Cheapest practical way to insulate under EXISTING wooden floor?

designermatt_2

27col wrote: »

The main thing, is to ensure that whatever insulation you use is held tight against the bottom of the floor boards. If it is allowed to slip at all there will be a draught of moving air against the bottom of the boards. This will completely negate the effect of the insulation. The principle of insulation is to maintain a mass of still air. I would recommend this basic precaution to anyone else who is proposing to insulate under a floor.

Actually this is the last thing you want to do. Suspended timber floors are ventilated and need to be to ensure he timber does not rot. This in a roof is referred to as a cold deck system.. where the structure is ventilated. A warm deck situation is where the structure is on the warm side of the insulation. The same is true for floors. If it is a suspended floor over the ground (not between rooms) then your insulation should only be beneath the joists, and should ideally form a vapour barrier too. the best away of doing this is as a previous poster suggested to use Celotex Kingspan or Extratherm, which is a rigid PIR insulant which you can fix to the underside of the joists and batten beneath and fix through the batten and insulation to the joists for extra security. All joints should be taped from underneath if at all possible (not possible everywhere obviously due to access). If you have any airbricks between the joists these should be ducted underneath the insulation using periscope vents, particularly true if you arenot insulating the whole of the ground floor. This is strangely enough something that I have just done and hope to reap the benefits of. It was made relatively easy for me by the fact that I have a good 700mm under my floor boards for me to rent out to an immigrant family or Anne Frank!.. It also serves as a good crawl space to do the insulating. Additionally you should inform Building Control as you are upgrading an existing thermal element and need to make an application... but who is gonna know!

27col

If there is a gap between the insulation and the bottom of the floor boards how are you going to get any insulating effect. I would think that it would be extremely difficult to seal the ends of the spaces between the joists so as to prevent a flow of air under the boards. Surely the basic principle of insulation is to maintain an area of still air.What you seem to be suggesting is to produce a whole row of air ducts running along the line of the joists. I agree with what you suggest, but only if the ends are well closed so as to produce a series of long sealed boxes between the joists.

designermatt_2

The ends of the joists where they dress into the wall would be insulated with sections of insulating board to ensure full insulation against the brick. Even if they aren't and it would be easy to do (I did it last week) this is a small area of uninsulated fabric that certainly should not have wind whistleing through as you seem to suggest. Whilst not ideal it wouldn't negate the efficacy of the subfloor insulation to such a degree that it becomes pointless carrying it out. It is common to insulate against 9" brick walls using PIR board ,battens and plasterboard. In this instance the battens and plasterboard can be omitted. Insulation is to prevent the loss of heat, as long as the air above the insulation is heated the insulation will keep that heat in whether or not it is moving or not. An unventilated airgap above the insulation such as you would get by undercloaking utilises the low emissivity of the foil face on the insulant, although 25mm is normally sufficient.

It is necessary to undercloak in the manner i have described to ensure that interstitial condensation does not occur within the structure.

Any insulation placed under the structure will, thickness against thickness, do more 'work' than that placed between. Additionally as the boards have straight edges, butting them against each other should create a much better joint than against structure. To do the insulation the way I have suggested doies require getting under the floor boards in a manner which I am lucky enough to be able to do. If it is your intention to insulate between I recommend you contact Celotex, (they have a phone technical advice line) or use a fee bit of software by Build desk to carry out the calculation to show if you are likely to suffer from interstitial condensation within the structure. Generally the rule is you can have insulation both between and below, but the primary insulant (thickest) should always be that below.

csps

designermatt wrote: »

Actually this is the last thing you want to do. Suspended timber floors are ventilated and need to be to ensure he timber does not rot. This in a roof is referred to as a cold deck system.. where the structure is ventilated. A warm deck situation is where the structure is on the warm side of the insulation. The same is true for floors. If it is a suspended floor over the ground (not between rooms) then your insulation should only be beneath the joists, and should ideally form a vapour barrier too. the best away of doing this is as a previous poster suggested to use Celotex Kingspan or Extratherm, which is a rigid PIR insulant which you can fix to the underside of the joists and batten beneath and fix through the batten and insulation to the joists for extra security. All joints should be taped from underneath if at all possible (not possible everywhere obviously due to access). If you have any airbricks between the joists these should be ducted underneath the insulation using periscope vents, particularly true if you arenot insulating the whole of the ground floor. This is strangely enough something that I have just done and hope to reap the benefits of. It was made relatively easy for me by the fact that I have a good 700mm under my floor boards for me to rent out to an immigrant family or Anne Frank!.. It also serves as a good crawl space to do the insulating. Additionally you should inform Building Control as you are upgrading an existing thermal element and need to make an application... but who is gonna know!

this is completely wrong i worked as a T.I.E. a thermal insulation engineer and the insulation must be touching the floorboards and definitly should not have a vapour barrier underneath i could go on and explain but just trust me i have insulated over 1000 houses

27col

You are wasting your time here csps. The last post on this thread was over 18 months ago.

stevall973

27col wrote: »

You are wasting your time here csps. The last post on this thread was over 18 months ago.

Nope! Not a waste of time! I'm going to insulate my floor,and reading the debate was very useful,definitely including the last one! Thanks everyone.

f11

I also am thinking about this. useful stuff, bit confused, will need to read properly another time!

aeromint

We are also going to be insulating underneath our wooden floors. Y/day we were visited by British Gas to see, if our house was suitable for wall cavity insulation - no. Apparently, the bloke was an ex-builder with lots of experience, & he said we were losing most of the heat through the wooden floor (cavity underneath). He also suggested to get some radiator panels for any external radiators. Hope all these changes will help us to reduce our future gas bills!

harberg

aeromint wrote: »

We are also going to be insulating underneath our wooden floors. Y/day we were visited by British Gas to see, if our house was suitable for wall cavity insulation - no. Apparently, the bloke was an ex-builder with lots of experience, & he said we were losing most of the heat through the wooden floor (cavity underneath). He also suggested to get some radiator panels for any external radiators. Hope all these changes will help us to reduce our future gas bills!

Be forever thankful for the advise not to insulate your walls. Many houses are not fit to have cavity wall insulation and would get damp walls because of it. As a landlord Ihave 4 houses of which one has cavity wall insulation. Also one of them has damp and mold problems. Can you guess which one?
On new build a small gap can be left between inner leaf and insulation. On a retofit it is hardly ever a good thing to do. I can not understand why energy companies and gouvernment are pushing it so much.

Financewiz

OK- this is thread necromancy but I would like to add my weight to harberg's warning about cavity wall insulation (CWI) - we purchased a house with it installed already and while the insulation certainly can improve heat retention, the chance of damp problems seems very high unless you have perfectly maintained brick/render exterior, a wall that meets every british standard going (and most don't) and do not live in a high rainfall/exposed house.

Energy Saving Trust does not recommend houses in exposed locations have it retro-fitted at all. We do, and the trouble trying to get CIGA (the 25-year government guarantee agency for cavity wall insulation) is amazing. They refuse to even acknowledge it could be an issue even when I have seen water dripping from the insulation with my own eyes and have proof of how much rubbish was already in the wall.