Floor insulation with a ventilated crawl space

[geraldthehamster]

100mm will be plenty and is widely available. Heat loss through floors is not great compared to ceilings, it's mostly draughts.

Hence the importance of squishing the rock wool tightly between the joists.

Don't forget to tell a grown-up where you're going, and take a mobile phone

Cheers
Richard

 

[manzana]

I have just seen a video on youtube where it is encouraged to also insulate the floor joists after insulating in between them.
http://www.youtube.com/watch?v=zd9RKeTrdXI
See towards the end of the video.

Has anyone done this? What material did you use for it?

 

[geraldthehamster]

I have just seen a video on youtube where it is encouraged to also insulate the floor joists after insulating in between them.
http://www.youtube.com/watch?v=zd9RKeTrdXI
See towards the end of the video.

Has anyone done this? What material did you use for it?

I don't think it's essential as long as your under floor space is well ventilated.

You could screw 25mm Celotex to the underside of the joists. But you'd be limited by the size of board you could get through whatever access hatch you'd created. If you do this I'd use some kind of washer to stop the screw heads pulling through the Celotex.

When I had my floorboards up, I fixed Celotex underneath the joists as well as between them. Probably overkill.

But really I wouldn't bother. Although some heat is lost through the floor, this is mostly about draughts.

Cheers
Richard

 

[JohnD]

I have just seen a video on youtube where it is encouraged to also insulate the floor joists

I would go so far as to say it is pointless. With mineral wool stuffed into the gaps, you now have about 7 inches of softwood in the joists, which is ample for a floor.

If you were using Celotex or kingspan between the joists, there would always be hairline, or bigger, cracks that cold air would find.

leaving them open will also facilitate ventilation.

the video looks like it is made by an insulation manufacturer, so the more product they shift, the more money they make. A Canadian outdoor deck will be much colder than a UK crawl space.

 

[manzana]

Hello,

I came across people talking about using a MOISTURE BARRIER when doing this job.
Can anyone shed some light if it is necessary to use it?

Also, how to use it?
Some people say use it between the wood floor and the insulation material.
Others say, at the bottom of the insulation material or just on the ground/earth itself.

Cheers!

 

[geraldthehamster]

Hello,

I came across people talking about using a MOISTURE BARRIER when doing this job.
Can anyone shed some light if it is necessary to use it?

Also, how to use it?
Some people say use it between the wood floor and the insulation material.
Others say, at the bottom of the insulation material or just on the ground/earth itself.

Cheers!

Vapour barriers are used on the warm side of insulation. But, in my opinion, a vapour barrier is not necessary in a suspended timber floor with a ventilated under-floor space.

Cheers
Richard

 

[jabuzzard]

100mm will be plenty and is widely available. Heat loss through floors is not great compared to ceilings, it's mostly draughts.

Personally if using mineral wool/fibreglass I would use a thickness that matched my joists. So if my joists are 100mm deep I would use 100mm mineral wool etc. That way the chicken wire/netting you are using to hold it in place will keep it against the floor boards.

 

[AronSearle]

Heat loss through radiation is equal in all directions, heat does not rise.

100mm of mineral wool is better than nothing, but it is still only 100mm of mineral wool.

 

[jabuzzard]

Heat loss through radiation is equal in all directions, heat does not rise.

100mm of mineral wool is better than nothing, but it is still only 100mm of mineral wool.

Except heat loss from a room is not 100% radiative. In fact it is not even close. In particular given that hotter gases are less dense which means that they do rise will always result in more heat being lost from a room through the ceiling than through the walls and floor. There is a reason build regulations require more insulation on a roof than on walls or floors.

100mm of mineral wool will make a huge difference to the heat loss from a room. The additional savings from using a rigid PU foam will take many years (most likely decades) to recoup.

 

[AronSearle]

Yes I Know it's not 100%, but it is not negligble, with modern membranes and airtightness practices it is much more important.

