Underfloor ventilation guidelines

[John Rumm]

Or put it another way, how much is enough?

The question:

Has anyone got any bright ideas about controlling the level of under floor void airflow, and also setting the appropriate levels?

The background:

When we moved in here, there were some damp areas near the bottom of walls, and a rather musty smell in a long deep (unheated with little airflow) cupboard under the stairs. Investigation in the cupboard found that condensation was forming on the cold outside wall and running down it into the skirting and floor. This was rotten and rather wet. Looking under the floor found a rotten joist against the wall, and more worryingly the undersides of the floor boards were literally wet, and not just close to the source of the water.

So I replaced the joists, and insulated that end wall in the cupboard to fix the condensation.

To address the general dampness under the floor I concluded that the main problem was lack of airflow in the substantial underfloor void (4' deep or so in places with lots of dwarf walls holding up the downstairs floor joists), and investigations showed that there were only air-bricks on one side of the house, and not many off them. So I stuck in an additional few where there were conspicuous by their absence.

The result after several years, is that the under floor space is now bone dry, and also as a bonus the walls that were previously a little damp at the bottoms[1] have also dried out nicely.

So far so good. However the downside of this solution is that we are in a windy location, and that extra ventilation under the floor wastes more energy. Even though the floors are carpeted, and have underlay, on a cold windy day you can feel the difference. One fix would be to throttle the air flow a bit - we probably need less to keep it dry than that required to actually dry it out, but how much?

[1] probably down to condensation again, since they are 9" solid walls with render on the outside - so less able to breathe than they might otherwise be

[Hugo Nebula]

[Default] On Sat, 19 Jan 2013 18:19:45 +0000, a certain chimpanzee, John Rumm , randomly hit the keyboard and wrote: ...

A better one would be to insulate between the joists.

[newshound]

Sounds like you could use a remote humidity sensor on a data logger and/or with an accessible display. Plenty of cheap sensors for use in reptile vivariums but can't recall seeing one with a remote display (although don't some of the wireless weather stations have them?) Then you could throttle the air flow with dampers over the airbricks provided it was dry enough. Alternatively you could cut down on the airbricks and install an automatic bathroom type extractor fan with a humidity sensor. But I think I would be interested in looking at a long term log of humidity and temperature (external and underfloor) first to try to decide what is going on.

[John Rumm]

Indeed, and long term that may happen (although it would be a pig of a job, so I don't expect it will be soon!)

[John Rumm]

Yes, logging it longer term might be interesting. It may well be that you only need half the airflow, or can block some vents off completely etc.

[GMM]

Insulating under a floor if you don't have easy access from below is a pain and causes a great deal of disruption but my experience says it would be well worth it. We had to replace the living room floor in the summer, and it went back down with 3" of celotex beneath. This mow makes it by far the warmest (/most heat efficient) room in the house. Now that it's cold, I really wish I had got a move on dealing with insulating the dining room floor (!)

Now, assuming that insulation isn't going to happen in time to deal with current chilliness, I wonder how consistent ventilation has to be. In other words, although not a perfect solution, would it be feasible (ie safe) to reduce or block ventilation (possibly at one end) for a few months, then unblock so it all dries out properly during the spring and summer? Clearly not an ideal solution but modern treated timbers should be good enough to withstand a little bit of damp, at least if they get dried out after a bit.

Not a very scientific answer I know, but readily implemented....

[John Rumm]

Most rooms (bar the main lounge) have plenty of space under - so gaining an access point may be enough to do the floor rather than lifting the whole thing. I will have to investigate next time I need to get under one.

Did you cut your celotex between the joists, or place it under them?

My next insulation job was going to be to clad the outside of the house with a couple of inches of PIR foam as I am guessing that will be a bigger ROI than the floors.

Yup kind of the problem was trying to think through. I was about to say that my joists etc will not be "modern treated timber" given the age of the house, but I know the previous owners replaced most of the floors, so they may well be.

[GMM]

The celotex (cut tightly) sits on battens running along the joists. This was driven mostly by the fact it was installed from above as the floor was built. In doing the dining room though, I'll do the same as although it's over the cellar, I don't really want to lose the headroom down there. I'm still not conbvinced that 3" celotex was necessary as my impression is that the draught proofing effect is much more important than the insulation properties.

I'm sure that's right in terms of the insulating properties but I have been very surprised how much of an effect the floor job had on our living room. I did add some 30mm insulated-plasterboard to a patch of outside wall at the same time but my guess is that only makes a tiny contribution (offset by the dirty great single glazed sash windows).

If mine wasn't red brick all round, I might well be tempted to wrap the place in insulation too! As it is, there will be a layer of PIR (probably 2") installed on the inner face of each outside wall as I work through the house (cornices permitting).

[Dave Liquorice]

The relative humidity of the air isn't the whole story though. You really need to know what the dew point is and if any surface is at or below that temperature. You will only get condensation on surfaces that are at or below the dew point temperature.

Yes "damp air" will try an normalise with porous surfaces but I don't think that will be wet anything like condensation is wet.

[<me9]

Having had a flood from the dishwasher[1] in the kitchen a couple of years ago, and removing a lot of the floor I decided to insulate between the joists. I was sceptical as to what improvement it would make, but have been surprised at te increase in comfort level by not having a cold floor.

