Insulate concrete floor

[mark138]

I’m renovating a 1950s detached house that has an extension to the rear. The original is suspended timber and the extension is concrete. I’d like to insulate the floor but the obvious thing to do seems to be to go under the floor boards and over the concrete. That would create a step between the two which I couldn’t live with. Building up over the floor boards creates problems with the stairs and front door. I’m assuming digging into the concrete to reduce the level would be a crazy amount of work? Is there an easy way to do that?

 

I’m also considering under floor heating while I’m at it but I’d only do that if it was insulated beneath. 
 

Is there a perfect solution? Or what is the best compromise?

[Iceverge]

49 minutes ago, mark138 said:

I’m assuming digging into the concrete to reduce the level would be a crazy amount of work? Is there an easy way to do that?

 

Probably not even though if it's an old extension there may not be as much concrete as you fear. Maybe drill a few trial holes with a masonry bit to find out how thick it is?  A day or two with a pneumatic breaker and a few men with barrows will see it through either way. Plenty of people here have tackled it. 

 

For the suspended floor this is the easiest and best way IMO. 

 

 

[mark138]

Thanks, very helpful. So drilling a trial hole should be easy enough. The extension is about 16 years old. 
 

That image led me to: https://www.ecologicalbuildingsystems.com/post/suspended-timber-floor-insulation-best-practice-installation-guide Looks good, I’m curious how it could be done from below as I have a large void beneath half the house (it’s on an incline) which would save me taking up the floorboards. Maybe it’s just better to remove the floorboards and replace with something like https://omnie.co.uk/torfloor-suspended for UFH?

[Iceverge]

At 16 years old your extension should have some insulation. I have no idea how much. 

 

Probably not enough for UFH as your downward losses will be too high but maybe enough to avoid digging it up. Impossible to say unless you can track down the plans or someone who was involved in the building.

 

Are radiators and a floor covering that is warm underfoot an option? 

 

Re the suspended floor. Theres no reason you can't push mineral wool up from below of you have access. Not pleasant job however! You'll find it more difficult to detail the edges of any membrane to guarantee airtightness without taking up the floor boards too. 

[mark138]

I’d be very surprised if it had insulation cause it was either diy or cowboy builders. A drill hole should confirm. Not insulating and using existing radiators is probably what people would typically do. I want to consider how much more it would cost to do a quality job.

 

I’m guessing membranes would be fairly pointless if you’re insulting the suspended floor from below as you can’t go over the joists with floorboards still nailed to them and you don’t want to cover the joists completely so they can breathe. I was considering using wood fibre batts instead of mineral wool as it would be nicer to work with and cheaper than hemp/sheeps wool.

[Iceverge]

A good breather membrane would still help vastly with airtightness. You'd need to be careful of condensation on the inside of it though. The Pro Clima detail above really is the best solution. 

 

You could probably determine how much insulation was there from a test hole.