Insulating under wooden floor boards???

[classybird]

Downstairs in my house I have exposed wooden floorboards which are great, however in winter time they do get a little cold. With heating prices going up I have decided to insulate underneath them. (There is a space about 1 metre high running under the house). Firstly would this cause any problems such as dry rot or condensation under the floorboards or in the house? It is quite cold and airy down there.

Then there is the question of what to use. I was thinking of the yellow foam insulating board covered in the silver foil, Polyiso is it called, that I could cut a little thicker than the joists and push into position. I have heard that Polyiso is the most expensive of the foam board insulation. Therefore are there effective cheaper alternatives or is that the best thing to use. I also want to bear in mind the ease of doing it rather than choosing a method that is a load of messing about.

Any advice, do's and dont's about this would be greatfully received. Thank you!

 

[r896neo]

the insulation needs to be tight up against the floor boards and the joists then have free cold air circulating underneath them to prevent rot.

Mineral wool is much cheaper and easier to get a good result as rigid board like celotex needs to be very well fitted as small gaps will cause draughts.

Working with wool in a confined space above your face is not nice and the celotex would be better from that point of view.

Electrical cables need to be either below the insulation in free space or left clear.

 

[classybird]

Hi, thanks so much for the reply!

 

[thermostadt]

Hi,

I ripped up my old floor some years ago and laid oak flooring – it looks lovely.

I insulated between the joists with Cellotex to stop drafts and try and keep the floor warmer because I was losing the carpet. Instead of butting the insulation tight against the floor-boards I installed it at the bottom of the joist, leaving an air-gap of a couple of inches. This is where all the radiator pipes run. Rather than them run under the insulation and heating the void under the floor I wanted them to sit above the insulation and below the floor to heat the house.

It’s like under floor heating where the pipes run.

I’m not expecting rot.

 

[r896neo]

Your hoping not to get rot.

 

[thermostadt]

Your hoping not to get rot.

Well, yes, I’m hoping not to get rot in the way that we all hope not to get rot under the floor. There is ample air movement under the floor void and there is a couple of inches between the insulation and the floor for air movement; helped by the radiator pipes causing convection currents.

 

[joe-90]

Can't see how you would get rot. Sounds fine to me.

 

[r896neo]

I was just playing devils advocate a little.

I agree it would be fine but I think you should always advise people to do things by the book unless there is a reason why they need a work-around.

Yes i know the 'by the book' comment is fuel on the fire for some but you know what i mean.

 

[classybird]

Thanks again to all those who have replied!

 

[gwernybwch]

I installed it at the bottom of the joist, leaving an air-gap of a couple of inches. This is where all the radiator pipes run. Rather than them run under the insulation and heating the void under the floor I wanted them to sit above the insulation and below the floor to heat the house.

It’s like under floor heating where the pipes run.

That sounds like an ingenious way of doing it!
A couple of questions - did you lay anything like netting or foil under the bottom of the joists to stop the insulation dropping down? Also did you lay anything like a plastic sheet / membrane on the top of the joists / bottom of the floorboards to stop any draft coming up through?

I agree it would be fine but I think you should always advise people to do things by the book unless there is a reason why they need a work-around.

Can you outline what you consider to be the correct way to insulate under a suspended wooded floor? Not a flame, just want to know what is considered best practice in these situations.

 

[r896neo]

The general principle of insulating spaces is never to have an unventilated air pocket as there is a possibility for condensation to form in it. Spaces should be fully filled or open somewhere and therefore ventilated.

For example a space such as between a joist should in theory either be completely filled with insulation or insulation packed tight against the underside of the floorboards leaving the bottom half of the joists exposed and ventilated from underneath.

If you were to board across the base of the joists with say kingspan boards and then put a membrane of some sort over the top of the joists as well you would have a large air pocket which would have warm air sitting in it. That air may also have moisture in it and this has nowhere to go so when the heating is off for a while it will condense onto the surface of the joists (or more likely the foil face of the kingspan) leaving dampness in this cavity. The dampness will then gradually rot your floor joists as there is no ventilation to let them dry out.


In reality however it would take a myriad of different factors all coming into play at the same time to give you problems but why take the risk. Insulation mftrs know what they are on about so check out their advice.