Spray foam can work.
**[quote=“Frank_Reif, post:21, topic:447, full:true”]
boulder-developments-super-foil-brochure-feb-21(1).pdf (1.8 MB)
Hi Frank and all,
I am going to use Superfoil SF19BB under my floor joists (as recommended by superfoil) as i am not talking the floorboards up. I do have 75mm of fiberglass put in years ago. Not much room to work there but will try my best to get airtight.
Hi John.
I’m not very confident in the advice from the article - or my understanding on the subject for that matter. But it does mention using rigid foil face foam insulation taped at the seams, which I would prefer. This will likely provide better results than the super foil because of the higher insulation value for your time, and because it provide support for the fiberglass insulation above it - preventing any sag over time and maintaining a good fit. I suspect that it would also be easier to make airtight than the super foil.
There are a number of other statement that I found surprising at first. Firstly that you can ignore ventilation rates from outside when considering crawl space surface temperatures, and that there is negligible heat transfer via convection or airflow from the floor to the ground. Instead, it’s dominated by radiative effects. Hence why foil faced insulation systems are suggested, because of the low emissivity of the aluminium. However, they are always vapor closed, so you have to figure our a means for the assembly to dry by other means, such as an air gap between the floor sheathing and the insulation.
What I don’t understand is that:
No details are provide as to where this air gap leads to and how air is recirculated.
The author also mentions careful details to protect form pests getting into your building from the crawlspace. Cement fiberbords are suggested. However, it’s not suggested that is should be foiled faced on it’s underside to prevent radiate coupling with the ground, which could cause it to be the condensing surface of choice in summer and making the hole foil/mold detail redundant.
Furthermore, I don’t understand why we can’t put the foil onto the ground instead of the poly membrane. It would eliminate the need for the foil insulation, and you can use a vapour open assembly that can dry into the crawspace, not requiring the ventilation gap in the floor system.
In other words, I’m not 100% whats happening here.
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Hi Frank,
I can understand your point of view, I may remove the old insulation and just use the sf19 bb this is vapor open.
The foil need a air space to work, it’s not to be put on a flat surface.
I have done there training course (1 day) and do like the advantages of the superfoil.
Your’s,
John
When correctly fitted I’m sure it’s fine as a vapour/airtightness barrier but I don’t believe that it is a substitute for insulation. You need to built up your R value/decrease your U value, which a foil product on its own won’t do, even with a layer of insulation sandwiched into it.
The insulation value is made up by the air it traps, it does work, it can’t be used “flat” but best with 2 air pockets front and back.
John
Sorry but to me that looks like sales blurb rather than peer reviewed scientific fact.
All insulation works like this or by reducing radiative heat transfer. Essentially it’s many tiny pores/pockets. If you compress insulation too much then these gaps get filled with the more conductive material.
What concerns me is that they do not include an example assembly with all the relevant details in the document. They show roofs, some floors, and walls, but not the crawl space.
At the moment we are relying on manufacturers to provide the information to use the miriad of different products.
Marketing should always be suspected. You should not be the only person who’s used their product for floors, so get them to send you an example build up of the floor, with air sealing and fastening details, what to do say walls when you need to attach it and seal it to brick.
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Hi, i did see the floor reading was missing. I have allready asked superfoil was it all right to use it under the jousts (I am not taking the floorboards up) and they said to use the bb, vapour open.
They did not give me a u value l will ask again and for full fixing instructions.
It only came on the market in the nineties and has passed all certification and is the only foil product with full approval.
Did they provide any diagrams? Details or products necessary for air sealing.
“Breathability” is a variable. Aluminium is closed. I suppose they get it to be breathable by having perforated layers offset so it’s radiatively opaque.
I suspect marketing for most manufacturers. The good ones will have tested each application and will provide information as to how it should be installed for warranty. See Gutex systems for reference - they provide detailed installation instructions.
As mentioned. The hard part will be maintaining air tightness. At the perimeter, sleeper walls, between rolls of insulation. Etc
May 2022 - John Obrien - Suspended Timber Floor - SF19BB - 0.32(1).pdf (180.1 KB)
Hi Frank and all,
1651690381065_May 2022 - John Obrien - Suspended Timber Floor - SF40BB - 0.22.pdf (180.1 KB)
Calculations are here but no drawing it looks like the foil is going straight under the jousts.
Will be held by 19mm staples or larger.and at the ends and overlap sealed with aluminium tape.
I wouldn’t trust any of that. There are incomplete tables, missing data, typos and missing lables/units all over it.
Someone correct me if I’m wrong here:
Vapour resistivity is the properties of the material regardless of thickness, and vapour resistance is taking the thickness or whole assembly into account. Therefore vapour resistance should not have the units GN.s/kg.m (m at the end being meters).
For example, the floor boards thickness they’ve assumed is 18mm. So 450 [GN.s/kg.m] * 0.018 [m] = 8.10 [GN.s/kg]
Anyway. I think you’ll be ok if you are very careful with taping the joints - they need to be rolled/pressed firmly on a dry/dust free surface.
The joints between the perimeter/sleeper walls will be from foil to masonry - which may be tricky to get good adhesion. I do not know what the best method to do this is - perhaps wire brush, dust off and then pva primer before taping with a dedicated tape? I don’t think standard foil tape is a good idea here.
If you want the fiberglass to effectively add to the thermal resistance I’d make sure it’s well fitted and does not sag into the foil too much.
I’ve changed my mind about a continous air gap above the cavity insulation and below the floor boards/chipboard. It might be helpful to redistribute concentrations of moisture. I suspect it would be attraced to the cold masonry around the perimiter (above the DPC) and then be wicked into the rest of the internal walls if it cant escape throught the floor or down into the ventilated void below the floor system. Perhaps you can use a spade bit to provide a few holes for the air to move past noggins. This is all conjecture mind you.
It might be worth using stainless steel staples - allign the staple parrallel to the joist so the tension accross the foil is spread more evenly across the staple’s width to limit the risk of tearing the foil or the staple rusting. As the staple will attract the moisture first.
I should not have to provide this advice, it should be part of using the product. So super foil need to up their game.
I agree wholeheartedly with all your caveats.
Regarding the tape I suggest a proprietary airtightness tape and primer. If in doubt Green Building Store are always good for advice.
I would use this tape for all perimeters and their own tape for lapping sheets of foil.
@Mike290 , hi, did you go ahead with the underfloor-spray solution (Q-Bot/ Elastospray)? Been thinking about it, for our 3-bed semi. I’m borrowing the borescope next week, to have a look without pulling up floorboards. Yet. It’s a small crawlspace (not a basement.)
I did! Expensive for a bungalow as the floor area is the whole property.
Did it make a difference? Hard to say. Given the energy crisis I dropped the house temperature down a degree or so anyway. I never expected it to pay for itself in my lifetime but I hoped it would make the house more comfortable, particularly on windy days; we are in quite an exposed location.
The insulation was installed in November 2022. Looking December gas use: 2021: 1624kWh 2022: 1329kWh. But, of course, there’s lots of factors affect how much you use in a given month.
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If you also have coarse old fashioned airbricks you can also look through those with the borescope.
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