Insulation under suspended wooden floor

[karljones]

Hi all... My first post ever!!

I have a 1920's bungalow and I am just about to begin various renovation projects. I am having a complete rewire and complete new heating system (neither by me!!). I have suspended wooden floors throughout with 2 to 3 foot space underneath. All the carpets etc are coming up and a number of floor boards will be lifted for wiring and pipework as everything is being replaced. I intend to insulate under the floor of the whole house once these are complete.

I was planning to use Kingspan/Celotex at 150mm thick and fix either using batons or just nails on the joists. I don't mind lifting the floorboards if it will make life easier. Would this be best?

Someone else suggested using regular loft insulation and suspend using the orange builders netting stapled to the underneath of the joists. Would this be better?

Also does the insulation sit under or above the pipes?

Any advice would be most appreciated

Thanks

 

[ladylola]

Which is best depends a lot on how you are doing the job. I'd always prefer the regular fibre glass method as long as I can do the job from above. It's possible to do it by crawling underneath the floor but it's not the most pleasant job in the world, masks and goggles are an absolute must to do it from below and recommended if you are doing it from above. Kingspan requires accurate cutting but will support itself moreso than fibreglass.
In practice wheather the pipes are above or below the insulation really depends on how your plumber fits them. On a suspended ground floor it can be common to find the pipes below the joists as it makes them so much easier to fit with less drilling and jointing, so your pipes will be below the insulation. Just insulate the pipes themselves and don't worry about it.

 

[karljones]

Thanks for the reply.

If I do go down the fibre glass route would I need to use a DPM below the fibre glass? Am I right in saying Kingspan/Celotex dont need a DPM?

Also, which one will provide the best insulation?

Thanks again

 

[ladylola]

I'd forget the dpm , never fitted it when I've done this job. I think the thinking behind not fitting it is that it would be liable to trap moisture that either condenses or even spills or leaks in the insulation. With no dpm air currants in the floor space which should of course be vented will keep everything dry.
Which one provides the best insulation? Fibreglass will be easier to ram between the joists so less likelyhood of draughts, celotex has I think a higher insulation value but does lose effeciency . Have a search on here , somewhere someone gives details of values and how celotex degrades until seven years later it's the same as standard polyestrene. It's also likely to be harder to fit snugly if there are pipes and wires in the way.

Happy and hopefully a warm new year

 

[flameport]

Fibreglass will be less effective than Celotex or other foam insulation so you would have to use a greater thickness to achieve the same insulation level, but as fibreglass rolls are at least 10x cheaper and you have several feet of available space, the celotex option would be reserved for those who like throwing money away for no reason.

If you want to fit a vapour barrier, it would go on top of the insulation (i.e. put insulation between joists, DPM over the top, put floorboards on top of that.

Any pipes need to be insulated anyway, so won't matter where they are.
Electric cables should be kept out of the insulation.

 

[karljones]

Thanks both for replies. Fibreglass is the way forward

So, just to clarify... some form of netting like the orange thick plastic stuff that builders use (don't know what it called!!) stapled or tacked to the underneath of the joists. Could even use batons running at 90 degrees to the joists every couple of feet. My joist are 8" thick so I will now have 8" of insulation under the floor.

Is this worth the time and effort or would I need to be 12-16" to achieve the same results as Celotex?

Back to the DPM.. and 2 differing opinions. Below the floorboards is a concrete base. I have lots of air bricks around the outside and plenty of ventilation. The whole of the concrete base is dry. Do I need a DPM or as ladylola says potentially by causing a barrier above the insulation will I actually be causing more problems by trapping the moisture against the insulation? That would surely cause problems in the future?

Thanks again both

 

[JasonF]

Thanks both for replies. Fibreglass is the way forward

So, just to clarify... some form of netting like the orange thick plastic stuff that builders use (don't know what it called!!) stapled or tacked to the underneath of the joists. Could even use batons running at 90 degrees to the joists every couple of feet. My joist are 8" thick so I will now have 8" of insulation under the floor.

Is this worth the time and effort or would I need to be 12-16" to achieve the same results as Celotex?

Back to the DPM.. and 2 differing opinions. Below the floorboards is a concrete base. I have lots of air bricks around the outside and plenty of ventilation. The whole of the concrete base is dry. Do I need a DPM or as ladylola says potentially by causing a barrier above the insulation will I actually be causing more problems by trapping the moisture against the insulation? That would surely cause problems in the future?

Thanks again both

Don't put dpm on top of joists it will cause moisture and rot the joists

 

[JasonF]

Don't put dpm on top of joists it will cause moisture and rot the joists