Expanding foam advice for air penetration

johnnyboy08

Hi, I need to fill some large gaps (approx 1 cm) between joists/trusses and walls in an attic and was just wondering if there are some brands better than others?

Foam generally isn't air tight but is there a more dense variant that would be a bit more restrictive to air flow than others?

Lumen

I would hesistate to comment on your specific application, but I've used Soudafoam "Window and Door SWS" which claims to be airtight to "DIN 18542 MO-01/1" whatever that means.

http://www.soudal.eu/soudalweb/productDetail.aspx?w=8&p=498&ID=4227

I apply it with a metal gun applicator.

Note that PU foam requires quite a lot of depth (I think I read 50mm) to be airtight, for smaller gaps and cracks you're better off with tape.

listermint

johnnyboy08 wrote: »

Hi, I need to fill some large gaps (approx 1 cm) between joists/trusses and walls in an attic and was just wondering if there are some brands better than others?

Foam generally isn't air tight but is there a more dense variant that would be a bit more restrictive to air flow than others?

Why are you foaming between joists and trusses?

johnnyboy08

listermint wrote: »

Why are you foaming between joists and trusses?

Not foaming in between them, just on the end ones where they are adjacent to the exterior gable walls between the joist and block only.

I'm planning to replace the existing 6" of fibreglass with 200mm of PIR between the joists and overlaying 100mm PIR perpendicular to the stud trusses over existing 100mm of fibreglass batts

listermint

johnnyboy08 wrote: »

Not foaming in between them, just on the end ones where they are adjacent to the exterior gable walls between the joist and block only.

I'm planning to replace the existing 6" of fibreglass with 200mm of PIR between the joists and overlaying 100mm PIR perpendicular to the stud trusses over existing 100mm of fibreglass batts

Will you be using membrane over the joists area ? Also what's the ventilation plan for the joists. Because 200mm pir will block the joists from breathing

johnnyboy08

listermint wrote: »

Will you be using membrane over the joists area ? Also what's the ventilation plan for the joists. Because 200mm pir will block the joists from breathing

No, not using a membrane. I think that's too big of a step for a first timer doing this kind of work. It's hopefully an improvement that can be made down the line.

The joists are 8.5 inches so 200 pir won't cover them completely, there'll be some joist still exposed at the top. Even if there wasn't then would breathing be a problem? I've looked at application guides for kingspan k7 and quinntherm qr boards and they have joists completely covered with fibreglass or pir between the joists and pir over the joists laid perpendicularly. I was thinking about doing the same but to play it safe I'll just stick with between the joist pir.

I'd be interested in hearing if that's still the way to go and your thoughts on what is in those application guides I mentioned. Both are available online.

For reference, page 7 in
https://www.kingspan.com/irl/en-ie/product-groups/insulation/downloads/product-literature/kooltherm-k7-pitched-roof-board

listermint

Rule of thumb is that 50 percent of a joist should be open to air circulation to prevent rot. If not use a breathable insulation between the rafters like steico and then put a vapour membrane over it on the attic side.

As with Anything it's all in the detailing. Simply sticking insulation between rafters will have some value but if it's not completed holistically then it's easily defeated by the lack of finishing.

johnnyboy08

listermint wrote: »

Rule of thumb is that 50 percent of a joist should be open to air circulation to prevent rot. If not use a breathable insulation between the rafters like steico and then put a vapour membrane over it on the attic side.

As with Anything it's all in the detailing. Simply sticking insulation between rafters will have some value but if it's not completed holistically then it's easily defeated by the lack of finishing.

I see your point, for example leaving at least a 50mm gap between felt and insulation when insulating between the rafters but the application guides I referenced earlier for insulating between and over the joists don't mention that so what you're saying is at odds with their documentation. I can double check with kingspan on Monday.

Understood with doing a full on holistic job but my main aim in this mini retrofit effort is to eliminate the main sources of cold air infiltration and add insulation where absolutely necessary (some of the ventilated dormer space has no floor insulation whatsoever so heat from downstairs is being lost). I would be very nervous about adding a membrane or vapour control layer without knowing exactly what I was doing to be honest.

Lumen

Joists or rafters? This is confusing. Joists on floor, rafters on roof.

Insulate one or the other, not both, unless you have a need to maintain a converted attic at a different temperature from the rest of the house, e.g. as an occasional guest bedroom.

You can't use 200mm PIR between 216mm rafters because that doesn't leave 50mm ventilation gap.

If the attic is going to be cold and you're using full fill non breathable insulation in the floor you had better be damn sure the ceiling underneath is air tight, or you're asking for interstitial condensation.

johnnyboy08

Lumen wrote: »

Joists or rafters? This is confusing. Joists on floor, rafters on roof.

Insulate one or the other, not both, unless you have a need to maintain a converted attic at a different temperature from the rest of the house, e.g. as an occasional guest bedroom.

You can't use 200mm PIR between 216mm rafters because that doesn't leave 50mm ventilation gap.

If the attic is going to be cold and you're using full fill non breathable insulation in the floor you had better be damn sure the ceiling underneath is air tight, or you're asking for interstitial condensation.

It's joists only, and to describe further it's the joists in a ventilated dormer roof that I would be looking to insulate between.

I'm not sure that I'm following you on the condensation point. If I was to go the pir route it would be between the joists and from the plywood block underneath the former stud wall to the wall plate, lying on the plasterboard of the downstairs ceiling. There would be no gap between that plasterboard and the warm side of the insulation so I can't see where condensation would form assuming the pir is butted in tightly against the joists and sealed with tape as the cold air would have a hard job making it to the plasterboard/warm side of the insulation.

