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Question DetailsAsked on 12/19/2013

What recommendation for for foam board and tape for gaps in insulation between studs in attic?

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7 Answers

Voted Best Answer

Am assuming you are installing foam board over existing insulation. Old standard for airsealing was accoustical sealant caulk, stays flexable but seals the leaks,, Other possiblilites are an expanding foam,, or if you are looking for a tape, I like the stainless steel tape, available at the big box stores (lowes, menards etc), for foam board use green, yellow or blue insulation,it has a higher R value than white polystyrene. Also is available in 2 foot by 8 foot panels so you can get it up an attic scuttle door,, Good Luck,,

Jim Casper old energy conervation guy

Answered 3 years ago by jccasper



Foam is used most often as they are probably already using it in the attic.

Are you sure about the application? Sounds like foam over some kneewall locations on a dormer or Cape home.

Can you post up a picture?

Acoustical sealant works well like jcasper said as well.

Foam seems to be more prominant just because it is used for other stuff when up there and it is easier to dispense.

Answered 3 years ago by WoWHomeSolutions


It is funny how everyone sees something different in a question, a picture would be worth a thousand words.

If by studs the previous answer about it being a knee wall would be correct, I might add that if that is the project before applying the foam board you should fill any gaps in the fibreglass insulation with fibregass as well. When that is done you could nail the foam board over the back ot the studs using the foil tape the joints and canned foam at the floor ot bigger gaps. I happen to like the foam products with foil on both sides as it has a higher R value than most of the others inch for inch. Parden the spelling but I believe it is a polyisrocyranate foam. It is most often yellow in color.

My take on the question is what your are calling studs are in fact the floor joists that supports your ceiling, the insulation in time developes gaps due to work done in the attic and improper installation. If that is the case the same advice would apply about filling in the gaps with additional fibreglass and then using the foam products.

Cutting the foam to fit between the joists is difficult to get a tight fit and care must be taken to not compress the insulation below and the edges and gaps would best be sealed with the spray foam as the tape will not hold too well to the joists since there is probably a good bit of dust on the surface and bending it for the long runs involved will be difficult if not impossible. If you were to lay the foam across the top of the joists it will be easier but more likely to get damaged by anyone entering the attic and a layer of plywood would have to be used for storage and at least working paths for any future service work in the attic. If you do go for the over the joist method fibreglass insulation should be added to fill the joist space at the ends of the runs so as to block cold air entering any space left, just do not block air flow to the vents in the eaves.

If you do use the spray foam pick up a bag of the cheapest plastic gloves you can find and if you do get it on you have nail polish remover handy, works like a charm.


Answered 3 years ago by ContractorDon


A few add-on thoughts to the other comments:

1) As I think it was Don said, it is almost impossible to cut the board to exactly fit between the joists or studs - plan, after all the foam board is in, on going along in each stud opening and foaming the open gap along all edges with foam in a can - likely to take a number of cans. A trick some of us use up here is to cut the foam boards the right width and length (with a 1/8" undercut) to a cut schedule on a table saw (cleaner cuts and faster when doing a lot), then take an electrical box cutout tool (mini router) and run a shallow groove down the length of each edge centered on the board thickness, and every foot or so across the edge. When installed, the crossways notches provide a place for the plastic tube of the foam can to fit in, and the longitudinal groove an easy way for the foam to expand along the edge, so it is spreading out and expanding into the gap from all along the edge, not just a few isolated injection points. Another way to do this, but takes two people and the boards have to be test fitted first because there is no going back, is to have one person apply a bead of the slower-setting low-expansion spray foam all along the contact surface on the studs, then the other person quickly shoves the foam board in place into the still soft foam, then apply more foam to the gap at the edge to seal it as necessary, cutting off the extra when hardened. Obviously, grooving the boards is not needed with this method.

2) The issue of making sure the boards have a walk-on ability if they are on the floor is vital - otherwise the first time a home inspector or electrician or whoever goes up there and walks on it, you will have broken boards and possibly someone putting a leg through the ceiling drywall.

