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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Insulating beneath Ground Floor https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1441655224 Message started by Sandy on Sep 7^th, 2015, 8:47pm

Title: Insulating beneath Ground Floor
Post by Sandy on Sep 7^th, 2015, 8:47pm

Hi all,

I've search quite a few threads about insulating on here, but can't find what I'm looking for.

I'm moving into a new house (c 1930's, not new build) that has floor boards over wooden joists, suspended between the inner courses of bricks about 18"/24" above the ground/soil/earth (now long dried).

I want to have underfloor heating installed and engineered timber flooring laid over this, but to gain the most and prevent as much heat escape as possible I want to provide as much insulation as possible underneath.

As I'll be lifting parts of the floor to allow easy access to wiring and plumbing, I am re-laying chipboard sheeting down to give a nice flat, bump free surface to work with. Whilst the floor's up and I can get between all the joists, is there a good method of inserting something (fixing/wedging) between the joists, maybe something like Kingspan to reduce heat leakage downwards? I realise that there is good reason to allow a good airflow under joists and floor boards to prevent condensation/damp/mildew etc, so I have considered this. I also realise that whatever I use should not touch the floor beneath to prevent wicking/capillary action etc.

Anyone any thoughts about the best way to insulate beneath the floor? Is it necessary? I've thought about foil backed materials, but heat can escape through conduction, right?

Thanks in advance.

Sandy.

Title: Re: Insulating beneath Ground Floor
Post by Lectrician on Sep 8^th, 2015, 6:26am

Building regs require this these days. Not sure on required thickness, but when I did my extension, 100mm was used.

Small battens nailed the length of the joists, 100mm down. Celotex/kingspan laid in tight.

Title: Re: Insulating beneath Ground Floor
Post by thescruff on Sep 8^th, 2015, 8:47am

What type of underfloor heating Sandy.

Assuming water.

The insulation Wedged between the joists on battens as above.

The next bit is important and contrary to your thinking.

Then add the manufacturers insulation should have groves to either clip the pipe too or better to take the steel/aluminium floor-plates which are fixed too the floor floor joists. ( in all cases I would recommend the floor plates)

Cover with a vapour barrier.

When you first test keep the heat to no more than 16c and build it up 1c a day to the design temperature.

Note;

Air between the heating surface and floor is bad as it acts as a barrier

Title: Re: Insulating beneath Ground Floor
Post by Sandy on Sep 8^th, 2015, 9:10am

Thanks both for your replies.

Board type insulation is simple enough by the sound of it.

Underfloor heating - I'm seriously considering the electric "mat" type of set up. It's thinner construction, no moving parts to break down or need repair, no potential for water leaks, ability to have different temp settings in each room.

I'm not suggesting you're wrong, but can I ask why you would assume water based underfloor heating?

From research I've done, I'm led to believe that £ for £, underfloor heating is more effective at heating than rads alone as the heat is distributed evenly across the whole surface area of the room, instead of a given radius around a wall mounted rad. At the moment, there is a central heating system in place and standard wall mounted rads. I appreciate that maybe in the coldest parts of winter underfloor heating might not be able to fully support heating a house, so there will be a need for some gas CH, but I just don't want to rely 100% on it if there is a more efficient option.

Of course, I could be completely wrong with my self-assessment, so I'm very much interested to hear other thoughts/opinions on it.

I also want to be able to take advantage of some renewable power sources (PV and/or wind) if I can get enough money together. Hopefully linked up will offer some support instead of relying 100% on mains power.

Regardless of whether I stick to wall mounted rads with gas CH, or move to an underfloor heating (water or electric), is insulating under the floor surface of any benefit, or likely to just be an unecessary expense?

Thanks again

Title: Re: Insulating beneath Ground Floor
Post by thescruff on Sep 8^th, 2015, 10:41pm

I assumed water because you mentioned water pipes in your opening post. However. electric present several problems in addition the running costs will be far greater than water although if you can get cheap electric then the gap should reduce, you should certainly get an economy meter for it.

I would put 50-100mm insulation between the joists then board it. On top of that I would use a flexible tile adhesive followed by a tile backer board with cement on both sides.

The heating mats would go next and covered with 8-10m of flexible Latex self-leveling compound.

Finally you would need a suitable adhesive to fix the wooden floor down or have it floating which I'm not sure is recommended.

Title: Re: Insulating beneath Ground Floor
Post by Sandy on Sep 10^th, 2015, 7:34pm

Ahh, ok, I see your point.

Ok, some good points. Thanks to you all for your input.