spray foam insulation

mattchippy

hi.just wondering does anyone have any comments/suggestions on spray foam on a new build.

mattchippy

but what about all these companies that claim that their foam is SO breathable you don't have to fit any card/vent

BryanF

mattchippy wrote: »

but what about all these companies that claim that their foam is SO breathable you don't have to fit any card/vent

Show me the iab cert ? Or BBA cert outlining their breathability. (By PM please first)

When you ring the manufacture of many of these products they are appalled that their 'trained' installers are recommending you don't need the vent cards

Macspower

Sorry for the late reply on this, but I think it's important to clarify the situation as this thread comes up in google searches.

The requirement of the card is nothing to do with the breathability of the product. Irish building regulations state that a 50mm gap must be left between the felt and the insulation.

This is not just for foam but for all other insulation such as rigid boards etc.

I'm afraid that my experience in the industry has brought me in contact with many inexperienced and some unscrupulous contractors. This is not to take away from the several very good contractors as well. Some will recommend spraying directly which is not the norm but can be done in certain circumstances.

I'll clarify more if anyone wishes to know.

4Sticks

Macspower wrote: »

I'll clarify more if anyone wishes to know.

What stops condensation forming on the soffit of the vent card ?

john f 4g63

Macspower wrote: »

Sorry for the late reply on this, but I think it's important to clarify the situation as this thread comes up in google searches.

The requirement of the card is nothing to do with the breathability of the product. Irish building regulations state that a 50mm gap must be left between the felt and the insulation.

This is not just for foam but for all other insulation such as rigid boards etc.

I'm afraid that my experience in the industry has brought me in contact with many inexperienced and some unscrupulous contractors. This is not to take away from the several very good contractors as well. Some will recommend spraying directly which is not the norm but can be done in certain circumstances.

I'll clarify more if anyone wishes to know.

Just a quick question about this,
If the roof is counter battened then is is ok to spray foam on to the underneath of the felt?

sydthebeat

john f 4g63 wrote: »

Just a quick question about this,
If the roof is counter battened then is is ok to spray foam on to the underneath of the felt?

no, what difference would the counter battening make?

cork2

john f 4g63 wrote: »

Just a quick question about this,
If the roof is counter battened then is is ok to spray foam on to the underneath of the felt?

The 50mm air gap should be on the internal side of the felt and a building regulation. Battening and counter battening is giving you a space on the external side of the felt, so you can create a space as big as you like and the air still won't reach the attic! I would never tell somebody not to use spray foam but speak to people who know what they're talking about, get a reputable company and if in doubt seek the advice of an engineer or architect. Professional advice isn't cheap but it's a fraction of the cost of a new roof which in the worst case scenario is the only solution when this goes wrong!

Macspower

4Sticks wrote: »

What stops condensation forming on the soffit of the vent card ?

If you look at the diagram below you will see that the vent card is breathable and any moist air that travels through the breathable foam can travel through the card and dissipate in the ventilation gap behind the card.

Macspower

john f 4g63 wrote: »

Just a quick question about this,
If the roof is counter battened then is is ok to spray foam on to the underneath of the felt?

this is one of the special circumstances I mentioned above.

This is not always the case but if a survey shows it is suitable the foam can be applied directly to breathable felt if the roof is counter battened. ( ie 50mm gap on outside.)

Without going into a big reply on this the most simple explanation is that the felt when fitted normally has a 25mm drape on it and if the foam is applied directly it "may" push the felt onto the tile/slate causing a moisture trap. If the roof is counter battened then the foam can in certain circumstances be applied directly. However this is not the normal procedure and vent card is fitted in most circumstances.

Macspower

cork2 wrote: »

The 50mm air gap should be on the internal side of the felt and a building regulation. Battening and counter battening is giving you a space on the external side of the felt, so you can create a space as big as you like and the air still won't reach the attic! I would never tell somebody not to use spray foam but speak to people who know what they're talking about, get a reputable company and if in doubt seek the advice of an engineer or architect. Professional advice isn't cheap but it's a fraction of the cost of a new roof which in the worst case scenario is the only solution when this goes wrong!

this is good advice.

john f 4g63

cork2 wrote: »

The 50mm air gap should be on the internal side of the felt and a building regulation. Battening and counter battening is giving you a space on the external side of the felt, so you can create a space as big as you like and the air still won't reach the attic! I would never tell somebody not to use spray foam but speak to people who know what they're talking about, get a reputable company and if in doubt seek the advice of an engineer or architect. Professional advice isn't cheap but it's a fraction of the cost of a new roof which in the worst case scenario is the only solution when this goes wrong!