3) Be cautious about covering electrical and plumbing and HVAC and such - because future access will be very difficult. If you have to cover up some utilities, it would be helpful to mark their approximate locations on a diagram or a set of photos, then transfer that to the insulation surface with heavy colored marker if the insulation will be exposed permanently. Remember, you cannot legally cover up any operating equipment like HVAC splitter valves, or electrical junction boxes.

4) if the insulation will not have drywall placed over it, check your local building code- in some jurisdictions, foam board (which is highly combustible in its native form) has to be either the much more expensive low flame spread and low-smoke variety (generally also not as good an insulator), or has to be covered by a fire rated sheathing like drywall. In some jurisdictions this applies only to habitated spaces, in others to attics also - especially in multi-unit buildings.

5) As far as surface seam sealing, in my career I have seen only one tape that lasted indefinitely - and that was a predecessor of duct tape used by the military in the 60's and 70's that is no longer available because it was highly combustible. Under attic moisture and heat changes, all tapes start disintegrating or peeling loose over the years, and especially on foam products as the outgassing solvents in the foam dissolve and weaken the adhesive on the tape, and dissolve the tape itself. Hence, the metal foil tape is better for this use (but expensive), but eventually the glue will stop adhering, so the acoustic caulk is much better. However, you cannot effectively apply that below about 45 degree surface temp as it will not stick, and you need about 60-65 degree tube temp for it to come out right. There are butyl ones that work at lower temps, but the ones I have seen are VERY pricey - on the order of $40-50/tube versus about 7-12 for contractor size (32 oz) tubes.

6) Pay attention to vapor control - is you are putting solid sealed foam board over joist spaces or existing insulation, you are creating a situation where you are installing a "negative side" vapor barrier - the foam board acts as a vapor barrier, trapping moisture coming in from the house side, which can condense and cause frosting and condensation and eventually rot, the same as if you put plastic sheeting over insulation in the attic. I have seen a number of homes in our severe winters area where foam board, or more commonly sprayed in place foam, was placed over existing fiberglass or cellulose insulation lying between joist spaces, and ended up rotting the joists - in one case to the point of causing a complete roof section collapse and wall splaying out in less than 10 years as the joists became so rotted that could not carry the tension loads from the roof any more. In addition to extreme care sealing all penetrations and seams against moist air flow from below, one solution used here is to actually drill 1/4" holes in the foam board every 6-12 inches each way to provide venting, even though this reduces the insulating value somewhat, and unless underlain by a continuous gap between the board and the underlying insulation, is not very effective.

7) The recommended solution, if the client insists on foam board rather than a more appropriate and more vapor permeable material like fiberglass, is to actually provide a ventilated air space between the underlying insulation and the foam board with firring strips. The best solution overall is to adhere to the general rule (except in a few perenially wet areas where reverse side vapor barriers are used) that each layer, from inside to outside, should be at least as permeable to moisture as the preceding layers. Obviously, an exception is made on the inside architectural surface as most people (and fire marshalls) would not like exposed plastic vapor barrier on the inside of the house, and commonly used outside sheathing products are less vapor permeable than batt insulation of course, as if house wrap. That is one reason why many houses with particle board or fiber board sheathing or impermeable house wrap have extensive rot problems in the walls - the sheathing, between the wood and the heavy glue, is so impermeable to water that while it may protect somewhat against water getting through the siding, it commonly has condensation problems from the inside moisture being unable to escape. This is one of the big problems with the EIFS systems, which use an exterior foam as the siding and are now creating such a large number of very severe house degradation problems both because interior moisture buyilds up between the foam and the underlying sheathing, and also because any water that gets through waterstops and flashing and down behind the foam exterior has no way to escape.

8) If you are actually taking about putting board insulation between the rafters (directly under the roof sheathing) do some research - putting insulation there increases the summer roof heat and can damage your shingles, plus puts the sheathing in a pressure cooker environment which can result in rapid sheathign degradation, especially if any moisture build up in it.

9) If you are talking truly between studs as in a knee wall, be sure you are not totally blocking off access to the area behind the kneewall - you should leave access panels and crawlspace for inspection, and ensure that area has adequate ventilation so it does not become a dead air space and build up moisture that can lead to mildew and eventually rot.