And it's walls that have the lowest U-value requirent, not floors.

 

[jabuzzard]

Yes I Know it's not 100%, but it is not negligble, with modern membranes and airtightness practices it is much more important.

A suspended timber floor is never going to be airtight.

By far the biggest heat loss through a suspended timber floor is through conduction through the floor itself and then forced convection carrying the heat away from the underfloor space. In this respect 100mm of mineral wool will and does make a massive difference. The difference between a cold windy day and a cold calm day was very noticeable in my house. With the former being much colder downstairs than the latter.

I would note further that the heat loss from radiation depends on the fourth power of the temperature differential (in addition to the emissivity of the surface) , the lower surface temperature of the floor underside that you will achieve from the 100mm of mineral wool will make a huge difference to the radiative heat losses.

And it's walls that have the lowest U-value requirent, not floors.

I never said otherwise, I was pointing out that they are both much lower than that for ceilings/roofs.

 

[AronSearle]

A suspended timber floor is never going to be airtight.

We won't agree on this, because I'm coming from the point of view of modern quality build and standards, not bob the builder working to 1980s standards.

You also make a confusing argument, you explain that rigid foam under mineral wool is not worth it, then explain exactly why it is by pointing out all the convenction loss that happens in a floor, that *still* happens with just 100mm mineral wool.

I never said otherwise, I was pointing out that they are both much lower than that for ceilings/roofs.

.013 roof
.028 walls
.22 floors

That is not "much lower", 100mm of mineral wool is better than nothing, but it is still not great.

 

[jabuzzard]

A suspended timber floor is never going to be airtight.

We won't agree on this, because I'm coming from the point of view of modern quality build and standards, not bob the builder working to 1980s standards.

Which is utterly irrelevant when considering someone talking about retrofitting insulation to a floor.

You also make a confusing argument, you explain that rigid foam under mineral wool is not worth it, then explain exactly why it is by pointing out all the convection loss that happens in a floor, that *still* happens with just 100mm mineral wool.

It would appear that both your reading comprehension and understanding of the laws of thermodynamics is lacking. I said that rigid foam is not worth it compared to mineral wool because the payback time is so long. Sure it will save on the heating bills but if it is going to take 20 years to get your money back coupled with the hassle of retro fitting rigid foam then it is probably not worth it.

Also convection of heat through the timber and then through the insulation happens for all types of insulation. Rigid foam does not magically not conduct heat.

I never said otherwise, I was pointing out that they are both much lower than that for ceilings/roofs.

.013 roof
.028 walls
.22 floors

That is not "much lower", 100mm of mineral wool is better than nothing, but it is still not great.

Assuming that you meant .022 and not .22 it is significantly lower than that required for a roof.

 

[JohnD]

explain exactly why it is by pointing out all the convenction loss that happens in a floor, that *still* happens with just 100mm mineral wool.

Tell us more about your weird idea that heat loss through a floor occurs by convection.

You think that the warm air travels by downwards convection, towards, through and away from the floor and into the void? Crazy.

And you think that mineral wool does not block convection currents when they do happen? (through a ceiling, obv, not through a floor)

 

[AronSearle]

Which is utterly irrelevant when considering someone talking about retrofitting insulation to a floor.

No, it is not, you can jump up into your loft whenever you fancy, but most householders will only rip up the floor once in their lifetime, if that. No reason not to spend a little more time and effort properly upgrading it.

Heat loss happens in a building through radiation and convection.

You describe exactly how air movement can effect a mineral wool insulated floor, air can be pulled through the insulation through the buildings convection, as well as cold air being blown through the insulation around vents, (which also happens in roofs).

Underlying the joists with a non air permeable insulation (can still be vapour permeable) stops this effect, with good workmanship and in if necessary membranes, then the major heat loss effect in the floor is then only through radiation, and 100mm of mineral wool is 100mm of mineral wool.

And you can use EPS instead of expensive rigid foam, significantly cheaper.