I think its probably a lot safer for me to just cut in pir insulation between the rafters and leave a 50mm gap between it and the felt.
Is there much of a difference in u-value between 125mm pir insulation and 125mm spray foam?

sydthebeat

Macspower wrote: »

If you look at the diagram below you will see that the vent card is breathable and any moist air that travels through the breathable foam can travel through the card and dissipate in the ventilation gap behind the card.

The condensation will form between the foam and the card, so it's immaterial whether the card is "breathable" or not.

If water vapor can travel the whole way through the foam, card and felt, then the insulation isn't doing its job.

Macspower

john f 4g63 wrote: »

I think its probably a lot safer for me to just cut in pir insulation between the rafters and leave a 50mm gap between it and the felt.
Is there much of a difference in u-value between 125mm pir insulation and 125mm spray foam?

It depends on the product used. PIR boards will have a better U value but insulation is not all about U value. Airtighness is also an important factor.

Well installed PIR boards are also an excellent insulator.

john f 4g63

Macspower wrote: »

It depends on the product used. PIR boards will have a better U value but insulation is not all about U value. Airtighness is also an important factor.

Well installed PIR boards are also an excellent insulator.

I plan on using an airtight membrane as well.
My plan is to use 120mm pir insulation in between the 175mm rafters, this being kept flush at the bottom of the rafters. Then i will fit the airtight membrane across the rafters and then a 50mm insulated slab. So basically my airtight layer is sandwiched between the insulation.

MicktheMan

john f 4g63 wrote: »

I plan on using an airtight membrane as well.
My plan is to use 120mm pir insulation in between the 175mm rafters, this being kept flush at the bottom of the rafters. Then i will fit the airtight membrane across the rafters and then a 50mm insulated slab. So basically my airtight layer is sandwiched between the insulation.

Why sandwich the a/t layer in the insulation when you have the opportunity to put it in the correct place (i.e. on the warm side)?

john f 4g63

MicktheMan wrote: »

Why sandwich the a/t layer in the insulation when you have the opportunity to put it in the correct place (i.e. on the warm side)?

How can i put it on the warm side?
The only reason i am putting it in between is so as i can fix it the rafters and then i can also fix the insulated slab to the rafters.

MicktheMan

john f 4g63 wrote: »

How can i put it on the warm side?
The only reason i am putting it in between is so as i can fix it the rafters and then i can also fix the insulated slab to the rafters.

Don't use insulated slabbing. Install the insulation (between & under rafters) , then airtight barrier, then service cavity (if required), then slab/skim finish.

john f 4g63

MicktheMan wrote: »

Don't use insulated slabbing. Install the insulation (between & under rafters) , then airtight barrier, then service cavity (if required), then slab/skim finish.

OK, Thanks for the advise. Might have another look at the detail again.

Sandman777

MicktheMan wrote: »

Don't use insulated slabbing. Install the insulation (between & under rafters) , then airtight barrier, then service cavity (if required), then slab/skim finish.

Whats the reason for not putting insulated slabs under airtight barrier?

MicktheMan

Sandman777 wrote: »

Whats the reason for not putting insulated slabs under airtight barrier?

General rule of thumb is that at least 2/3 of the effective insulation should be on the cold side of the air tight barrier in order to avoid the risk of interstitial condensation.
This is all well & good, however, I would be concerned about what happens at the rafters (i.e. where there is no insulation on the cold side).

Sandman777

MicktheMan wrote: »

General rule of thumb is that at least 2/3 of the effective insulation should be on the cold side of the air tight barrier in order to avoid the risk of interstitial condensation.
This is all well & good, however, I would be concerned about what happens at the rafters (i.e. where there is no insulation on the cold side).

Thats ok, i have 2/3 of mine on the cold side of the barrier! I put the insulated slabs across the ceiling for cold bridging as i have floored out 2/3 of attic for storage!

Terryd1987

What kind of money(ballpark) would it cost to do this job on a 900 sq ft cottage with a hip roof?

Macspower

Terryd1987 wrote: »

What kind of money(ballpark) would it cost to do this job on a 900 sq ft cottage with a hip roof?

To even give a guideline price more details would be required.. Feel free to send me PM and I'll get the required measurements to give you an idea.

The industry seems to be charging from about 18 euro per m2 to around 22 per m but there are a lot of variables such as rafter depth U value required etc etc..

there is also a huge variance in the quality of the work done so look for NSAI and SEAI registration as a minimum and look for references. You will find like most items that you get what you pay for.

hope this is of some help.