Answered 3 years ago by LCD


Thank you all for your advice. I went to lowes last night to purchase the blue board and informed the lowes worker of my project. I live in the Deep South. We have high humidity year round. He is concerned that if I secure blue board over the existing insulation that condensation will occur in the summer months and cause mold. I spoke with my energy auditor and she said that the blue board is designed to breathe some and therefore we wouldn't have moisture issues.

Any advice would be appreciated.

Answered 3 years ago by Guest_9534934


Without know what material you were looking at, its impossible to know the vapor permeability rating and what the recommendation should be.

What is the exterior construction of your home, the location, and the insulation type and depth.

You are correct in that an improperly placed vapor retarder level and type can destroy a home from within.

WoW Home Solutions

Answered 3 years ago by WoWHomeSolutions


I guess my mind wandered when I firsst answered your question, because I failed to ask one of the most important questions - why are you considering blue board rather than just adding more insulation of the type already there to fill in the gaps ? Board insulation is pretty much guaranteed to be much more expensive, and you said you are putting it between the studs so you do not need the self-supporting or flat backing board characteristics like you would if you were adding it OVER the studs and then adding drywall as a finish surface over that. I would recommend using the same insulation you have, increased thickness to full stud depth if that is what you are going for.

Particularly in a primarily cooling rather than a heating environment, I don't think you will really notice the difference in R value between the products. With forced air A/C (as opposed to radiant systems), a normal house loses far more of the cold through air losses than through conductive heat transfer anyway, unless it has been made a "tight" house. Too complicated to explain here, but it is different than with heating because of the different convective situation and the "stack effect" considerations.

As for the blue board being moisture permeable - I am surprised at that answer. Unless it is a very cheap low-density "popcorn" foam (which will have an R value comparable to or less than fiberglass), it is NOT going to have adequate moisture permeability to take care of any moisture that accumulates in the wall cavity. This is particularly true if it is closed cell like the Dow Hi series boards, which have virtually no air or moisture permeability. Again, we are sort of shooting in the dark here not knowing your exact situation, but unless this board is going to be right behind the drywall and underlying vapor barrier, on the house side of the wall from the existing insulation, I would NOT consider using anything that could act as a vapor barrier - and the includes spray in foam and faced insulation also - if you use batt, use an unfaced batt - no paper or foil on either side.

One other consideration not yet discussed but particularly pertinent in attics is the fact that you WANT some air circulation in your walls - because of the differences in air pressure and temperature daily and seasonally, you WILL get conditions at times in attics where you have air reaching the dew point (where condensation starts) - the issue is to get rid of it as soon as possible so mildew and mold does not take hold, and in habitated areas so you do not start getting a "musty" smell just from buildup of odors from the building materials in their natural state. While your wall might theoretically have adequate moisture pemeability to let moisture dissipate, without some minor airflow you WILL get odor and local mildew situations - typically near the top and outside surface of the walls in cold outside conditions, and toward the bottom and inside surface in hot conditions with cool or air conditioned interior surfaces. Therefore, firberglass tends to be the best solution in those areas, because it allows some air migration, unlike board foam and the commonly sold and misnamed easily matted "cellulose" which is actually shredded newspaper rather than actual cellulose fibers. Also, because of the guaranteed sporadic mildew growth conducive conditions, fiberglass (or rock wool around high temperature areas like furnace ducts and chimneys) is tobe favored because it is non-organic so does not readily support growth of mildew or mold, and can actually be bought pre-treated with mildewcide.

An aside, but critical to the overall situation, is to be sure whatever you do (assuming this is a typical peaked roof atti with open eaves) is to maintain the airflow from outside into the "unconditioned" part of the attic, and along the underside of the roof to (hopefully) ridge vents, or at least to a ventilator or gable vented attic space, so the hot and moist air in the attic can be vented continuously out to the outside. Without that type of continuous airflow you have to have all other aspects of vapor control and insulation perfect to avoid problems, with good attic space ventilation it can significantly mitigate against the bad effects of defects elsewhere in the building envelope.

Answered 3 years ago by LCD


Once again, thank you for your responses. You have given me much food for thought. I have decided to not install the blue board but to instead add insulation as suggested. Have a Merry Christmas!

Answered 3 years ago by Guest_9534934

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