Again I have to go back to the installation guides that are out there, they mention condensation when doing between and over joists in combination but not for a single layer application. Maybe they are too simplistic but I can check with a pir manufacturer next week just to make sure.

Lumen

johnnyboy08 wrote: »

It's joists only.
The attic is ventilated so would be a cold space and the ceiling underneath wouldn't be airtight but again there's no mention of that in any installation guides I've come across. I could play it safe and fill between the joists with mineral wool rather than pir but my thinking there was to go for a cleaner insulation type with low u-value to thickness ratio.

OK. On a point of practicality, how neat are the spaces between your joists? In my attic it would be quite difficult to fit rigid insulation, as there are wires, downlighters and transformers all over the place, but odd cross-members at different heights.

johnnyboy08

Lumen wrote: »

OK. On a point of practicality, how neat are the spaces between your joists? In my attic it would be quite difficult to fit rigid insulation, as there are wires, downlighters and transformers all over the place, but odd cross-members at different heights.

You read my mind, that question was coming. I've pipes and cables to worry about.

So for the cables, I can move any of those which are running along the top of the downstairs ceiling plasterboard to the top of the joists so they wouldn't be covered in pir so heat can dissipate safely.

The pipes are a different matter though. Is it possible to alter between insulation types between different joists e.g. joists 1 & 2 use pir, joist 3 mineral wool, joist 4 pir etc?

Lumen

I would just use mineral wool between the joists. 200mm of mineral wood is loads, with rigid insulation over the top.

Where you should spend the effort is around the eaves.

johnnyboy08

Lumen wrote: »

I would just use mineral wool between the joists. 200mm of mineral wood is loads, with rigid insulation over the top.

Where you should spend the effort is around the eaves.

How so?

Lumen

johnnyboy08 wrote: »

How so?

I recently went round a friend's refurbished (non-dormer) bungalow with a relatively cheap FLIR camera gadget attached to my smartphone.

He's spent a fair amount of time and expense insulating the attic, but every room had about 1m of cold area on the ceiling up to the external wall, and in some cases the cold area spread even further back into the room.

You have to think about the fact that you have cold air rushing up under the eaves, ventilating the roof timbers (good), but that air movement and ventilation can play havoc with your insulation attempts if your detailing isn't exactly right.

You can't just ram insulation into every crevice, or else you'll ruin the ventilation.

I have read that the best approach is vent cards, but I haven't got round to using them myself.

This is a cross section which shows the air flow but not how they're fitted.

(random pic off Google, I'm not making a product recommendation)

johnnyboy08

Lumen wrote: »

I recently went round a friend's refurbished (non-dormer) bungalow with a relatively cheap FLIR camera gadget attached to my smartphone.

He's spent a fair amount of time and expense insulating the attic, but every room had about 1m of cold area on the ceiling up to the external wall, and in some cases the cold area spread even further back into the room.

You have to think about the fact that you have cold air rushing up under the eaves, ventilating the roof timbers (good), but that air movement and ventilation can play havoc with your insulation attempts if your detailing isn't exactly right.

You can't just ram insulation into every crevice, or else you'll ruin the ventilation.

I have read that the best approach is vent cards, but I haven't got round to using them myself.

This is a cross section which shows the air flow but not how they're fitted.

(random pic off Google, I'm not making a product recommendation)

Get ya. I had thought of doing something kind of similar where I was going to put plywood between the joists at the outer side of the wall plate so wind couldn't go through the insulation if that makes sense.

I then thought that that would only be a factor if the insulation was loose like mineral wool or blown, pir wouldn't allow the same volume of wind through. I could still place the plywood and pir to make sure that is ok but putting in the plywood would be a tough job. There's not much room to move around in that dormer space and you'd probably need to be some kind of yoga master to fit that in.

Lumen

johnnyboy08 wrote: »

Get ya. I had thought of doing something kind of similar where I was going to put plywood between the joists at the outer side of the wall plate so wind couldn't go through the insulation if that makes sense.

I then thought that that would only be a factor if the insulation was loose like mineral wool or blown, pir wouldn't allow the same volume of wind through. I could still place the plywood and pir to make sure that is ok but putting in the plywood would be a tough job. There's not much room to move around in that dormer space and you'd probably need to be some kind of yoga master to fit that in.

Plywood would be really expensive. I think vent cards are usually cardboard or plastic.

It's true that air can't pass through rigid insulation (at least not foil backed or closed cell stuff), but it can go around the edges.

I think trying to find one product to do both insulation and control of air movement is a bit of a flawed strategy.

Good luck anyway! :-)

johnnyboy08

Lumen wrote: »

Plywood would be really expensive. I think vent cards are usually cardboard or plastic.

It's true that air can't pass through rigid insulation (at least not foil backed or closed cell stuff), but it can go around the edges.

I think trying to find one product to do both insulation and control of air movement is a bit of a flawed strategy.

Good luck anyway! :-)

Cheers, thanks for your help. There's plenty of food for thought there.

MicktheMan

johnnyboy08 wrote: »

I'm not sure that I'm following you on the condensation point. If I was to go the pir route it would be between the joists and from the plywood block underneath the former stud wall to the wall plate, lying on the plasterboard of the downstairs ceiling. There would be no gap between that plasterboard and the warm side of the insulation so I can't see where condensation would form assuming the pir is butted in tightly against the joists and sealed with tape as the cold air would have a hard job making it to the plasterboard/warm side of the insulation.

It's not the cold air thats at issue, it's the warm moist internal air diffusing up through your ceiling past your insulation and then condensing on the colder attic side, getting trapped and rotting your timbers.