[1] Dishwasher waste pipe was the immediate flood which we noticed, but the plastic bowl above the compressor in the fridge had split, allowing defrost water to leak under the skirting board, possibly for many years, resulting in a very wet underfloor. It took some time to work out where the water had come from, but from its location it seemed to be impossible to be from the dishwasher as there was a dry section between the wettest section and the dishwasher. At first I suspected rising damp, and runoff from higher ground to the front of the property into the cavity (in wall between garage and house) and out where there was a void near the fridge, but investigation showed the cavity was dry.

One thing that amazed me was the amount of shrinkage in the floorboards over about a month as things dried. There was a hatch to give access to the stop tap under the floor, this was three boards wide, when removed (with difficulty) it was too wide by about 1/4 inch. on re-fitting the boards that section of floor, (about 2 metres wide) if cramped up would have resulted in a gap of about 50 mm left open. I had to space the boards between 3 and 5 mm apart to fill the space. __

[harry]

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You need to insulate the floor from underneath. You have four feet, so plenty of working space. I have 8"/200mm of rigid board under my floor.

[John Rumm]

Quite possibly - although its fairly low down the priority list at the moment.

The working space sounds good - but its subdivided by many dwarf / sleeper walls, so its not just a case of make a hole, climb in and get busy.

8" would be pointless here since the most I can get on the walls is about 2".

[Thomas Prufer]

There's a few solutions that turn on a fan or open a vent only if the net effect is to dry the cavity: there's a humidity sensor inside, a temperature sensor on the wall or floor, and a temperature sensor outside. A microcontroller calculates dewpoint/abs. humidity, and only allows airflow if the absolute humidity outside is lower than inside, and there won't be any condensation...

Many of these solutions are homebrew, but there are some commercial ones out there.

Thomas Prufer

[GMM]

I was thinking about this issue whilst trudging through the snow on my way home from work tonight.

I never really understand these draughts as it's not like there's a massive pressure differential across a floor, especially if the house is quite well sealed, so it's possible that at least a component of the flow is convection as the cold air beneath the floor replaces warm air rising inside the house. Is that another go at saying insulation would be a good thing? Maybe, though I don't mean to nag as it's a major task and not one for the winter for sure.

That said, one interim approach might be to baffle the air bricks, in much the same way as the black hole ventilators work: Just putting (say) a piece of ply, a little bigger than the air brick, on stand-offs of an inch or so would stop the wind flow without stopping ventilation as such, so should reduce the draught problem in an easily reversible way without too much concern about damp accumulating on the timbers.

In my experience, timbers post about 1960-ish have quite remarkable powers of recovery from damp. A few years ago in our last house a flat roof on a small unheated extension leaked during the cold weather, resulting in ice build up in the ceiling void. When this thawed, it ruined the palsterboard ceiling, which I took down. The timbers were, predicatably, absolutely soaked and it looked like they would need replacing. After a week or two of drying out though, they were good as new and I rebuilt the ceiling after fixing the leak. Clearly persistent dampness will always cause problems but short term variations are probably not something to be overly concerned about.

[John Rumm]

The drafts don't (particularly) get into the house - but in effect pass under it cooling the floor. With original setup (most air bricks just on two sides) there was no real cross flow of air and hence they did not really do much. Adding ones on the sides they were missing made a significant difference - presumably because the pressure will tend to be higher on the windward side of the house than on the trailing side.

Indeed, insulation would - I suspect even a little bit would make a big difference.

Yup, it is probably the approach I was going to follow short term anyway.

Perhaps one options would be to use a section of wall that was previously slightly damp as a tell tail - i.e. take moisture readings from it, then add baffles, and monitor what if any effect they have on the moisture.

[GMM]

Sounds like a good plan: With something to monitor, you have an early warning of any issues.

Ventilation seems (to me) to be a black art. In many situations, there is a specifiication like '100mm2 of purpose provided ventilation', which takes no account of the potential for the air to actually flow. When our gas fire was fitted (by a Gas Safe pro - don't want any comebacks on that, no matter how trivial it looked and how expensive it was) he installed a baffled vent. It doesn't allow any noticable draught but fulfills the ventilation requirement. This makes almost no sense to me (I would expect the volume of air exchanged to be the important parameter) but it keeps the regulators happy.

Presumably, a lower air flow under a floor would allow that air to warm a bit, which might carry any moisture (from the oversite) out before it can condense too much. Too little flow and everything stays wet, so there must be a sweet spot somewhere.

[RobertL]

I agree. I have the same job waiting to be done. I am sure there is a big market available for a product for retrofitting insulation between the joists without removing too many boards.

Robert

[RobertL]

I agree. These solid block insulating boards actually have thermal conduct= ivity that is about same as air. Under the floor, if there were no drafts= or leaks, then the static air would stratify (hot at the top, cold at the = bottom) and the insulation would not be needed. In the loft of course the h= ot surface is at the bottom so the air does not stratify and you need somet= hing to stop the convection.

I often wonder if underfloor insulation could be achieved by simply stretch= ing a membrane across wall-to-wall under the joists and filling all the gap= s between boards.

Robert

effect.

[RobertL]

Ideally you'd like the airflow to be limited to a set maximum. On windy days the limiter would limit the flow but on still days it wouldn't. This would seem to do the job ...

[John Rumm]

those on the typically windy side I suppose.