flat roof will not work."

  29. Edward Lord | | #29

    For now the attic will be a place for nice views,storage, and maybe potential living space for grandchildren in 5-10 years.For now it feels like I could play basketball there ( did not seem this big at the design stage). Thus for the very near future(next 3-5 years) we will not be actively using it.
    I had not read Lstibureks article till now(interesting read) . It seems to me that the issue was the cellulose "loosing or never obtaining" its dense packing and falling away from the roof decking with thus inadequate venting below the roof deck. Then voila! failure!

    In my case I will have a "20 foot Vent space " above my attic floor and thus any moisture that did migrate to the underside of the attic flooring could dry to the attic space.Is this correct?

    To use cellulose in the 16" truss space with can lights , occ ductwork(mostly appliance vents) plumbing vent stacks seems like a good idea.Can this be dense packed with cellulose with the use of scrim or insulweb below?? I am not sure I understand where to put the air seal (at the level of the underside of the attic floor or at the first floor ceiling level?)

    Also what would be the rough cost be to dense pack cellulose into about 3500 sq feet on 16 " deep floor trusses? ( 4700 cubic feet)

    What about doing a tradional roof (techsheild decking) with vents and then dealing with conditioning this attic space later if truly used.?

    What about the techshield , my rough estimate is that is about 1000$ extra to install vs tradional decking? Will this keep the attic at a lower temp and help with the rest of house conditioning and make the attic more comfortable for now untill (if ever) it is properly insulated??

    Lot of questions I know but as you guys help me think thru this ,my questions are becoming more focused!!

    Thanks everybody!

  30. John Brooks | | #30

    Ted, I am extra interested with your situation because I think you may have have accidentally built the "perfect ceiling".... much more buildable than the "perfect wall"
    Too bad you will have to screw it up someday when you expand into the attic.

  31. John Nooncaster | | #31

    I'm in a similar position to Ted and not trying to hijack but understand a hybrid approach to "our" problem.

    The other John wrote:
    "The Thermal Barrier should be in Full contact with the Air barrier... so if you choose to blow between the trusses ...then be sure to FULLY fill the 16" cavity and keep your fingers crossed."

    Wouldn't this problem not exist if the roof deck had a minimum amount of foam insulation?
    (Minimum as required by zone to prevent condensation)

    Does it make sense to pile cellulose in area that will be used in a few years (on the advantech)? Especially if some roof deck insulation would prevent the Lstiburek dense pack problem? Am I missing something?

    Isn't the jest of Lstiburek's article that in general, foam is a necessary evil for roof insulation. It provides air sealing in turn protecting the sheathing. Ted still has the option to use rigid foam above the roof deck or spray foam below to create a conditioned attic space.

    Ted, how would you insulate the second story if you were to complete it now? Are there a bunch of knee walls. Any attic space? Vaulted areas? Where is the duct work located? I know this is still down the road but it might impact today's decisions.

  32. Edward Lord | | #32

    Now there are very few knee walls in the attic and for now there will be no hvac in the attic.My hvac will be in the basement and then drop conditioning into the basement and push conditioning into the main floor as i have floor trusses between the main floor and first floor. IF i decide to condition the attic area i am prepared to put a separate hvac unit for its sole purpose.When it comes time to insulate the attic for conditioning it i am cosidering two approaches
    1) create a 2" vent channel under the roof deck--insulate with either fiberglass or cellulose and then place 2" foam across the roof joist and held in place with strapping(giving me 7 in of insulation and 2 inches of foam--see Martins article about creating a conditioned attic and referenced above)).I will use knee walls in places and then insulate up the vertical knee wall and along the roof decking(as above) and then a horizontal reg ceiling at about 9 feet tall and then blow insulation on top of this horizontal new attic ceiling
    2) foam the whole darn roof decking BUT I am a ittle untrusting of using r20 foam( as manufactures recommend) vs r40 (seems right to me).The cost to foam the whole roof to r38 open cell is 20000$

    I thought about foam over the roof decking(we are placing exterior foam on all the walls(1.5 inch dow and .5in sis dow with 1 inch dow at the structural corners and where bracing is necssary).This will be covered in tyvek and then a rain screen .Both the foam and tyvek will be taped. The cost to do foam on the roof rafters and then a decking is about 6000$-10000$(my rough estimate)

    Any help on the
    techsheild questions and
    the ability to dense pack foam in a 16 inch truss space and
    how to provide an air barrier if the attic floor truss space is my "roof insulation" until i use the attic

    Great help people!!

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